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Guide for In-Place Treatment of Wood in Historic Covered and Modern Bridges

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Guide for In-Place
Treatment of Wood in
Historic Covered
and Modern Bridges
Stan Lebow
Grant Kirker
Robert White
Terry Amburgey
H. Michael Barnes
Michael Sanders
Jeff Morrell
United States
Department of
Forest Service
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200 mL
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In cooperation
with the
United States
Department of
March 2012
Lebow, Stan; Kirker, Grant; White, Robert; Amburgey, Terry; Barnes, H.
Michael; Sanders, Michael; Morrell, Jeff. 2012. Guide for In-Place Treat-
ment of Wood in Historic Covered and Modern Bridges. General Technical
Report FPL-GTR-205. Madison, WI: U.S. Department of Agriculture, For-
est Service, Forest Products Laboratory.
43 p.
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Historic covered bridges and current timber bridges can be
vulnerable to damage from biodeterioration or fire. This
guide describes procedures for selecting and applying in-
place treatments to prevent or arrest these forms of degrada-
tion. Vulnerable areas for biodeterioration in covered bridg-
es include members contacting abutments, members near
the ends of bridges subject to wetting from splashing and
members below windows or other openings that allow entry
of wind-blown precipitation. Pressure-treated timber bridge
members can be vulnerable when untreated wood is exposed
by field fabrication or by the development of drying checks.
The objective of an in-place preservative treatment is to
distribute preservative into areas of a structure that are
vulnerable to moisture accumulation and/or not protected
by the original pressure treatment. Types of field treatments
range from finishes, to boron rods or pastes, to fumigants.
A limitation of in-place treatments is that they cannot be
forced deeply into the wood as is done in pressure-treatment
processes. However, some can be applied into the center
of large members via treatment holes. These preservatives
may be available as liquids, rods or pastes. Bridge members
can be treated with fire retardants to delay ignition, reduce
heat release, and slow the spread of flames. In-place coating
products are available to reduce surface flammability, but
these coatings may need to be reapplied on a regular basis if
exposed to weathering. For more integrated protection, fire
retardant treatment of bridge members may be combined
with other forms of protection such as lights, alarms,
sprinklers and monitoring systems.
Keywords: guide, covered bridge, timber bridge,
deterioration, fire, wood preservatives, in-place treatment
This study is part of the Research, Technology and
Education portion of the National Historic Covered
Bridge Preservation (NHCBP) Program administered
by the Federal Highway Administration. The NHCBP
program includes preservation, rehabilitation and resto-
ration of covered bridges that are listed or are eligible
for listing on the National Register of Historic Places;
research for better means of restoring, and protecting
these bridges; development of educational aids; and
technology transfer to disseminate information on cov-
ered bridges in order to preserve the Nation’s cultural
This study is conducted under a joint agreement be-
tween the Federal Highway Administration–Turner
Fairbank Highway Research Center, and the Forest
Service – Forest Products Laboratory.
Federal Highway Administration Program Manager–
Sheila Rimal Duwadi, P.E.
Forest Products Laboratory Program Manager–
Michael A. Ritter, P.E.
Introduction ............................................................................................. 1
Causes of Biodegradation ................................................................... 1
Effects of Climate ............................................................................... 2
Role of Wood Structure ....................................................................... 3
Problem Areas for Biodegradation in Wooden Bridges ...................... 3
Good Practices .................................................................................... 7
Types of Supplemental Treatments ......................................................... 8
Water-Diffusible Preservatives ........................................................... 9
Non-Diffusible Liquid Treatments .................................................... 12
Fumigants .......................................................................................... 13
Applying Supplemental Treatments ...................................................... 14
Internal Treatments ...........................................................................14
Water-Diffusible Internal Treatments ................................................ 14
Fumigants .......................................................................................... 15
Non-Diffusible Liquids ..................................................................... 16
External Treatments .......................................................................... 16
Research on the Use of Supplemental Preservative Treatments for
Covered Bridges .................................................................................... 17
Summary of Supplemental Treatment Concepts ...................................21
Liquid Surface Treatments ................................................................ 21
Paste Surface Treatments .................................................................. 21
Internal Treatments ...........................................................................21
Example Supplemental Treatment Applications ............................... 22
Who Can Apply Supplemental Preservative Treatments? .................... 31
The EPA Pesticide Label is the Law ................................................. 31
Collection of Drill Savings ............................................................... 32
Fire Prevention ...................................................................................... 32
Contributing Factors ......................................................................... 32
Research on the Use of Supplemental Fire-Retardant Treatments
for Covered Bridges .......................................................................... 34
Fire Protection Technology ............................................................... 35
Conclusions ........................................................................................... 35
References ............................................................................................. 37
Additional Sources ................................................................................ 38
Appendix—Contact Information for State Offices Conducting the
U.S. EPA’s Certified Pesticide Applicator (CPA) Program ................... 39
Guide for In-Place Treatment of Wood in
Historic Covered and Modern Bridges
Stan Lebow, Research Forest Products Technologist
Grant Kirker, Research Forest Products Technologist
Robert White, Research Forest Products Technologist
Forest Products Laboratory, Madison, Wisconsin
Terry Amburgey, Professor Emeritus, Department of Forest Products
H. Michael Barnes, Professor, Department of Forest Products
Michael Sanders, Senior Research Associate, Department of Forest Products
Mississippi State University, Starkville, Mississippi
Jeff Morrell, Wood Science and Engineering Professor
Oregon State University, Corvallis, Oregon
Wooden bridges have a long history of use throughout the
world, and like bridges made of other materials, the service
life of wooden bridges can be enhanced through proper
construction, inspection, and maintenance. Wooden bridges,
whether historic covered bridges or current highway timber
bridges, can be vulnerable to damage from biodegradation.
Because of the long history of using wooden bridges, the
causes of wood biodeterioration are well documented, as are
the means to mitigate their effects. Biodeterioration is mini-
mized through design and construction practices, and in the
case of modern timber bridges, through pressure treatment
of the timbers with wood preservatives. However, the po-
tential for degradation remains, and over time many bridges
need maintenance that may include in-place treatment with
preservatives. Fire is also a major threat, particularly for
covered bridges. In this manual, we describe procedures
for selecting and applying in-place treatments to bridges to
prevent or arrest degradation. This guide focuses on preser-
vative treatments to protect against biodeterioration, but also
briey discusses approaches for minimizing damage caused
by re.
Causes of Biodegradation
Some understanding of the causes of biodegradation is help-
ful when considering in-place treatment. Because there are
many excellent sources of information on the organisms that
damage wooden structures (see References), this guide pro-
vides only a brief summary.
In most applications of wooden construction materials, de-
cay fungi are the most destructive organisms. Fungi are mi-
croscopic thread-like organisms whose growth depends on
mild temperatures, moisture, and oxygen. In part, the high
degree of damage by wood-decay fungi reects their
ubiquitous presence in all locations. Given suitable
conditions, attack by some type of wood-decay fungus is
assured. Numerous species of fungi colonize wood, and
they have a range of preferred environmental conditions.
Decay fungi are often separated into three major groups:
brown-rot fungi, white-rot fungi, and soft-rot fungi. Brown
rot and white rot are usually the most destructive and are the
fungi most likely to be found in wood above ground. These
two groups of decay fungi have differences in wood species
preferences and in the manner that they degrade the wood,
but the optimal environmental conditions to cause wood
decay are fairly similar for both groups. Both decay types
can cause substantial damage in a relatively short amount of
time. Soft-rot fungi, in contrast, generally prefer wetter, and
sometimes warmer, environmental conditions. Damage by
soft-rot fungi resembles that by brown-rot fungi, but typi-
cally occurs more slowly and nearer to the wood surface.
Termites follow fungi in terms of the amount of damage
to wood structures in the United States. Their damage can
occur more rapidly than that caused by decay, but their geo-
graphic distribution is less uniform. Numerous termite spe-
cies are native to the United States, and like decay fungi, the
type and severity of attack varies by species. Termite species
in the United States can be grouped into the categories of
ground-inhabiting (subterranean) or wood inhabiting (non-
subterranean) termites. Most damage in the United States is
caused by species of subterranean termites. The threat from
subterranean termites has increased with the spread of the
non-native Formosan subterranean termite (FST) in some
areas of the southeastern United States. Non-subterranean
termites are less damaging because they have a narrower
geographic range and degrade wood more slowly due to
smaller colony size.
Other types of insects such as powderpost beetles and car-
penter ants can cause notable damage in some situations, but
their overall signicance pales in comparison to decay fungi
General Technical Report FPL–GTR–205
and termites. Other organisms, including bacteria and mold,
can also cause damage in some situations. Several types of
marine organisms degrade wood placed in seawater. On an
economic basis, however, decay fungi and termites are by
the far the most destructive pests of wood used in terrestrial
Effect of Climate
We have long recognized that exposed wood deteriorates
more rapidly in warm, wet climates than in cold and/or dry
climates. Historically, use of wood as a construction mate-
rial mirrored this effect, with greatest use occurring in north-
ern latitudes. The two greatest factors inuencing regional
biodeterioration hazard are temperature and moisture. The
growth of most decay fungi is negligible at temperatures
below 2 °C (36 °F) and relatively slow at temperatures from
2 to 10 °C (3650 °F). The growth rate then increases rap-
idly, with most fungi having optimum growth rates at tem-
peratures between 24 and 35 °C (7595 °F). Soft-rot fungi
typically tolerate warmer temperatures than brown- and
white-rot fungi. Fungal growth rate declines steeply
at higher temperatures, with little growth above 40 °C
(104 °F) and no growth above 46 °C (115 °F). In most loca-
tions and applications in the United States, the lower end
of this temperature range has the greatest effect on fungal
growth. Northern regions of the United States may have sev-
eral months of the year when temperatures are continuously
too low for growth of decay fungi and other months when
conditions for growth are only intermittently favorable. The
result, as we see from practical experience, is that decay
progresses more rapidly in warmer regions of the United
States. Although temperatures on the surface of wood ex-
posed to sunlight can exceed those favored by decay fungi,
the inner portions of wood products are usually cooler.
Decay tends to develop more rapidly in wood in shaded
locations, but this is usually associated with a slower rate of
drying rather than with protection from excessive heat.
The role of moisture in biodeterioration, especially by decay
fungi, cannot be overemphasized. Decay fungi require a
moisture content of at least 20% to sustain any growth, and
higher moisture contents (over 29%) are required for initial
spore germination. Decay fungi cannot colonize wood with
a moisture content (MC) below the ber saturation (average
of 30% MC). Free water must be present. Most brown- and
white-rot decay fungi prefer wood in the moisture content
range of 40% to 80%. Growth at lower moisture contents is
much slower, and typically wood with a moisture content
of less than 25% cannot be attacked unless the fungus has
another source of moisture nearby. Previously established
fungi are not necessarily eliminated at even lower moisture
contents. Some species of decay fungi produce thick-walled
resting spores and have been reported to survive (without
further growth) for years on wood at moisture contents
around 12%. As the moisture content exceeds 80%, void
spaces in the wood are increasingly lled with water. The
subsequent lack of oxygen and build-up of carbon dioxide
in free water limits fungal growth. Soft-rot fungi, however,
tolerate higher moisture contents and lower oxygen levels.
As with temperature, the lower end of the moisture content
limitations have the greatest effect on regional decay hazard.
Humidity alone is not sufcient to raise wood moisture con-
tents to levels needed by decay fungi, although equilibrium
moisture contents of over 20% can occur in cool, moist
climates (Fig. 1). Air is able to hold more moisture at warm-
er temperatures, lowering the relative humidity and equi-
librium wood moisture content. Humidity does play a key
role in slowing the drying of wood once it is wetted because
wood will dry more quickly at lower relative humidity.
The drying rate also depends on the length of dry periods
between wetting and on construction details that affect the
uptake of free water and the loss of water vapor from wood.
Temperature affects not only the degree of activity, but also
geographic distribution of termite species within the United
States. The natural range of native subterranean termites is
generally limited to areas where the average annual temper-
ature exceeds 10 °C (50 °F), although they have been found
farther north in areas where human activities create pockets
of warmer temperatures. Within much of their range in the
Midwestern and Eastern United States, termite activities
gradually decline above ground, and little activity occurs in
the winter. Termites cease their activities when temperatures
fall below freezing, and in colder climates may burrow over
1 m into the ground to avoid prolonged freezing tempera-
tures. The net effect of temperature on termite degradation
of wood is similar to that of decay fungi; conditions are
most favorable in regions with warmer climates. The tem-
perature effect may be more extreme for termites than fungi,
however, as some regions of the northern United States have
virtually no risk of termite attack.
The effect of moisture on termite attack varies with ter-
mite species. To some extent, the type of termite and their
dependence on moisture does vary with climate, but it is a
loose correlation. Dampwood termites require wood with
EMC (%)
Barrow (AK)
Figure 1. Examples of equilibrium moisture content
(EMC) of wood exposed outdoors and protected from pre-
cipitation in Barrow, Alaska; Hilo, Hawaii; and Phoenix,
Guide for In-Place Treatment of Covered and Timber Bridges
high moisture levels and typically only attack wood that is
in direct contact with the ground. As a result, their effect on
wooden structures is relatively minor. Their high moisture
requirements coincide with their preferred habitats in the
northwestern United States and southern Florida, but they
are found in the southwestern United States as well. Native
subterranean termites require moisture to prevent desicca-
tion, but can attack wood at moisture contents well below
the ber saturation point by building shelter tubes upward
from their nests in the ground. Native subterranean termites
are widely distributed in the southern two-thirds of the Unit-
ed States, although their distribution is less uniform along
the Pacic Coast. Formosan subterranean termites also
require a source of moisture to attack wood above ground
but are less reliant on proximity to soil for survival. They
may establish colonies on upper oors of buildings if a con-
sistent source of moisture is present. Drywood termites are
so-named because they are able to survive in wood above
ground, and can often derive sufcient moisture solely from
the wood. They are commonly found in southern California,
Arizona, and in coastal areas from South Carolina to Texas.
Role of Wood Structure
Wood structure affects both susceptibility to decay and
the movement of preservative through the wood. On the
most basic level, wood can be thought of as a collection of
elongated, hollow straws arranged in a series of concentric
circles along the length of the tree (Fig. 2). As a tree de-
velops, new cells grow around the outer circumference of
the stem forming the conductive tissues that comprise the
sapwood. Tree growth is fastest in the spring, producing
relatively thin-walled cells (earlywood), while thick-walled
cells are formed late in the season (latewood). These alter-
nating bands of thick- and thin-walled cells form growth or
annual rings. The older inner sapwood cells eventually stop
functioning and form a darker core of nonconductive tis-
sues called heartwood. The thickness of this sapwood band
varies greatly by species. Heartwood differs from sapwood
most notably in its much higher extractive content and much
lower permeability. Because of this structure, uids move
much more readily along the grain than across it. Exposed
end-grain serves as conduit for rapid movement of moisture
deep inside large members. This structure also allows
preservatives to move more readily along rather than across
the wood grain. Although the majority of wood cells are
aligned to maximize ow parallel to the grain, the wood
structure does allow some ow across the grain. This trans-
verse ow is accomplished through ray cells and through
openings between longitudinal cells. The heartwood of some
species contains toxic extractives that prevent attack by de-
cay fungi or termites, and the heartwood of other species has
water-repellent structural elements that limit water uptake
and thus minimize decay. Many historic bridges were at
least partially constructed with these durable wood species.
Problem Areas for Biodegradation in
Wooden Bridges
Signicant decay can occur in any untreated portion of
a bridge where oxygen is present and the wood moisture
content is above 20% to 25% for sustained periods. Suf-
cient oxygen and moisture for decay are always present
in members placed in contact with the ground, or near the
waterline area of members placed in water. In most climates
there is also sufcient moisture for decay in members that
are not directly in contact with soil or water or protected
by a covering. In general, larger members are most prone
to developing decay because water becomes trapped inside
the wood during precipitation and is slow to dry during
subsequent dry weather. Liquid water is rapidly absorbed in
end-grain during rain, and subsequent drying can be slowed
if air movement is limited in that area. Unfortunately, these
conditions commonly exist at connections where members
are joined by fasteners or other means.
Problem Areas in Covered Bridges
Because covered bridge members typically were not treated
with wood preservatives before installation, they are vulner-
able to biodeterioration in any areas with sustained exposure
to moisture. One of the most common, and critical, areas of
Figure 2. Typical structure of softwood species.
Figure 3. The area where covered bridge members contact
the abutment or supports at the ends of the bridge often
provides conditions for deterioration.
General Technical Report FPL–GTR–205
deterioration in covered bridges is where the support mem-
bers (bottom chord or bedding timbers) contact some form
of an abutment (Fig. 3). Although the abutment area may be
largely protected by the roof of the bridge, several factors
combine to increase the risk of moisture accumulation:
1) the stone or masonry used to construct abutments can
wick and hold moisture, 2) the location near the end of the
bridge increases the likelihood that water will enter through
the bridge deck above, 3) high humidity and lack of air
movement in this area retards drying. Similarly, all large
members near the end of the bridge may be vulnerable to
wind-blown or splashed precipitation. The deck members,
the lower portions of the end posts, the ends of the bot-
tom chords, and the ends of the diagonal bracing may all
be exposed to wetting, depending on construction and site
conditions. Wetting deck members near the ends of bridges
is especially likely in bridges with vehicular trafc (Fig. 4).
Areas below windows or other designed openings in the side
of a bridge provide additional potential avenues for moisture
intrusion (Fig. 5). Although these openings are typically
placed relatively high on the side of a bridge, the roof over-
hang is not always sufcient to exclude moisture.
Other areas of covered bridges become vulnerable to mois-
ture as a result of leaks or vandalism. Sources of moisture
from openings in the roof or cladding can occur almost any-
where in a bridge and are not always easily detected. How-
ever, water stains or general discoloration may be visible
indicators of water leaks or concealed decay. The area where
decay develops may not be immediately adjacent to where
water enters the structure. As with other sources of moisture,
problems are most likely to develop in larger members or at
connections where wood is slow to dry. Vandalism is a fre-
quent cause of water intrusion. Cladding may be repeatedly
removed to allow access for shing or swimming, exposing
the bottom chords to precipitation (Fig. 6). Any portion of a
bridge where the cladding has been lost for an extended pe-
riod (or even for several shorter periods) may be vulnerable
to decay.
Problem Areas in Modern Timber Bridges
The preservative treatments standardized for use in timber
bridges are generally very effective in protecting the treated
wood. However, in many cases and especially with larger
members, the preservative does not penetrate all the way
to the center of each piece. The structure and chemistry of
wood affect the ability of preservatives to penetrate into the
wood, as well as the efcacy of some types of preservatives.
The outer sapwood in most tree species is the part of the tree
that is most easily treated with liquid preservatives, whereas
the heartwood is much more resistant to preservative
Complete penetration of the sapwood should be the goal
in all pressure treatments. It can often be accomplished in
small-size timbers of various commercial woods, and it
may also be obtained in piles, ties, and structural timbers.
Practically, however, the pressure treater cannot always
ensure complete penetration of sapwood in every piece
when treating large pieces of round material with thick
sapwood. Therefore, specications permit some tolerance.
For instance, AWPA Processing and Treatment Standard
T1 for Southern Pine Piles requires penetration of 75 mm
(3 in.) or 85% of the sapwood thickness. The penetration
requirements vary, depending on the species, size, class, and
specied retention levels. The proportion of sapwood varies
greatly with wood species, and this becomes an important
factor in obtaining adequate penetration. Species within the
Southern Pine group are characterized by a wide sapwood
zone that is readily penetrated by most types of preserva-
tives. Other important lumber species, such as Douglas-r,
have a narrower sapwood band in the living tree, and as a
result products manufactured from Douglas-r have a lower
proportion of treatable sapwood. The treatment standards
recognize this, and require only penetration of
19 mm (0.75 in.) and 85% of the sapwood in Douglas-r
piles. The proportion of heartwood varies in lumber and
timbers. During sawmilling, larger dimension timbers tend
Figure 4. Members at ends of covered bridge are vulnerable
to wetting from wind-blown rain. The picture at the bottom
shows decay in a stringer under the bridge decking shown
on the top.
Guide for In-Place Treatment of Covered and Timber Bridges
to include the center of the tree and thus may have a
substantial area of untreatable heartwood (Fig. 7).
Proper preservative treatment creates an excellent barrier
against fungi and insects. However, this barrier can be com-
promised during on-site installation or as a result of checks
and cracks from normal weathering and moisture changes.
One of the most common sources of exposure of untreated
wood is cutting piles to height after driving. In almost all
cases this practice exposes untreated heartwood in the center
of the pile, and there is an increased probability that internal
decay will develop if this exposed surface is left unprotect-
ed. Attempts to protect this cut surface may only be partially
successful (Figs. 811). Cut-off piles that do not appear to
have been adequately protected are among the most likely
candidates for application of supplemental treatments.
Check (crack) formation in both piles and large sawn tim-
bers is another route for exposure of untreated wood in the
center of members. These checks also allow water to collect
and be trapped within the wood. Because wood does not
shrink and swell equally in all directions, formation of some
drying checks is not unexpected. Ideally, thoroughly drying
the members before pressure treatment would encourage
Figure 5. Windows and similar openings in covered bridg-
es provide avenues for moisture entry. Larger and lower
windows allow the most access.
Figure 6. Vandals removed sections of cladding on this
bridge, exposing the support members to moisture and
Figure 7. During pressure treatment,
preservative typically penetrates only
the sapwood. Round members have a uni-
form treated sapwood shell (upper photo-
graph), but sawn members may have less
penetration on one or more faces (lower
General Technical Report FPL–GTR–205
these checks to form before treatment and allow them to be
well protected with preservative. However, it is generally
not feasible to dry large timbers or piles to their in-service
moisture content prior to treatment. Small drying checks
also may not be a concern if they do not penetrate past
the treated shell. However, the appearance of large drying
checks in timbers or piles can be an indication of conditions
favorable for internal decay, and these are areas that warrant
closer inspection and possible eld treatment (Figs. 12, 13).
Another common source of breaks in the treated shell is
eld fabrication of treated members. Examples include
cutting to length, notching, and boring holes for fasteners
(Fig. 14). The extent of eld fabrication during construc-
tion should be minimized by specifying as much fabrication
as possible before treatment, but some eld fabrication is
usually necessary. The wood exposed during construction
should be protected by application of a preservative such
as copper naphthenate to the cut surface, but this practice is
not always followed (Fig. 15). In some cases, construction
personnel are concerned about the loss of excess liquid pre-
servative into water beneath the structure. When inspecting
an existing structure, it is often difcult to determine if cuts
were made in the eld and whether or not a preservative was
applied to the cut surfaces (Fig. 16).
The placement of a member within a structure may also
affect its susceptibility to decay (Fig. 17). Some evidence
shows that members that are protected by the bridge deck
and not placed into standing water are less likely to develop
decay. This supposition is logical if these members are pro-
tected from wetting, but caution is needed in applying this
nding categorically. Bridge designs and conditions vary,
Figure 8. Tar-like coatings may not provide suf-
cient long-term protection for cut-off pile tops.
These piles are likely candidates for development
of internal decay.
Figure 9. Examples of internal decay in vertical
members that were cut to height after installation.
Only the preservative-treated zone remains sound.
In some sawn members with heartwood faces, the
shell may not be complete (see photograph on
bottom). Fortunately, most pressure-treated mem-
bers used in bridges have greater penetration than
shown in these examples.
Guide for In-Place Treatment of Covered and Timber Bridges
and in some cases members beneath the deck will have suf-
cient moisture for decay to develop.
Good Practices
Although the purpose of this manual is to discuss protec-
tion of bridges with preservative treatments, it is essential
to note that preservatives are not a substitute for other types
of maintenance that can minimize the conditions favoring
Because water is the key to biodegradation, maintenance
of covered bridges to minimize water intrusion is essential.
This includes prompt repair of leaks in roofs and replace-
ment of sections of cladding that have been damaged or
removed by vandals. An additional source of moisture that
is sometimes overlooked is water splashed from puddles
Figure 10. These piles are at risk of developing inter-
nal decay, and it is likely that the pile on the bottom
already has substantial decay. Although cutting piles
to height after installation is a common practice, the
exposed untreated wood should be eld treated and
then protected with a durable cap. In this case, the
tops are only partly protected, and organic matter is
accumulating on the pile tops.
Figure 11. The older pile pictured on the bottom
demonstrates how decay eventually develops in the
untreated core of piles that have been cut to height
on the job site. Cutting the piles at an angle with the
intent of encouraging water to run off does not in-
hibit decay.
Figure 12. Deep seasoning checks can be an indica-
tion of potential problem areas unless the checks
formed prior to pressure-treatment.
General Technical Report FPL–GTR–205
that may form near the bridge entrance (Fig. 18). If possible,
puddles in this area should be eliminated, especially for
bridges that carry vehicle trafc.
A more challenging maintenance task can be removal of
dirt and organic debris that builds up in cracks and crevices
over years of service. This organic material helps to trap
moisture and provides a source of nutrients for decay fungi
in both covered bridges and highway bridges (Figs. 19–22).
Unfortunately, this debris tends to accumulate in joints and
connections, where the risk of decay is already relatively
high. Although it is often not practical to remove all of this
material, it is benecial to remove obvious accumulations.
Types of Supplemental Treatments
The objective of supplemental treatment is to distribute
preservative into areas of a structure that are vulnerable to
Figure 13. Checks allow ready access for moisture
from rain and snow to reach the interior of large
timbers. An accumulation of organic matter further
increases the potential for decay.
Figure 14. Metal fasteners are sometimes associ-
ated with decay pockets if holes are drilled after
treatment and expose untreated wood. Field-fab-
ricated bolt holes should be eld treated with a
preservative such as copper naphthenate during
Figure 15. The upper bolt in this pile was either re-
moved or never installed. The hole now serves as a
ready access for fungi to the untreated interior of the
Figure 16. Unless it was pressure-treated after fabrica-
tion, this type of connection is a recipe for decay. It ex-
poses untreated wood and creates an area that traps and
holds moisture.
Guide for In-Place Treatment of Covered and Timber Bridges
moisture accumulation and/or not protected by the original
pressure treatment. A major limitation of supplemental treat-
ments is that they cannot be forced deep into the wood un-
der pressure, as is done in the pressure-treatment processes.
Types of supplemental treatments range from nishes to
boron rods to fumigants (Table 1).
Water-Diffusible Preservatives
Water-diffusible preservatives or diffusible components of
preservatives move slowly through water within the wood
structure. Water-diffusible preservatives do not “x” in the
wood and thus are able to diffuse through wood as long as
sufcient moisture is present (Fig. 23). The distance or ex-
tent of diffusion is a function of preservative concentration,
wood moisture content, and grain direction. A concentration
gradient is needed to drive diffusion, and concentration can
become a limiting factor with surface- (spray-) applied sur-
face treatments because the volume of actives applied to the
Figure 17. The groundline area of piles represents a
severe decay hazard.
Figure 18. Puddles near bridge entrances cause lower
members to be repeatedly wetted by splash from
vehicle trafc.
Figure 19. Vegetation growing on a structure is an indi-
cation that conditions are favorable for decay.
Figure 20. This type of accumulation of dirt and organic
matter makes decay more likely and more rapid if mois-
ture reaches this area.
General Technical Report FPL–GTR–205
surface is limited. The most commonly available diffusible
preservatives are based on the use of some form of boron.
Sodium uoride is less widely used as a diffusible treatment.
This chemical is effective against decay fungi, but less ac-
tive against insects. It is currently available in the form
of a solid rod and as a component of a liquid or paste
Boron-based supplemental treatments are widely used
because they have several advantages. Boron has efcacy
against both decay fungi and insects but has relatively low
toxicity to humans. The sodium borate formulations used as
supplemental treatments are also relatively simple to dilute
with water prior to application. Borates are also odorless
and colorless and when diluted typically do not interfere
with subsequent application of nishes. In addition, borates
are corrosion inhibitors and have been shown to prevent fas-
tener corrosion in some situations.
Borate eld treatment preservatives are available in a range
of forms including powders, gels, thickened glycol solu-
tions, solid rods, and as a component of preservative pastes.
The concentration of actives is usually expressed as a per-
centage of disodium octaborate tetrahydrate (DOT),
although concentration is sometimes reported as a
Figure 21. Dense vegetation slows drying after rain and
contributes to the deposition of organic matter under
and in the bridge.
Figure 22. Galvanized pile caps (top) can provide protec-
tion of cut surfaces if applied at the time of construc-
tion. However, pile caps alone do not stop decay that
has already started, and they must be inspected to en-
sure that they have not been damaged (bottom).
Figure 23. Examples of boron diffusion into lumber with
20% moisture content over time. The red color is an indica-
tor that reacts when boron is present. A 15% DOT solution
was applied to the surface of the lumber.
Figure 24. Powdered disodium octaborate tetrahydrate
Guide for In-Place Treatment of Covered and Timber Bridges
percentage of boric acid equivalents (BAE) or boric oxide
(B2O3) equivalents. Typically, wood moisture contents of at
least 20% are thought to be necessary for boron diffusion
to occur. Whereas this moisture level is often surpassed for
wood exposed outdoors, some members in a covered bridge
may be below this moisture content. Diffusion appears to be
substantially more rapid at wood moisture contents in ex-
cess of 40%. At higher moisture contents, diffusion is much
greater along than across the wood grain, but this effect may
be less apparent at lower moisture contents.
Powdered borates typically contain 98% DOT and are often
the least expensive product on the basis of active ingredi-
ent purchased (Fig. 24). The powder is mixed (by weight)
with water for use in spray or brush applications. Solution
concentrations in the range of 15% DOT (by weight) can be
achieved with the combination of warm water and vigorous
agitation. Powdered borates can also be poured or packed
into holes for internal treatments, but this method of applica-
tion can be labor intensive and increases the risk of spillage.
Thickened glycolborate solutions typically contain 40%
DOT and polyethylene glycol, although one product con-
tains 50% DOT. The syrupy liquid is then diluted 1:1 or
1:2 with water, yielding a solution containing approximately
22% or 15% DOT (Fig. 25). Lower concentrations can also
be prepared if desired. The glycol formulations allow a
greater borate solution concentration than the powders, and
the resulting dilutions tend to resist precipitation longer than
those prepared from powders. Dilution by volume rather
than weight can also be advantageous in some situations.
The more viscous and more concentrated glycol–borate
solutions are also thought to allow deposition of higher con-
centrations of boron on the wood surface during spray appli-
cations. This effect was recently evaluated with spray treat-
ments of Southern Pine lumber specimens. Specimens were
briey sprayed with either a 15% DOT solution prepared
from powder or 15% and 23% DOT solutions prepared from
glycol–borate formulations. After spraying, the specimens
were allowed to sit in humid conditions for 26 weeks, and
then boron content was assayed at three depths from the
wood surface. The specimens sprayed with the 23% DOT
thickened solution had signicantly greater boron in the
outer 6 mm and slightly greater boron concentration within
7–12 mm from the surface than specimens sprayed with
either 15% DOT formulation. The 15% DOT glycol–borate
solution also resulted in slightly higher boron concentra-
tions than the 15% DOT solution prepared from powder.
The glycol benet appears to primarily be a function of
increased surface loading, as there is some evidence that the
glycol does not increase the rate of penetration of the boron
through the wood.
Glycol–borate solutions can be applied by spray or brush,
or used to ood cut-ends or holes. Because the solution
contains water, some diffusion can occur even in dry wood.
This effect is greatest for applications that provide a reser-
voir of solution, such as in lling treatment holes. With the
addition of foaming agents and specialized equipment, these
formulations can also be applied as foams. This approach
has been used by the National Park Service in treatment of
difcult to access areas of historic wooden ships.
Borate gels contain 40% DOT and are available in tubes for
ease of application in standard caulking guns. An advan-
tage of the gel formulation is that it can be applied to voids,
cracks, and treatment holes that are oriented horizontally or
downward and would not retain liquid borates. They are also
convenient to apply but are typically the most costly form of
borates on the basis of active ingredient purchased.
Rods contain active diffusible preservatives compressed or
fused into a solid for ready application into treatment holes.
The most common active ingredient is boron, although one
product is composed of sodium uoride (Fig. 26) and anoth-
er contains small percentage of copper (Fig. 27). Fused bo-
rate rods are produced by heating DOT until it is molten and
pouring this material into molds of various diameters. The
boron cools into a glass-like rod with a high percentage of
boron (Fig. 28). Both systems produce a maximum amount
of boron per volume of rod. The advantage of rod formula-
tions is their ease of application and low risk of spillage.
They can also be applied to holes drilled upward from
below a member. A disadvantage of the rods is that their ap-
plication does not include water to assist the initial diffusion
process. Because of this lack of moisture, some applicators
will drill slightly over-sized treatment holes and ll the void
space around the rod with a borate solution. This additional
borate solution does appear to provide benet in increasing
boron concentrations in the wood around the treatment hole.
Paste formulations typically contain at least one component
that diffuses into the wood and at least one other component
that is expected to provide long-term protection near the ap-
plication. The most common diffusible component is some
form of borate, although one formulation uses uoride. The
less mobile component is commonly some form of copper.
Pastes tend to be a more complex mixture of actives than
other types of supplemental treatments. The paste treatments
are most commonly applied to the ground line area of poles
or terrestrial piles. In some products, the paste is incorpo-
rated directly into a wrap for ease of application. Labeling
Figure 25. A 22% DOT glycolborate solution was applied
to the right side of this specimen and then allowed to dry
before this picture was taken.
General Technical Report FPL–GTR–205
also allows most of the paste products to be used for internal
treatment of holes by application with a caulking gun. The
paste would need to be loaded into rellable caulking tubes
for application in this manner. The pastes can also be spread
on the tops of cut piles before application of pile caps.
Because of their copper components, pastes have a blue or
green color and thus may not be appropriate for areas where
maintenance of a natural or historic appearance is important.
Pastes also leave a residue on the wood surface in their area
of application.
Sodium uoride is a diffusible component of a liquid for-
mulation that is primarily used for treatment of internal
voids in poles and piles. The formulation also contains a
water-based form of copper as a less mobile preservative
component. It is applied by drilling a hole into the void and
forcing the solution into the wood using air or mechanical
pump pressure. Because the solution is applied under pres-
sure, extra care must be taken to ensure that the void does
not have other openings that will allow the formulation to
exit the pile and into the surrounding environment.
Non-Diffusible Liquid Treatments
The oldest and simplest method for applying supplemental
preservative treatment during fabrication or routine main-
tenance involves brushing or spraying a preservative onto
the untreated wood or suspected problem area (i.e., joints,
fasteners, pile tops). Flooding of bolt holes and the tops of
cut-off piles is particularly useful. Often the treated surface
will be covered or closed during construction and will no
longer be available for surface treatment. The solutions do
not penetrate more than 1 or 2 mm across the grain of the
wood, although greater penetration is possible parallel to
the grain of the wood. In general, however, these treatments
should not be expected to move great distances from their
point of application.
The preservatives in this category are applied as liquids but
have some mechanism that allows them to resist leaching
once applied to the wood. The most typical examples are
the oilborne preservatives, which resist leaching because of
their low water-solubility. For decades, pentachlorophenol
and creosote solutions were used for this purpose but their
use is now restricted to pressure-treatment facilities. Most
current liquid treatments use some form of copper, (i.e.,
copper naphthenate or copper-8-quinolinolate) although
zinc naphthenate is also available in some areas. Because
of the limited volume solution applied and their supercial
application, the efcacy of these treatments will gradually
decline over time. One study found that a pentachlorophenol
solution applied to bolt holes provided only 8 years of
Oil-based copper naphthenate, the most common form of
liquid eld-treatment preservative, is available in copper
concentrations ranging from 1% to 8% (as elemental cop-
per). The solution is typically applied at 1% to 2% copper
Figure 26. Sodium uoride rod.
Figure 27. Example of 19-mm- (0.75-in.-) diameter rod that
contains both copper and boron.
Figure 28. Borate rods are available in a range of sizes in-
cluding the 19-mm (0.75-in.) and 13-mm (0.5-in.) diameters
shown here.
Figure 29. A solvent-based copper naphthenate solution (1%
copper) was applied to the right side of this specimen and
allowed to dry before this photograph was taken.
Guide for In-Place Treatment of Covered and Timber Bridges
concentration, and more concentrated solutions are diluted
with mineral spirits, diesel, or a similar solvent (Fig. 29).
Oil-based copper naphthenate is commonly used for treat-
ing areas of untreated wood exposed during fabrication of
pressure-treated wood. These solutions impart an obvious
green color to the wood, although some of the 1% copper
solutions are tinted to dark brown or black. The green color
weathers to brown with exposure (Fig. 30). Oil-based
copper naphthenate solutions also have a noticeable odor.
Water-based copper naphthenate is currently less widely
used than the oil-based formulations. It available as a con-
centrate containing 5% copper, and can be diluted with
water. The water-based formulation has a somewhat less
noticeable odor, and the color is slightly bluer (Fig. 31).
The water-based formulation is slightly more expensive
than the oil-based form and may not penetrate as deeply
into the wood as the oil-based form does.
Oil-based copper-8-quinolinolate was recently standardized
by the American Wood Protection Association for eld-
treatment of cuts, holes, or other areas of untreated wood
exposed during construction. It is available as a ready-to-use
solution containing 0.675% copper-8-quinolinolate (0.12%
as copper metal) as well as incorporated water repellents.
It has a light greenish color, although it can be tinted to
some extent. It can be applied by immersion, brushing,
or spraying.
Zinc naphthenate is similar to copper naphthenate, al-
though zinc is less effective than copper in preventing decay
from wood-destroying fungi or growth of mold fungi. How-
ever, an advantage of zinc naphthenate is that it is clear and
does not impart the characteristic greenish color of copper
naphthenate. It does, however, have a noticeable odor. Zinc
naphthenate can be formulated in both water-based and
solvent-based formulations.
Fumigants are gases that are used to internally treat large
piles or timbers. Like some water-diffusible formulations,
fumigants are applied in liquid or solid form in predrilled
holes. However, they then volatilize into gasses that move
much greater distances through the wood than do the water-
diffusible treatments. One type of fumigant has been shown
to move over 2.4 m (8 ft) along the grain from point of
application in poles. Fumigants tend to arrest fungal attack
more quickly than water-diffusible systems and are not de-
pendent on being applied to moist areas of the wood to func-
tion. To be most effective, a fumigant should be applied at
locations where it will not readily volatilize out of the wood
to the atmosphere. All but one of the commercial fumigants
(chloropicrin) eventually decomposes to produce the active
ingredient methylisothiocyanate (MITC). One of the prod-
ucts is the solid melt form of 97% MITC that is encapsu-
lated in aluminum tubes. Other MITC products use metham
sodium (sodium N-methyldithiocarbiamate), or the granular
dazomet (tetrahydro-3, 5-dimethyl-2-H-1,3,5, thiodazine-
6-thione). One of the dazomet products is available in pre-
package tubes that can be placed into treatment holes with
minimal handling or risk of spillage. It and the solid-melt
form of MITC have the advantage of placement in holes that
are drilled upward. Fumigant treatments are generally more
toxic and more difcult to handle than the diffusible
treatments. Some are considered to be Restricted Use
Pesticides (RUPs) by the U.S. Environmental Protection
Figure 30. The green color of copper naphthenate tends to
weather to brown over time. The photograph on the top is
soon after construction and that on the bottom one year
later. This wood was pressure-treated.
Figure 31. A water-based copper naphthenate solution (1%
copper) was applied to the right side of this specimen and
allowed to dry before this photograph was taken.
General Technical Report FPL–GTR–205
Agency (EPA), requiring extra precautions and application
by specially trained personnel.
Liquid fumigants are poured into pre-drilled treatment
holes, necessitating that they be applied from above. The
most commonly applied liquid fumigant is metham sodium
(33% sodium N-methyldithiocarbamate). Like several fu-
migants, this liquid formulation decomposes to produce the
active ingredient MITC. It tends to be less expensive than
other sources of MITC, but also contains a lower proportion
of the active ingredient. One of the oldest fumigants, chloro-
picrin, is only available in liquid form. It is the most effec-
tive, long-lasting fumigant but also difcult to handle safely
because of its volatility. It is a RUP and its use is generally
conned to critical structures in rural areas.
Granular fumigants are poured into pre-drilled treatment
holes in a manner similar to liquids. The current
formulations use granular dazomet (98% tetrahydro-3,
5-dimethyl-2-H-1,3,5, thiodazine-6-thione), which decom-
poses to produce MITC. The granular fumigant formulations
offer relatively easy handling compared with liquid metham
sodium and also contain a higher percentage of the active in-
gredient. However, they decompose to produce MITC more
slowly than the liquids, and in some cases, liquid accelerants
such as copper naphthenate (containing 1% copper) are also
poured into the treatment hole to promote decomposition.
Encapsulated fumigants are pre-packaged for convenient
application and have the added advantage of allowing holes
to be drilled from below. In addition to convenience, these
encapsulated fumigants minimize the risk of spillage when
applications are made over water or any other sensitive
environments. One encapsulated product contains the same
granular dazomet that is poured into holes. It is encased in a
tube-shaped, air-permeable membrane that contains the par-
ticles while allowing MITC gas to escape (Fig. 32). Another
encapsulated product consists of an aluminum tube lled
with solid 97% MITC (Fig. 33). At the time of application,
a special tool is used to remove the air-tight cap from the
tube, and MITC vapors are released through this opening.
A disadvantage of the encapsulated fumigants is their higher
costs and that they require a minimum treatment-hole
diameter and depth for application.
Applying Supplemental Treatments
Internal Treatments
Internal decay in larger timbers is a function of their ten-
dency to check, and for these checks to provide points of
water ingress. Wood wets through sorption of liquid water
but dries by evaporation of water vapor. As a result, wood
almost always wets faster than it dries, particularly far from
the surface. This creates elevated, relatively stable mois-
ture conditions deeper in the wood. In most cases, bridge
members are too thick to effectively treat the interior of the
member with surface application of preservatives. Internal
treatments are typically applied to these timbers by drilling
holes into the wood, but there are many variations on this
approach (Table 2).
Water-Diffusible Internal Treatments
Water-diffusible internal treatments generally do not move
to the same extent as do fumigants, and so their application
locations and spacing are critical. Although they could be
used to treat the length of piles or beams, they may be better
suited to protection of specic vulnerable areas such as near
pile tops or the groundline, connections, and areas adjacent
to fasteners. The extent of movement of these diffusible
treatments has been shown to vary with wood moisture con-
tent and wood species, although wood moisture content is
probably the most important factor. Wood moisture content
is typically lower for wood above ground than wood used in
ground contact, and studies of boron movement from inter-
nal treatments have indicated somewhat limited mobility in
above-ground timbers.
Research evaluating the mobility of boron from solid rods
in above-ground softwood timbers suggests that rods would
need to be placed no more than 50 mm (2 in.) apart across
the grain and 300 mm (12 in.) apart along the grain. Some-
what tighter spacing may be needed for red oak. Substantial
variability in boron mobility has been reported in timbers
treated with combinations of liquid and solid internal
treatments. However, the results indicate that spacing of
approximately 75 mm (3 in.) across the grain and between
75 and 125 mm (3–5 in.) along the grain would be needed to
achieve overlapping boron penetration in southern pine tim-
bers. The manufacturer of one of the boron rod products rec-
ommends parallel to the grain spacing of between 150 and
380 mm (6–15 in.) depending on the size of the timber and
the size of the rod installed. They recommend that the across
Figure 32. A granular fumigant pre-packaged in a vapor-
permeable membrane.
Figure 33. A solid fumigant encapsulated in a metal tube.
The cap is removed at installation.
Guide for In-Place Treatment of Covered and Timber Bridges
the grain distance between treatment holes not exceed
150 mm (6 in.).
Liquid borates may be applied in a manner similar to rods,
except that their use is generally limited to holes oriented
downward. The concentration of boron in the liquid treat-
ments is not as great as that in the rods, but the potential for
diffusion is greater at lower wood moisture contents. The
liquid borates also provide protection more rapidly than
do the rods, but the duration of protection is more limited.
Liquid borates also allow more exibility in the size of the
treatment hole, and in some cases it may be desirable to
drill many small holes instead of a few large holes. The liq-
uids can be readily applied to smaller treatment holes with
squeeze or squirt bottles. The holes can be temporarily left
unplugged to allow relling as the liquid moves out of the
treatment hole and into the wood in situations where the
treatment holes are protected from precipitation and public
access. Alternatively, a rod can be placed into the treatment
hole after the liquid has drained into the wood. It is worth
noting, however, that movement of liquid is slow through
the heartwood of many wood species, and the time required
for the hole to empty may be longer than anticipated. Rods
and liquid borates can also be simultaneously added to treat-
ment holes by drilling holes slightly larger than needed to
accommodate the rod. This approach can provide both an
immediate boost of liquid boron as well as the longer term
slow release from the rod, but it does require drilling a larg-
er treatment hole than would otherwise be necessary.
Liquid borates have also been injected into small treatment
holes in horizontal timbers using a low-pressure sprayer,
with the nozzle pressed tightly against the treatment hole to
prevent leakage. Under these conditions, a diamond pattern
was recommended, with 300 mm (12 in.) between holes
along the grain and 100–150 mm (4–6 in.) across the grain.
Likely penetration achieved using this approach would de-
pend greatly on wood permeability. Risk of spillage into the
area below the structure may be higher with this approach
than with non-pressure applications because the treatment
holes may cross seasoning checks.
Gels and paste products may also be applied as diffusible
internal treatments in a manner similar to liquids and rods.
Depending on the properties of the individual product, they
may be applied to holes that are horizontal or even oriented
upward. Application to treatment holes is typically accom-
plished with use of a caulking tube and caulking gun. In the-
ory, these formulations provide somewhat of a compromise
between the liquid formulations and the solid rods, with
slower distribution than the liquids but more rapid distribu-
tion than rods. However, there is little published research
comparing the penetration or longevity of these formula-
tions to that of the other formulations.
In some instances, water-based external treatments that
contain both non-diffusible and diffusible components may
be injected under low pressure. These products are most
effective when inspection determines that a void has formed
in the wood. These products are typically grease-like in na-
ture and will not run out of the wood as quickly or easily as
non-diffusible liquids do.
There is less information on the mobility of internal dif-
fusible preservatives other than boron. Both uoride and
copper have been incorporated into internal treatments,
and uoride has been used as a stand-alone preservative
in a rod form. The mobility of copper when applied in this
manner appears very limited, probably as a result of lower
water solubility and its tendency to react with and “x” to
the wood structure. Although uoride is considered to be a
diffusile preservative component, it may have slightly less
mobility than boron. Fluoride tends to be a better fungicide
than boron and would be expected to remain in the wood for
a longer time if it is less mobile than boron.
To be most effective, a fumigant should be applied at loca-
tions where it will not leak away through checks or be lost
by diffusion to the atmosphere. When fumigants are applied,
the member should be inspected thoroughly to determine an
optimal drilling pattern that avoids metal fasteners, season-
ing checks, and severely rotted wood. Manufacturers have
developed specic guidance for application of their products
to round vertical members such as piles. Although these ap-
plication instructions vary somewhat between products, they
generally specify drilling holes of 19–22 mm (3/4–7/8 in.)
diameter downward at angle of 45° to 60° through the
center of the pile. The length of the hole is approximately
2.5 times the radius of the pile. A minimum hole length of
305 mm (12 in.) is required for the use of the MITC-Fume
tube, necessitating the use of a steeper drilling angle in
smaller piles. In terrestrial piles, the rst hole is drilled at or
slightly below the ground line. Subsequent holes are drilled
higher on the pile, moving up and around the pile in a spiral
pattern. Depending on the product and size of the pile, holes
should be spaced at either 90° or 120° around the pile. The
recommended vertical distance between treatment holes
varies from 152 to 305 mm (6 to 12 in.) near the groundline,
with 305-mm (12-in.) spacing used higher on the pile. Al-
lowable uses of fumigants for aquatic piles are not always
specied on the product labels, but at a minimum the lowest
part of a treatment hole should be above the waterline, and
considerable care should be taken, as most fumigants can be
toxic to sh.
There is much less information on application of fumigants
to large timbers or glued-laminated beams. Holes are typi-
cally drilled into a narrow face of the member (usually
either the top or bottom). Holes can be drilled straight down
or slanted; slanting may be preferable because it provides a
larger surface area in the holes for escape of fumigant. As
a rule, the holes should be extended to within about 51 mm
(2 in.) of the top or bottom of the timber and should be no
more than 1.22 m (4 ft.) apart. The treatment holes can be
drilled upward in a similar manner with the encapsulated
solid fumigants. Solid fumigants provide a substantial
General Technical Report FPL–GTR–205
advantage in treatment of timbers and beams because access
is often limited to the bottom face. A disadvantage of the
pre-encapsulated fumigants is that they require a minimum
size of treatment hole, and thus cannot be used on smaller
When treating with fumigants, the treatment hole should be
plugged with a tight-tting treated wood dowel or remov-
able plastic plug immediately after application. Sufcient
room must remain in the treating hole so the plug can be
driven without squirting liquid chemical out of the hole
or impacting the solid fumigant. The amount of fumigant
needed and the size and number of treating holes required
depend on timber size. Fumigants will eventually diffuse out
of the wood, allowing decay fungi to recolonize. Additional
fumigant can be applied to the same treatment hole, a pro-
cess that is made easier with the use of removable plugs.
Fumigants should not be applied into voids or when applica-
tion holes intersect voids or checks, thus limiting the risk
for accidental release of the product into the environment.
Structures where fumigants have been applied should be
marked to indicate its presence. Care should be taken in the
removal of wood structures that have been treated with solid
fumigants to ensure that the chemical has moved out of the
treatment hole and into the surrounding wood. Some pro-
ducers of solid fumigants have procedures for recovery of
their tubes when a structure is removed. Consult the manu-
facturers of the formulation for specic information.
Non-Diffusible Liquids
Non-diffusible liquid treatments, typically containing some
form of copper, are sometimes used for internal treatments.
Although these treatments do not diffuse in water within
the wood, they can move for several inches parallel to the
wood grain in permeable sapwood. Movement across the
grain is minimal. The advantage of these liquids relative to
the diffusible treatments is their resistance to leaching. Thus,
they may have applications where resistance to weathering
is of greater importance than volume of wood protected. An
example is bolt holes positioned in a manner that is likely
to subject the hole to frequent wetting (in more sheltered
locations, a concentrated water-diffusible treatment is likely
to provide greater protection). Treatment holes can also be
drilled above existing connectors, lled with preservative,
and plugged. This type of treatment may be desirable if
subsequent fabrication or construction activities will make
that area difcult to access in the future. These preservative
liquids may be used to ood internal voids such as decay
pockets in poles and terrestrial piles, but the risk of spillage
may make this type of application less suitable for aquatic
applications. In addition, much of the chemical is absorbed
by wood that is already decayed rather than adjacent sound
External Treatments
External treatments generally have the greatest applicabil-
ity for members that have not been pressure treated, but
also have value in protecting pressure-treated wood when
untreated wood is exposed during fabrication. Many of the
same formulations used for internal treatments can also be
used for external treatment. Protection is generally limited
to within a few millimeters of the wood surface although
greater movement does occur when solutions are applied
to the end-grain of wood. Surface-applied gels, pastes, and
water-diffusible treatments can also achieve deeper penetra-
tion under some conditions. However, broad-scale surface
sprays can be highly problematic from the viewpoint of en-
vironmental contamination, and the potential benets from
this approach must be weighed against the risks. In many
cases, it may be more practical limit surface applications to
localized areas.
Water-diffusible liquid preservatives (borates) are typically
applied with low-pressure sprayers or by brushing in smaller
areas. The greatest benet is achieved by ooding checks,
cracks, and other openings, potentially allowing diffusion
into decay-prone areas where water tends to collect within
the wood. Because of this, it is often desirable to apply the
solution after a prolonged dry interval, when checking in
the wood is at a maximum. Borates applied to the wood
surface can be rapidly depleted if the wood is exposed to
precipitation or other forms of liquid water. Borate depletion
from exposed members can be slowed (but not completely
prevented) with application of a water-repellent formulation
after the borate treatment has dried. This may necessitate
tarping or otherwise protecting the treated members until
they have dried sufciently to allow application of the wa-
ter-repellent. Use of preservative-based water repellent (for
example containing copper or zinc naphthenate) can provide
further protection to the wood surface. This process can be
repeated after the wood surface loses its water repellency.
Surface application of non-diffusible liquid treatments is
most benecial in situations where penetration into the
wood is less important than resistance to leaching. Perhaps
the most obvious example is eld-treatment of untreated
wood exposed during fabrication of treated wood. Protection
of pile tops is especially important, and in these situations a
copper-containing solution should be applied to the exposed
surface. Zinc naphthenate can also be used if a clear treat-
ment is required. As discussed above, the non-diffusible
liquids can also be applied after a diffusible treatment to
slow leaching of the diffusible preservative and to provide
long term protection. For example, a cut pile top can rst be
treated with a concentrated borate solution, and then treated
with and oil-based copper after the borate solution has dried.
At least one product uses pads soaked in a copper solution
as part of a groundline wrap/bandage system. It is essential
that any pile or pole top also be protected with a water shed-
ding cap to prevent the wood from checking and allowing
water and fungal spores to enter beyond the protected zone.
The most common external use of gels and pastes is in the
protection of the ground-line area of support posts or piles
as part of a wrap system. Soil is excavated from around the
support to a depth of approximately 0.46 m (18 in.) and the
Guide for In-Place Treatment of Covered and Timber Bridges
formulation is brushed or troweled onto the exposed wood
to form a 3–8-mm- (0.125–0.375-in.-) thick layer that ex-
tends 5176 mm (23 in.) above the ground line. The layer
of preservative is then covered with a water-impervious
wrap to hold the chemical against the wood, and the ex-
cavated area is relled. The diffusible components of the
formulation (for example boron) gradually diffuse into the
wood, while the less mobile components remain near the
wood surface. Although these treatments are primarily used
to supplement the groundline area of preservative-treated
utility poles, they have also been shown to offer substantial
protection to the groundline area of untreated wood. This
type of system should not be used in areas where standing
water is expected. The same principal can also be used to
protect wood above ground that is covered with metal or a
simmilar barrier. For example, these products can be spread
on to pile tops before ashing is applied, or on the timbers
that are subsequently wrapped with metal ashing. Metal
ashing can cause moisture to condense between the metal
and the wood, so treatment in this area is desirable. How-
ever, many of these formulations are not colorless, and pre-
servative that wicks along the grain and extends beyond the
cover could slightly discolor untreated wood.
Research on the Use of
Supplemental Preservative
Treatments for Covered Bridges
In 2001, Oregon State University conducted a study funded
by the Federal Highway Administration (FHWA), “Iden-
tication of Preservative Treatments and Fumigants for
Treating Historic Covered Bridges” (project DTFH61-
01-C-0059), which included both eld and laboratory evalu-
ations of remedial preservative treatments. The laboratory
research compared the ability of numerous types of internal
treatments to move through wood as a function of mois-
ture content, wood species, and dosage. Movement of the
water-diffusible preservatives BoraCare (boron, Nisus Cor-
poration, Rockford, TN), ShellGuard (boron, Perma-Chink
Systems, Inc., Knoxville, TN), Tim-bor (boron, Tim-bor
Professional, Rockford, TN), CuRap 20 (boron, copper, ISK
Biocides, Inc., Memphis,TN), Impel rods (boron, PRG, Inc.,
Rockville, MD) Cobra rods (boron, copper, Perma-Chink
Systems, Inc., Knoxville, TN) and FluRods (uoride, Os-
mose Utilities Services, Inc., Buffalo, NY) was evaluated, as
were the fumigants dazomet (methylisothiocyanate), MITC
(methylisothiocyanate) and chloropicrin (trichloronitrometh-
ane). Preservative mobility was compared for Douglas-r,
Southern Pine, eastern white pine, eastern hemlock, red oak,
and white oak, and the diffusibles were evaluated at three
wood moisture contents (30%, 60%, and 100%). As ex-
pected, movement of the water-diffusible preservatives was
strongly related to moisture content, with relatively little
diffusion noted at 30% MC. The study noted that because
overall moisture contents in covered bridge members will be
relatively low, care will be needed to place water-diffusible
treatments where they are likely to be
wetted. Diffusion was also positively correlated with
concentration of chemical applied, both within and between
types of preservatives. Wood species also affected diffusion,
with the less permeable wood species having the lowest
concentrations of actives at greater distances away from the
treatment. This effect was particularly notable for imperme-
able white oak heartwood, which had only limited diffusion.
Mobile concentrations of uoride tended to be lower than
those of boron, and diffusion of copper was very limited.
The laboratory evaluations revealed that fumigant moved
quickly through blocks treated with MITC or chloropic-
rin, reaching maximum levels within one week and then
declining as the volatile fumigant moved out of small test
specimens. In contrast to the diffusible treatments, fumigant
levels tended to be higher in the less permeable species such
as Douglas-r than in the highly permeable Southern Pine.
This nding indicates that longer intervals between reappli-
cation may be appropriate for covered bridges constructed
with less permeable species such as Douglas-r or white
oak. Movement of fumigant from the blocks treated with
dazomet was much slower, with only very low levels de-
tected after one week, and slightly increased levels detected
after 4 weeks. However, no fumigant was detected in some
species treated with dazomet, and when concentrations were
detected they were many times lower than the one-week
concentrations observed for the MITC or chloropicrin-treat-
ed specimens.
The eld portion of the research was conducted by install-
ing two internal water-diffusible treatments (boron rods and
uoride rods) and two internal fumigant treatments (MITC
and dazomet) in timbers in ve covered bridges. The bridges
were located in California, Vermont, Wisconsin, and Illinois
and included timbers of white pine, spruce, Douglas-r, and
sugar pine. Mobility of the treatments was determined by
assaying the treated timbers at 1 and 2 years after treatment.
Sampling holes were drilled into the treated members at
distances of 30, 60, and 90 cm (fumigant treatments) or 10,
20, 30 cm (water-diffusible treatments) from each side of
the treatment hole.
With few exceptions, no movement of boron and uoride
from the rods was detected in the eld-treated bridges.
Concentrations in assay samples were either not above
background levels or not detected. The possible exceptions
were low levels of uoride detected in a few assay samples
removed 10 cm from treatment holes after 2 years exposure
in the California bridges. The general absence of boron and
uoride in the assay samples is in agreement with the lack
of weight loss observed in the rods after 2 years exposure.
The poor mobility observed in this study is probably attrib-
utable to the low moisture content of the bridge members.
The highest moisture content detected in the members when
the rods were placed in the bridge was 27%. Although the
moisture content in the members likely uctuates with pre-
cipitation events, it appears that moisture was never consis-
tently elevated to the point that diffusion could occur from
the rods.
General Technical Report FPL–GTR–205
In contrast to the water-diffusible treatments, MITC was
detected in many of the samples removed from locations
adjacent to the MITC treatments holes. Concentrations were
generally greatest and most consistently elevated in samples
removed from closest (30 cm) to the treatment holes, but
elevated concentrations were also detected at distances of
60 and 90 cm. Concentrations detected in samples removed
from 4 of the 5 bridges were relatively similar. The high-
est concentrations after 1 year were detected in a California
bridge located in hot, dry climate, while concentrations
detected after 2 years were higher in the northern bridges.
Sublimation of solid MITC is faster at higher temperatures
and the higher temperatures at the warmer California loca-
tion may have accelerated MITC release from the tubes.
Weight losses measured after 2 years suggest that nearly all
the MITC had been released from tubes at that bridge. In-
terestingly, MITC concentrations detected at the other Cali-
fornia bridge were notably lower than for the other bridges.
The reason for this is unclear, as the MITC weight loss from
the tubes at this bridge after 2 years was similar to the other
None of the wood assay samples corresponding to the
dazomet treatments contained detectable concentrations of
MITC at any distance, bridge, or time point. Weight loss
from the dazomet treatments was also minimal, indicating
that little decomposition and release of MITC had occurred
after 2 years. Dazomet requires moisture to decompose to
MITC, and as with the water-diffusible treatments, the low
wood moisture content may have limited the dazeomet’s
mobility. Some suppliers recommend addition of accelerants
to dazomet treatments to speed decomposition, which was
not done in this study. It is possible that greater decomposi-
tion would have been observed with the use of these
The laboratory and eld tests conducted in this study
illustrate that movement of preservative away from solid-
rod water-diffusible treatments is highly dependent on
moisture. Because the majority of covered bridge members
are generally dry, the use of these solid water diffusibles
would be most efcient if they were closely targeted to loca-
tions where moisture is suspected. The use of solid-water-
diffusible treatments in covered bridges may also be seen
as a type of insurance against future moisture problems. In
theory, if these moisture problems do occur, the preserva-
tive in the rods would become activated and spread into the
moistened area. Movement from the rods could have also
have been given an initial boost by adding water or a liquid
borate solution (in the case of the boron rod) to the treat-
ment hole. Of the two fumigants evaluated, the MITC tube
was clearly the most effective at moving into the wood dur-
ing this 2-year study. Because MITC and chloropicrin treat-
ments do not rely on moisture for their mobility, they have
greater potential for movement in dry bridge timbers. MITC
from the MITC treatments in this study routinely moved 60
cm (24 in.) from the treatment hole, suggesting that instal-
lation of this treatment with a spacing of 120 cm (48 in.)
would provide for adequate protection of members. MITC
was also detected at 90 cm (36 in.) from the treatment holes,
but the concentrations detected were often below the
20 ug/g thought to be needed to prevent growth by decay
fungi. Average MITC concentrations increased during year
2 of the study, suggesting that the treatments will be effec-
tive for at least 3 years. Research on utility poles indicates
that MITC levels in wood decline gradually over time and
fall below effective concentrations 5 to 7 years after treat-
ment. In covered bridges, the longevity of the treatment will
be less predictable because of the wide range of designs and
member dimensions. However, the lack of soil contact for
most covered bridge members should also slow fungal colo-
nization once the fumigant has dissipated.
In another study directly applicable to timber bridges, re-
searchers at Mississippi State University (MSU) evaluated
the efcacy of eld treatments in protecting timber connec-
tions (joints) or pile sections. In the connections study the
MSU researchers evaluated the installation of saturated felt
pads within the joints, the application of preservatives to the
joint surface, and the application of preservatives to holes
drilled into the timber near the connection. The joint treat-
ments were evaluated on southern pine, red oak, and yellow
poplar wood species. The researchers found that pads satu-
rated with a solvent based copper naphthenate solution were
largely effective in protection the joint area from decay,
while pads saturated with a solvent-based water repellent of-
fered little protection. The efcacy of treatments applied to
holes near the joint varied by wood species. Boron solution,
boron rods, copper borate paste and liquid fumigant (33%
sodium N-methyldithiocarbamate) all provided substantial
protection for red oak, but the copper borate paste was less
effective in protecting southern pine. None of the treatments
applied to holes were effective in protecting yellow poplar
specimens. Borate solution or copper borate paste applied
directly to the outer surface of the connection was generally
effective in protecting the joints, although the borate solu-
tion was much less effective in protecting the yellow poplar
specimens. Copper borate paste applied to surface of the
joint was the most effective overall treatment.
The MSU researchers also evaluated the protection of un-
treated southern pine pile sections with both internal and ex-
ternal treatments. The internal treatments (liquid fumigant,
boron rods or uoride rods) were applied to holes drilled
into the center of the piles, while the external treatments
(copper borate paste, uoride paste or a pentachlorophenol
grease) were applied as groundline wraps. The exterior
pastes applied as wraps were generally effective in protect-
ing the groundline area of the piles. In contrast, the internal
treatments were much less effective, although a combination
of uoride rods and fumigant provided moderate protection.
The poor performance of the internal treatments probably
resulted from their inability to protect the exterior of the
pile, and it should be noted that internal treatments are typi-
cally applied to pressure-treated piles that have a protected
exterior shell.
Guide for In-Place Treatment of Covered and Timber Bridges
Table 1—Summary of supplemental preservative treatments properties and applications
as Actives
as Dilution
EPA hazard
category Uses
in wood
Examples of
trade name(s)
Liquid 98% DOT Powder Dilute 10%–15%
in water
(by weight)
Caution Surface spray, brush,
or foam,
internal injection,
oured in holes
High Board Defense, Borasol,
Timbor, TimberSaver,
Liquid 25%–40% DOT Water/glycol
Dilute 1:1
with water
Caution Surface spray or brush,
poured into holes
High Bora care, Bor-Ram,
BoraThor, Shell-guard
Liquid Copper naphthenate,
1%–2% as Cu
Oil or water
RTU Warning Surface spray or brush,
poured into holes,
pads for bandages
Low Tenino, Cuprinol No. 10
Green Wood Preservative,
Jasco Termin-8, CU-89
Liquid 9.1% DOT,
0.51% boric acid,
0.96% copper hydroxide
(0.6% copper)
Water based RTU Caution Surface spray, brush,
or foam,
internal injection
B high, Cu low Genics CuB
Liquid Copper naphthenate,
5% as Cu
Water based Dilute 1:4
or 1:1.5
with water
Danger Surface spray or brush,
poured into holes
Low Aqua-Nap 5
Liquid Copper naphthenate,
8% as Cu
Oil based Dilute 1:3.0–3.8
or 1:7.5–8
with oil
Warning Surface spray or brush,
poured into holes
Low Cu-Nap Concentrate,
Liquid Zinc naphthenate,
(1%–2% as Zn)
Oil or water
RTU Warning Surface spray or brush,
poured into holes
Low Jasco ZPW
Liquid Copper-8-quinolinolate
Oil based RTU Caution Surface spray or brush,
poured into holes
Low Outlast Q8 Log Oil
Liquid 33% Sodium
RTU Danger Internal fumigant treatment,
poured into holes
Gas, very high WoodFume,
Pol Fume
Rod 100% Anhydrous
disodium octaborate
Rod RTU Caution Placed into holes High Impel Rod
Rod 93% Sodium fluoride Rod RTU Warning Placed into holes High FluRod
Rod 90.6% DOT,
4.7% Boric acid,
2.6% Cu
Rod RTU Caution Placed into holes B high, Cu low Cobra Rod
Granules 98% Dazomet Granule RTU Danger Internal fumigant treatment,
laced into holes
Gas, very high Dura-fume
General Technical Report FPL–GTR–205
Table 1—Summary of supplemental preservative treatments properties and applications—con.
as Actives
as Dilution
EPA hazard
category Uses
in wood
Examples of
trade name(s)
Paper tube 98% Dazomet Paper tube RTU Danger Internal fumigant treatment,
placed into holes
Gas, very high Super-Fume
Capsule 97% Methylisothiocyanate Capsule RTU Danger,
Internal fumigant treatment,
placed into holes
Gas, very high MITC-FUME
Paste 43.5% Borax,
3.1% Copper
hydroxide (2% Cu)
Paste RTU Warning With exterior wrap
for groundline area,
spread under pile caps,
injected into holes
(caulking gun)
Cu low, B high Cu-Bor
Paste 40% Borax,
18% Copper
naphthenate (2% Cu)
Paste RTU Warning With exterior wrap
for groundline area,
spread under pile caps,
injected into holes
(caulking gun)
Cu low, B high CuRap 20
Paste 43.7% borax,
0.2% tebuconazole,
0.04% bifenthrin,
0.3% copper
(0.05% Cu)
Paste RTU Caution With exterior wrap
for groundline area,
spread under pile caps,
injected into holes
(caulking gun)
B high,
others low
70.6% Sodium fluoride Bandage RTU Danger Applied as self-contained
bandage to groundline
area of poles
High Pole Wrap
17.7% Copper
aphthenate (2% Cu)
Bandage RTU Caution Applied as self-contained
bandage to groundline
area of poles
Low Cunap Wrap
solid disks
30%–60% DOT,
Sodium fluoride
Bandage RTU Caution Applied as self-contained
bandage to groundline
area of poles
High BioGuard
Gel 40% DOT Gel RTU Caution Internal, injected into holes High Jecta
Guide for In-Place Treatment of Covered and Timber Bridges
Summary of Supplemental
Treatment Concepts
Liquid Surface Treatments
• Surface-applied liquid treatments should not be expected
to penetrate more than a few millimeters across the grain
of the wood, although those containing boron can diffuse
more deeply under certain moisture conditions. They will
not effectively protect the interior of large piles or timbers.
• Liquid surface treatments are most efciently used to
ood checks, exposed end-grain, bolt holes, etc. They may
move several inches parallel to the grain of the wood if
the member is allowed to soak in the solution.
• Surface treatments with water-diffusible components will
be leached by precipitation if used in exposed members.
However, their loss can be slowed if a water-repellent
nish is applied after the diffusible treatment has dried.
Paste Surface Treatments
• Paste surface treatments can provide a greater reservoir of
active ingredients than liquids. When used in conjunction
with a wrap or similar surface barrier, these treatments can
result in several centimeters of diffusion across the grain
into moist wood over time. They are typically used for
the groundline area of posts or piles that are not usually
exposed to standing water, but can also be applied to end-
grain of connections or pile tops. Some formulations can
be applied under low pressure as a void treatment.
Internal Treatments
• Internal treatments are typically applied to the interior of
larger members where trapped moisture is thought to be a
Table 2—Application characteristics for internal preservative treatments
of treatment
Target retention
in wood
(oz/ft3 or kg/m3)
of treatment holes
Diameter Length Posts/piles Timbers
Boron rod 1.7–5, as DOT 5/16–13/16 in.
(8–21 mm)
2.5–13 in.
(64–330 mm)
7–15 in.
(178–381 mm)
90–120° intervals
6–14 in.
(152–356 mm)
along the grain,
3–6 in.
(76–152 mm)
across the grain
Boron/copper rod 1.7–5, as DOT 1/4–3/4 in.
(6–19 mm)
1.5–5.5 in.
(38–140 mm)
Vertical spacing
not described,
120° intervals
6–14 in.
(152–356 mm)
along the grain
Sodium fluoride rod 1.4, as NaF 7/16–5/8 in.
(11–16 mm)
3–5 in.
(76–127 mm)
6 in. (152 mm)
90–120° intervals
Not described
Borate, liquid glycol 1.1, as DOT Variable Variable 7–15 in.
(178–381 mm)
90–120° intervals
12–16 in.
(305–406 mm)
along the grain,
4–6 in.
(102–152 mm)
across the grain
CuNaph liquid 0.96–2.4, as Cu Variable Variable Not described Not described
CuNaph/NaF liquid
A Variable To cavity Flood
internal cavity
Not labeled
for this use
hydroxide liquid
A 0.5 in.
(13 mm)
To decay
decay pockets
decay pockets
hydroxide paste
3.7–14.7, as borax
+ Cu(OH)2
Up to 1 in.
(25 mm)
Variable Not described Not described
Borax/CuNaph paste
ot provided 3/4 in.
(19 mm)
Variable 24 in. (610 mm)
90° intervals
Not labeled
for this use
Borax, tebuconazole,
bifenthrin, oxine
ot provided Variable Variable Not described Not described
DOT gel 1.1, as DOT Variable To center 12–24 in.
(305–610 mm)
12–24 in.
(305–610 mm)
along grain
Fumigants Approximately 0.01
for MITC-based,
for chloropicrin
3/4–7/8 in.
(19–22 mm)
12 in.
(305 mm)
minimum for
6–12 in.
(152–305 mm),
90–120° intervals
of 4 ft
(1.23 m)
along grain
General Technical Report FPL–GTR–205
current or future concern. These treatments can be applied
to smaller members in some situations.
• Water-diffusible treatments move with moisture in the
wood. They are generally easier to handle, but do not
move for as great a distance as do fumigants and do not
move in dry wood. The diffusion distance in moist wood
is approximately 50–100 mm (2 to 4 in.) across the grain
and 150–300 mm (6 to 12 in.) along the grain. Diffusible
treatments may be best suited for focusing on specic
problem areas such as near exposed end-grain, connec-
tions, or fasteners.
• Rod-diffusible rod treatments provide a longer, slower
release of chemical while liquid-diffusible treatments
provide a more rapid, but less long-lasting dose of preser-
vative. Paste and gel internal treatments fall somewhere
between rods and liquids in terms of rate of release and
• Fumigant treatments: Fumigants volatilize and move as
a gas through the wood. They have the potential to move
several feet along the grain of the wood, but have greater
handling and application concerns.
• Void-ooding treatments can be used where large decay
pockets have been detected. Care must be used to avoid
loss of these preservatives from the wood into the envi-
ronment from checks or during the application process.
Example Supplemental Treatment
Exposed End of Pile or Other Pressure-Treated
Vertical Member
The exposed end-grain of vertical members such as piles
can be an entry point for precipitation or other sources of
moisture. In some cases, these members are cut to height on
site, potentially exposing untreated wood in the center of the
member. These members can be supplemental treated using
combinations of internal treatments and application of liquid
or paste preservative to the top surface, along with a water
shedding cap (Fig. 34). For relatively recent construction in
which no decay is suspected, application of liquid or paste
treatments to the top surface, followed by ashing or pile
cap is probably sufcient. In older structures, supplemental
treatments further down the member may be necessary, es-
pecially in areas of critical connections. If accessible, holes
can be drilled vertically down from the top face and lled
with some combination of liquids, rods, and/or pastes. Treat-
ment holes may also be drilled downward at an angle from
the vertical faces. Generally, treatment holes should extend
through approximately 2/3 of thickness of the member.
Ideally, internal treatments other than fumigants would be
applied just above fasteners as the diffusion distance is typi-
cally greater downward than upward. Fumigant treatments
can be applied above or below fasteners and at a greater
Metal flashing/pile cap
Copper borate paste
250 mL
200 mL
150 mL
100 mL
50 mL
Figure 34.
of exposed
end of pile
or other
Guide for In-Place Treatment of Covered and Timber Bridges
spacing than diffusible treatments. Application of internal
treatments can be continued down the pile to the water or
soil line (Fig. 34).
Covered Post/Pile
In situations where the top of the post/pile is capped or cov-
ered with another member (Fig. 35), treatment options are
limited to internal treatments. Water-diffusible treatments
can be placed into downward angled holes near the top of
the post/pile cap, or rods can be applied to holes drilled hor-
izontally under the cap. Fumigant treatments can be placed
lower on the vertical support.
A horizontal member supported on the pile can also be pro-
tected with internal treatments, but solid treatments may be
the most practical option if limited access does not allow
drilling of downward-angled holes. Rods can be installed
in holes drilled horizontally into the member or into holes
drilled upward from the underside the member. Solid fumi-
gants can also be applied in this manner if the depth of the
member allows it.
Bridge Bents
The piles and horizontal members associated with bents
are areas of concern for application of supplemental treat-
ments. Bridge bents are the substructure unit supporting
each end of a bridge span. They are typically composed of
two or more piles connected by a beam or girder that supp-
ports the bridge. Typically the end of the horizontal support
extends well beyond the bridge deck and is fully exposed to
the precipitation, as is the end pile supporting that member.
Ideally, the pile top was protected with a permanent cap, but
this is not always the case, and sometimes the horizontal
member is expected to provide sufcient protection. Access
to the ends of bents is typically good, allowing exibility
in approaches to treatment. Internal treatments can be used
to protect both the pile and the beam. Fumigants and/or a
combination of rods, pastes, or liquids can be applied to
treatment holes. If decay near the top of the pile is a con-
cern, holes can be drilled horizontally into the pile near the
top and treated with rods. The remainder of the pile can be
treated down to the water line using holes drilled downward
at an angle and lled with diffusible treatments. The remain-
der of the pile could also be treated with fumigants using
fewer holes. Because fumigants have greater mobility, the
rst fumigant hole can be placed further from the top of the
pile. The horizontal cap beam could also be protected using
diffusible treatments and/or fumigants. For example, any
of the forms of diffusible treatment could be applied to
holes drilled down into the top of the beam approximately
150 mm (6 in.) from the end. Fumigant could also be ap-
plied to a hole drilled into the top of the beam to provide
protection that would extend back under the bridge deck.
Glued-Laminated (Glulam) Member
In many ways a glulam stringer or beam can be viewed as
a large timber, although the glueline can affect chemical
250 mL
200 mL
150 mL
100 mL
50 mL
Figure 35. Example ap-
proach for supplemental
treatment of piles and hori-
zontal supports under the
bridge deck at an abutment.
General Technical Report FPL–GTR–205
movement. Areas of primary concern are at connections
and fasteners such as at the bridge abutments (Fig. 36). If
glulams are pressure-treated after lamination, they may have
a sizable core of untreated wood in a manner similar to a
large timber. However, glulam members are different from
sawn timbers in some respects when applying supplemental
preservative treatments. The depth of checking in glued-
laminated members in service is usually less than that in
solid-sawn timbers, and there is typically less value in ood-
ing checks or otherwise applying liquid preservatives to the
surface. Although there is some evidence that preservatives
can move across the glue lines, it is also reasonable to ex-
pect that the glue line will impede movement of preservative
between laminates. Thus, even though the sides of glued-
laminated members are often accessible for drilling, drilling
through the narrow faces and through as many laminates
as possible is preferred. Unfortunately, this usually limits
applications to the bottom face of the beam because the up-
per face is not accessible. Encapsulated fumigants are well
suited for this situation. Multiple fumigant capsules can be
used in one hole or void space can be left in the upper part
of the hole because fumigant vapors can travel upward to ll
this space. Drilling horizontal holes into the bottom laminate
may be warranted if surface decay in the contact area of the
abutment is suspected. These holes will be too small for fu-
migant application but could be treated with rods, pastes, or
gels. A bead of paste or gel could also be applied along the
edge of the beam, but it is uncertain how far chemical ap-
plied in this manner will move under the beam.
Town Lattice Under Opening
Town lattice bridges present somewhat unique challenges
for in-place treatment because lattice members have smaller
(thinner) dimensions than many other types of covered
bridge supports. Their narrow dimensions discourage water
entrapment, but town lattice connections can trap moisture,
especially in areas below bridge openings and in lower
chords near the roadway. Internal treatments can be used
to provide some protection for these connections. The dif-
fusible internal treatments can be applied into the narrow
face of each member on each side of the connection
(Fig. 37). Rods can be purchased in various diameters al-
lowing use of relatively small-diameter treatment holes. Liq-
uids, pastes, and gels can also be applied to small-diameter
holes, and drilling holes downward from the upper face al-
lows use of liquid treatments either alone or in combination
with rods. However, drilling from the top of the member
may also create a more visible treatment hole for members
below eye level. Visibility of the holes can be minimized
by drilling downward for connections above eye level and
upward for connections below eye level, but drilling upward
limits treatments to solid rods. Drilling holes with diameter
sufcient for fumigant treatments may not be desirable in
narrower members, and the high surface-to-volume ratio
Figure 36. Example approach
for supplemental treatment of
glued-laminated beam con-
tacting an abutment.
Guide for In-Place Treatment of Covered and Timber Bridges
is likely to result in more rapid loss of fumigant from the
Town Lattice Chord Connection
The bottom chord of a town lattice bridge near the road deck
is in an area where wetting is possible, and the chord/lattice
connection provides ample area for water entrapment. The
large number of these connections and the fact that each
connection involves six individual planks makes treatment
a challenge (Fig. 38). One approach is to drill downward-
sloping holes into each member to allow introduction of
diffusible preservatives. Any combination of liquids, pastes,
rods, or gels could be used. Treatment holes can be placed
near the treenails, but not so close that they weaken the
connection. In this situation, a surface-applied preservative
liquid may also be benecial. A clear solution such as a bo-
rate liquid could be used, with emphasis placed on working
the liquid down in between the individual planks around the
connection. Borate solutions are not xed to the wood and
may be washed off the wood surface with splashing from
vehicular trafc. Depending on the conguration, the appli-
cation of a more water-resistant preservative to the portion
of the chord near the road deck may be a consideration. A
clear zinc naphthenate solution is one possibility, or if the
area is not highly visible, a copper-based solution could be
used to provide additional protection. An especially vulner-
able area of the chord nearest the road deck is the butt joint
between two planks. The end-grain in this butt joint will
readily absorb and hold water splashed from the roadway.
Treatment solution can be worked into this joint where it
will also be absorbed into the end-grain to provide protec-
tion from decay associated with wetting.
Members of a Burr Arch Bridge Below a Window
Depending on the extent of roof overhang, conguration of
the opening, and climate, areas below bridge openings may
receive sufcient wetting to sustain decay. If the opening
is low enough to serve as a viewpoint, the likelihood of the
public viewing and touching these members is increased
(Fig. 39). Typically, the water-trapping areas around connec-
tions are most likely to retain sufcient moisture to support
decay. These areas can be treated using a combination of
boron or copper boron internal treatments and a surface ap-
plication of a liquid boron solution. Treatment hole plugs
may attract attention and it may be desirable to use a type of
plug, such as a driven wooden dowel that cannot be easily
removed by vandals.
250 mL
200 mL
150 mL
100 mL
50 mL
Figure 37. Example in-place treatment of
town lattice members below a window
General Technical Report FPL–GTR–205
250 mL
200 mL
150 mL
100 mL
50 mL
Figure 38. Approach to in-place
treatment of the connection
of diagonal members to chord
members at the bridge deck.
250 mL
200 mL
150 mL
100 mL
50 mL
Figure 39. Example
approach to in-place
treating members of a
Burr arch bridge ex-
posed to wetting from
a window opening.
Guide for In-Place Treatment of Covered and Timber Bridges
Members Contacting Abutment
Areas where wooden bridge members contact stone or
masonry abutments are among the most prone to decay or
termites, and bridge designers often included sacricial
members in these areas that could be periodically replaced.
In many cases, previous restorative work has addressed this
issue by changing the contact point so that the untreated
covered bridge timber rests on pressure-treated wood or
some other type of support that is less conducive to moisture
accumulation. However, untreated structural members in
some bridges do rest on stone or masonry, and these can
challenging but important areas to protect with eld treat-
ments (Fig. 40). Access is often limited, and unlike in most
connections, the area of moisture accumulation is on an
exterior surface that is inaccessible. However, depending
on the situation, substantially increased protection may be
possible. Fumigants or other internal treatments can be used
to protect the bulk of the interior, and rods containing dif-
fusible preservatives can be placed in a series of horizontal
holes just above the bearing surface. In some cases, it may
be possible to inject preservative liquid, paste, or gel be-
tween the bearing surface and masonry, or a caulking gun
can be used to deposit paste or gel of a water-diffusible pre-
servative along the edge of the member where it meets the
masonry. However, this latter approach requires discretion
as it does leave the preservative deposit exposed (Fig. 41).
Heel Connection of Diagonal Truss to Bottom Chord
Several covered bridge designs incorporate some type of
heel connection where a diagonal support member rests on
the bottom chord. There are many forms of this connection
but often the diagonal sits in a notch in the bottom chord to
prevent it from sliding along the chord (Fig. 42). These
connections are well-suited to trapping and holding
Figure 40. Example approach for in-
place treatment of the area of a bridge
chord contacting an abutment.
Figure 41. Example of application of a copper-borate paste
to the members contacting a bridge abutment.
General Technical Report FPL–GTR–205
250 mL
200 mL
150 mL
100 mL
50 mL
Figure 42. Possible approach-
es to protection of heel con-
nection with (a) fumigants only
or (b) fumigant and borates
targeted in the area of the
Guide for In-Place Treatment of Covered and Timber Bridges
moisture if exposed to wetting. Of particular concern is
the large area of end grain of the diagonal member where
it rests in the notch. As is often the case, the wood in this
connection could be treated using a range of approaches.
Fumigants could be applied close to the connection in both
the chord and diagonal members (Fig. 42a). Because the
holes are oriented downward, liquid, granular, or encapsu-
lated fumigants could be applied. Fumigants would have the
advantage of protecting a large volume of wood, but may
not be as efcient in targeting the notch area of the connec-
tion. If existing wetting/decay is suspected, application of
diffusible preservatives in the areas of the connection may
be benecial (Fig. 42b). Holes can be drilled into the end of
the diagonal member and ooded with borate solution with
the objective of permeating the end-grain at the connection.
Borate rods or gel could then be placed into the hole before
plugging. Borate gel could also be applied around the pe-
rimeter of the connection.
Timber Frame Connection
Structural support timbers may be exposed to moisture ei-
ther as result of the original design or loss of siding or roof-
ing materials. As in other structures, areas around fasteners
and connections are most likely to warrant preservative
treatment. Because moisture conditions conducive to decay
are likely to be inside the large members, surface treatments
alone may not be particularly effective. However, applica-
tion of concentrated solutions of a diffusible preservative
to the end-grain areas may have value because subsequent
wetting and wicking may draw the preservative a consider-
able distance into the wood. Drilling the holes needed to
apply internal treatments may not always be acceptable, but
in this example it is assumed that the holes can be drilled as
long as they are not visible from inside the bridge (Fig. 43).
Solid diffusible rods can be applied from beneath the large
beams and angled upwards towards the connection. Down-
ward sloping treatment holes can accommodate liquid-
diffusible treatments or solid-diffusible treatments or both.
Some beams may be large enough for application of a solid
fumigant, which can also be applied to an upward-angled
treatment hole. Fumigants protect a much larger volume of
wood than diffusible treatments do and are not dependent
on localized moisture conditions for movement through the
wood. However, their use may not be appropriate in many
structures and particularly those with limited air exchange
or human habitation.
Immersion of Portions of Covered Bridge Substructure
During Flooding
Immersion of portions of the substructure during high water
is an extraordinary circumstance. Brief immersions occur-
ring rarely are unlikely to introduce sufcient water into
a structure to sustain decay. However, prolonged or more
frequent immersion could possibly be a concern if sufcient
water is wicked into the end-grain of large support members
at connections and around fasteners. Water absorbed into
large members can be slow to dry and it is possible that suf-
cient moisture to support decay would exist if ooding oc-
curred regularly.
Because of the large volume of wood involved, potential
treatments for this problem are complex. If the bridge is suf-
ciently damaged by the ooding that repair or rehabilita-
tion is required, then serious consideration should be given
to replacing the larger, more critical members with pressure-
treated wood as part of this process. Members sent for
pressure-treatment should be cut to length and pre-drilled
prior to treatment to ensure that wood at the end grain and
connector holes is treated. If no member replacement is war-
ranted, existing members can be treated with borate prod-
ucts while still wet. The moisture in the wood will allow the
borates to diffuse more deeply into the wood than might oth-
erwise occur. Concentrated borate solutions can be brushed
or sprayed onto the wood surface, with special emphasis on
ooding checks as well as treating end-grain at joints and
around connectors. Holes can be drilled near joints for in-
stallation of boron or uoride rods and/or boron solutions.
Once the wood surface has dried, a penetrating water-
repellant nish can be applied to slow subsequent boron
Figure 43. Possible approach to treatment of a timber connection.
General Technical Report FPL–GTR–205
Terrestrial Pile with Cross-Bracing
A common support conguration for the shoreline area of
timber bridges is the placement of bents supported by two
or more piles. Diagonal cross-bracing will often connect the
piles (Fig. 44). In most cases, the piles will have been pres-
sure-treated with preservative, resulting in a shell of pro-
tected wood surrounding a core of untreated wood. In this
situation, the ground-contact area of the pile is most vulner-
able especially if drying checks have penetrated through
the treated zone to expose untreated wood. However, the
area where the cross-bracing attachment penetrates through
the treated shell can also be of concern if that area receives
sufcient wetting. The large interior volume of wood associ-
ated with a pile can perhaps be most efciently treated with
fumigants because they move greater distances through the
wood than water-diffusible treatments do. In this situation,
fumigants can be applied to 19- to 22-mm- (¾- to 7/8-in.-)
diameter holes that are sloped downward at an angle of
45° to 65°. The holes should extend through the center of
the pile and to about 2/3 of the pile diameter. Treatment
holes are started at or slightly below the groundline and
continue up the pile in spiral pattern. The vertical distance
between holes is typically 150–300 mm (6–12 in.) with the
holes staggered by 90 to 120 degrees. The closer spacing
is used for larger diameter piles. Because of the downward
slope to the holes, liquid or granular fumigants can be
applied. If exterior decay is a concern, the pile can also be
protected with an external groundline treatment (see exam-
ple “Groundline area of terrestrial pile” below).
Groundline Area of Terrestrial Pile
The external groundline area of terrestrial pots or piles
can be treated using groundline wraps, or bandages, in a
manner similar to that commonly used for utility poles
(Fig. 45). Soil around the wood is excavated to depth of
approximately 0.46 m (18 in.), and remaining soil and any
decayed wood is scraped off the surface. The preservative
paste is then applied from the bottom of the hole to slightly
above the groundline using a stiff brush or trowel.
A water-resistant wrap (often supplied with the paste) is
then wrapped around the paste and stapled to the pile. The
wrap serves the important purpose of trapping the paste
against the wood, creating a reservoir of preservative that
slowly diffuses into the wood. Alternatively, some products
are supplied as prepared bandages with the preservative
incorporated in a treated pad. The interior cover of the pad
next to the wood is slit or otherwise opened immediately
prior to installation. Although primarily used to replenish
the groundline area of preservative-treated posts or piles,
wraps can provide some level of protection to the ground-
line area of untreated wood. Decay will still occur above
ground unless the upper portion of the member is protected
from moisture. The labels of the paste products do not
always explicitly provide their suitability for use relative to
Figure 44. Example use of
fumigants for internal treat-
ment of bridge pile with
Guide for In-Place Treatment of Covered and Timber Bridges
the water levels of streams or rivers. However, they do state
that the product should not be applied to areas with surface
water or below the mean high water mark of intertidal areas.
One product, MP400-EXT, also states that it should not be
applied in aquatic environments.
Who Can Apply Supplemental
Preservative Treatments?
Wood preservatives are dened as pesticides under the Fed-
eral Insecticide, Fungicide, and Rodenticide Act (FIFRA),
and thus are regulated by the EPA. The EPA regulations
provide a minimum set of requirements, and each state may
have additional requirements for use of a pesticide. The
EPA is most concerned with the Restricted Use Pesticides
(RUPs). Two of the fumigants discussed in this publica-
tion (chloropicrin and methylisothiocyanate) fall into this
category. EPA regulations require that RUP applicators be
certied as competent to apply RUPs in accordance with
national standards. Certication programs are conducted
by states, territories, and tribes in accordance with these
national standards. Training of certied applicators covers
safe pesticide use as well as environmental issues such as
endangered species and water quality protection. Certied
applicators are classied as either private or commercial,
and there are separate standards for each. All states require
commercial applicators to be recertied, generally every
3 to 5 years. Some states also require recertication or other
training for private applicators.
States vary in their regulations about application of non-re-
stricted use pesticides. Most states require that commercial
applicators become licensed to apply these products. How-
ever, a private applicator (property owner) can purchase and
apply these pesticides on their own property without any
type of licensing. Application of supplemental treatments
to bridges by state, county, or local government employees
can be somewhat of a grey area. Although these employees
could be considered as applying the treatments to their own
property, the property itself is public. Thus, many states do
require that government workers be trained and licensed as
pesticide applicators.
The best source of information for applicator licensing re-
quirements is the state agency responsible for conducting
the EPA’s pesticide applicator program. Contact information
for each state can be found in Appendix.
The EPA Pesticide Label is the Law
Pesticide product labels provide critical information about
how to safely and legally handle and use pesticide products.
Unlike most other types of product labels, pesticide labels
are legally enforceable, and all of them carry the state-
ment, “It is a violation of Federal law to use this product in
a manner inconsistent with its labeling.” Labeling can also
include material to which the label (or other labeling mate-
rial) refers. For example, if a label refers to a manual on
how to conduct a procedure, that manual is also labeling that
the user must follow. Although all portions of the EPA label
contain critical information, and the entire label should be
read before use, some sections are particularly relevant for
eld application of preservatives:
Ingredient Statement
The ingredient statement identies the name and the per-
centage by weight of each active ingredient and the percent-
age by weight (but generally not the names) of other/inert
Net Contents/Net Weight
The net contents/net weight section identies the weight or
volume of pesticide in the container expressed in conven-
tional U.S. units of measurement. It should not include any
packaging materials.
Precautionary Statements
Precautionary statements are designed to provide the pes-
ticide user with information regarding the toxicity, irrita-
tion, and sensitization hazards associated with the use of a
pesticide, as well as treatment instructions and information
to reduce exposure potential. Four kinds of precautionary
statements may appear on a typical pesticide label.
1. Hazards to humans and domestic animals statement:
Where a hazard exists to humans or domestic animals,
Figure 45. Steps in application of an external groundline wrap treatment.
General Technical Report FPL–GTR–205
precautionary statements that describe the particular haz-
ard, route of exposure, and precautions to be taken must
appear on the label.
2. First aid statement: This section of the label provides
information to the pesticide user concerning appropriate
rst aid for the various routes of exposure associated with
accidental exposure. If the rst aid statement appears
on the back panel, then there must be a statement on the
front panel indicating that rst aid information can be
found on the back panel.
3. Environmental hazards statement: Where a hazard exists
to non-target organisms, precautionary statements that
identify the hazards and necessary precautions must ap-
pear on the label.
4. Physical or chemical hazards statement: Hazards such
as ammability, explosive potential, or electric insula-
tor breakdown, as well as the various precautions to be
taken, must be identied, as applicable.
Directions for Use
Directions for use provide instructions to the user on how
to use the product and identies the pest(s) to be controlled,
the application sites, application rates, and any required ap-
plication equipment. Just as importantly, this section also
includes a use-restrictions statement. General (non-site-spe-
cic) precautions, restrictions, or limitations of the product
are stated, as are any precautions and restrictions that apply
to specic sites. An endangered species statement may also
be included if applicable.
Worker Protection
The worker protection section provides information for
the applicator to minimize their potential exposure to the
supplemental treatment. The four types of worker protection
statements that generally appear in the precautionary state-
ments include the following:
1. Handler personal protective equipment (PPE) statement:
Addresses handler PPE requirements such as gloves and
safety glasses.
2. User safety requirement statements that address how
to handle contaminated PPE: Provides instructions for
cleaning and maintaining PPE, and sometimes for dispos-
ing of heavily contaminated PPE.
3. Engineering controls statement: Describes any reductions
or modications to handler PPE requirements that may be
made in the presence of certain engineering controls (e.g.,
closed systems, enclosed cabs, lock and load containers).
4. User safety recommendations: Provides additional user
safety information.
Storage and Disposal Instructions
Storage and disposal instructions provide instructions for
storing the pesticide product and for disposing of any un-
used pesticide and the pesticide container.
Collection of Drill Shavings
When holes are drilled into pressure-treated wood to apply
internal treatments, the resulting shavings contain preser-
vative treatment. Because of their greater surface area to
volume ratio, preservative leaching from these shavings
is many times greater than from the treated wood itself. If
the shavings generated during construction are allowed to
enter the water below a treated wood structure, they make a
disproportionately large contribution to the overall preserva-
tive release from that structure. However, this concern can
be minimized if reasonable efforts are made to collect the
shavings. Many approaches can minimize discharge of drill
shavings into the environment. Tarps are commonly used
and may be the most practical approach for collecting shav-
ings from large numbers of treatment holes. However, the
use of tarps becomes more difcult under windy or rainy
conditions. Plastic bags, tubs, or trays are also useful collec-
tion devices for collecting shavings from individual treat-
ment holes, and vacuum cleaners are also sometimes used.
Regardless of the method used, it is inevitable that collec-
tion of construction debris will add some time and expense
to a eld treatment project. The importance of collection
should be stressed in planning and budgeting for the project
so that the application crew is clear that debris collection is
an integral part of the job.
Fire Prevention
Fire is another serious threat for covered bridges and is a
leading cause of loss and damage. Although some res are
accidental, many are set by vandals or arsonists. Fire is dif-
cult to prevent, especially if intentionally set. The cause
of the re is relevant, as it affects the likelihood that steps
taken to prevent re damage will be successful.
Contributing Factors
All covered bridges are potentially vulnerable to re, but
several factors can increase the risk for damage.
Location can inuence vulnerability to re in multiple ways.
Bridges that are isolated or are on roads with light trafc can
be more vulnerable to re deliberately caused by arsonists.
They can also become an attractive gathering place for par-
ties, which can lead to accidental or purposeful res. The
remoteness of a location also increases the time before a
re is reported and the time required for responders to reach
the bridge. Location in a hot, dry climate can also increase
susceptibility to re by lowering the moisture content of the
wood and of any ammable material that has accumulated
inside the bridge. It also increases the possibility of re
spreading to the bridge from external vegetation.
Bridge Design
Large heavy timbers are more difcult to ignite than smaller
members with a higher proportion of surface area. Larger
timbers also sustain less damage during brief exposure to
Guide for In-Place Treatment of Covered and Timber Bridges
re. Thus, in theory, bridge designs with larger members
such as king post or Burr arch designs would be less vulner-
able than would a bridge built with a town lattice design.
However, the cladding on all bridges remains vulnerable
and even the largest timbers can be ignited by a determined
arsonist. Wood species can also play a role in both in ease of
ignition and in the rate of ame spread. Among softwoods,
some pine species tend to have greater ame spread than
do species such as spruce, hemlock, or Douglas-r. Among
hardwoods, the higher density species such as oak tend to
have lower ame spread than do less dense species such as
yellow poplar.
Accumulation of Organic Debris
The presence of large amounts of dry organic matter, such
as leaves, can increase the susceptibility of bridges to ac-
cidental re caused by cigarettes or sparks. Although it is
not practical to keep a bridge completely free of this type of
debris, obvious accumulations should be removed.
Selection of Roong Material
In areas where wildre is a concern, use of metal roong
or shakes/shingles protected with re retardant (discussed
below) can reduce the risk of ignition from wind-blown
External Vegetation
Combustible vegetation should be cleared away from the
bridge in areas where wild re is a concern.
Protection with Fire-retardant Treatments
Wood products can be treated with re retardants to improve
their re performance. Fire-retardant treatment results in de-
layed ignition, reduced heat release rate, and slower spread
of ames, but it does not make the wood non-combustible.
Fire-retardant treatment of wood generally improves the re
performance by reducing the amount of ammable volatiles
released during re exposure or by reducing the effective
heat of combustion, or both. The wood may then self-extin-
guish when the primary heat source is removed.
In terms of re performance, re-retardant treatments are
marketed to improve the ame spread characteristics of the
wood products as determined by standardized ammability
tests. These tests are used to generate a ame-spread index
(FSI), which is a measure of the overall rate of ame spread-
ing in the direction of air ow. Lower FSI values correspond
to a lower rate of ame spread. In the building codes, the
classes for FSI are A (FSI of 0 to 25), B (FSI of 26 to 75),
and C (FSI of 76 to 200). The classication labels of I, II,
and III have sometimes been used instead of A, B, and C.
The FSI for most domestic wood species is between 90 and
160. Some domestic softwood species meet the Class B FSI
without treatment. Other domestic softwood species have
FSIs near the upper limit of 200 for Class C. All available
data for domestic hardwoods are for Class C. Fire-retardant
treatments are necessary when a Class A ame spread index
is desired for a wood product.
A wood product labeled as “re-treated wood” for purpose
of compliance with applicable sections of the building code
must meet certain performance requirements in the codes
and related specications (AWPA & NFPA). The re per-
formance requirement for FRT wood is that its FSI is 25 or
less when tested according to the ASTM E 84 ame-spread
test and that it shows no evidence of signicant progressive
combustion when this 10-min test is continued for an addi-
tional 20 min. In addition, it is required that the ame front
in the test shall not progress more than 3.2 m beyond the
centerline of the burner at any given time during the test.
Fire-retardant treated roof covering materials are designated
Class A, B, or C based on their performance in a different
type of test as well as their use with other building material.
The roong material protocol includes intermittent ame
exposure, spread of ame, burning brand, ying brand, and
rain tests. Each of the three classes has a different version
of the pass–fail test. The Class A test is the most rigorous.
Class C the least. FRT wood shingles and shakes are avail-
able that carry a Class B or C re rating. The Class A rating
can be achieved by placing FRT shingles over specic types
of underlayment assemblies.
Fire-retardant treatments that contain boron can potentially
impart some degree of preservative protection. Some prod-
ucts make claims about preservative efcacy while others
do not, even though they may contain boron. As with boron-
based wood preservatives, the efcacy of these treatments
against decay and insects depends on the concentration of
boron delivered to the wood as well as the penetration of
boron into the wood. Boron-containing FRTs applied by
pressure treatment would be expected to provide the greater
preservative efcacy than surface treatments.
Pressure Treatment with Fire Retardants
To be most effective, wood members should be pressure-
impregnated with re-retardants in a manner similar to
the processes used to produce preservative-treated wood.
However, considerably heavier absorptions of chemicals are
necessary for re-retardant protection. Penetration of chemi-
cals into the wood depends on species, wood structure, and
moisture content. Because some species are difcult to treat,
the degree of impregnation needed to meet the performance
requirements for FRT wood may not be possible.
Inorganic salts are the most commonly used re retardants
for interior wood products, and their characteristics have
been known for more than 50 years. These salts include
monoammonium and diammonium phosphate, ammonium
sulfate, zinc chloride, sodium tetraborate, and boric acid.
Guanylurea phosphate is also used. Traditional re-retardant
salts are water soluble and are leached out in exterior ap-
plications or with repeated washings. However, water-in-
soluble organic re retardants have been developed to meet
the need for leach-resistant systems. These water-insoluble
systems include (a) resins polymerized after impregnation
into wood and (b) graft polymer re retardants attached
General Technical Report FPL–GTR–205
directly to cellulose. An amino resin system based on urea,
melamine, dicyandiamide, and related compounds is of the
rst type.
Wood pressure-treated with re-retardants can be consid-
ered for replacement members in covered bridges. Pressure
treatment will generally provide greater protection than the
surface-applied treatments. Negatives associated with re-
retardant pressure treatment include increased cost as well
as some reduction in mechanical properties. Fire-retardant-
treated wood is often more brash than untreated wood. For
structural applications, information on mechanical proper-
ties of the FRT wood product needs to be obtained from
the treater or chemical supplier.
In-Place FRT Applications (Coatings)
Commercial coating products are available to reduce the
surface ammability characteristics of wood. The two types
of coatings are intumescent and nonintumescent. The widely
used intumescent coatings “intumesce” to form an expanded
low-density lm upon exposure to re. This multicellular
carbonaceous lm insulates the wood surface from high
temperatures. Intumescent formulations include a dehydrat-
ing agent, a char former, and a blowing agent. One potential
dehydrating agent is polyammonium phosphate. Ingredients
for the char former include starch, glucose, and dipentae-
rythritol. Potential blowing agents for the intumescent coat-
ings include urea, melamine, and chlorinate parans. Non-
intumescent coating products include formulations of the
water-soluble salts such as diammonium phosphate,
ammonium sulfate, and borax.
Because coatings are not pressure impregnated or incorpo-
rated during manufacture, re-retardant coated wood is not
FRT wood as dened in most codes or standards including
National Fire Protection Association (NFPA) 703. In NFPA
703, a re-retardant coating is dened as a coating that re-
duces the ame spread of Douglas-r and all other tested
combustible surfaces to which it is applied by at least 50%
or to a ame spread classication value of 75 or less,
whichever is the lesser value, and has a smoke developed
rating not exceeding 200 when tested in accordance with
ASTM E 84 or UL 723. There is no requirement that the
standard test be extended for an additional 20 min as re-
quired for FRT wood. NFPA 703 differentiates between a
Class A coating
as one that reduces ame spread index to 25 or less and a
Class B coating as one that reduces ame spread index to
75 or less.
Fire-retardant coatings for wood are tested and marketed
to reduce ame spread. Clear intumescent coatings are
available. Such coatings allow the exposed appearance of
old structural wood members to be maintained while provid-
ing improved re performance. This is often desirable in the
renovation of existing structures such as covered bridges.
Studies have indicated that coatings subjected to outdoor
weathering are of limited durability and would need to be
reapplied on a regular basis.
Although their use to improve the resistance ratings of wood
products has been investigated, there is no general accep-
tance for using coatings to improve the re resistance rating
of a wood member. There is a lack of full-scale ASTM E
119 test data to demonstrate their performance and validate
a suitable calculation methodology for obtaining the rating.
Even pressure-applied re-retardant treatments should not
be considered an absolute solution for the threat of re to
covered bridges. A determined arsonist will still be able
to cause substantial damage to a bridge treated with re
retardants. However, FRTs should lessen the risk or extent
of damage from accidental res or from less determined
Research on the Use of Supplemental
Fire-Retardant Treatments for Covered
In 2000, Mississippi State University conducted a study
funded by the FHWA (Fire Retardant Treatments for His-
toric Covered Bridges, contract DTFH61-00-C-0017) inves-
tigating the potential of several types of pressure-treatment
applied FRTs and an on-site borate spray for re protection
of wood used in covered bridges. One exterior and one in-
terior commercial re-retardant were used to pressure-treat
specimens of Southern Pine, yellow poplar, and white oak.
The treated specimens were then evaluated for strength
properties and for relative re performance using a cone
calorimeter. Additional specimens were spray-treated with a
borate solution to simulate potential in-place FRT treatment,
and subsequently tested for re-retardant properties.
The mechanical properties of the specimens pressure-treated
with re retardants were reduced in comparison to the con-
trols, especially with the exterior re retardant. Work to
maximum load (WML) suffered the greatest impact, with
approximate 50% reduction noted for Southern Pine and
yellow poplar and 24% reduction in white oak. Modulus
of rupture (MOR) was reduced by about 25% in Southern
Pine and yellow poplar but only slightly reduced in white
oak. However, the FRT treatments had little effect on stiff-
ness (modulus of elasticity). These reductions in mechanical
properties are worthy of consideration, but do not exclude
the use of pressure FRT in many applications.
Cone calorimeter tests indicated that pressure applied re-
retardant treatments substantially reduced heat release and
substantially increased time to ignition, for both southern
pine and yellow poplar. Estimates of the ame spread index
(FSI) indicated that the pressure-treated Southern Pine and
yellow poplar would meet the specications for Class I re-
retardant material (FSI below 25). The white oak specimens
were less well protected by pressure treatment, probably
because white oak is resistant to pressure treatment and
absorbed less re-retardant. The estimated FSI for white
Guide for In-Place Treatment of Covered and Timber Bridges
oak was lowered from 75 for the untreated specimens to
between 43 and 47 for the treated specimens. The results of
this portion of the study indicate that when covered bridge
members are to be replaced, pressure-treatment of the re-
placement members with a re retardant may be justied.
The borate spray applications were much less effective in
reducing heat release or increasing time to ignition, although
yellow poplar showed some improvement. The average
estimated FSI for yellow poplar was lowered from over
130 for untreated specimens to between 77 and 91 for the
spray-treated specimens. Average estimated FSI decreases
for Southern Pine and white oak were less evident, perhaps
because both species have lower estimated FSI values for
untreated wood. It is difcult to determine if the marginal
re-retardant benets seen with borate spray treatments
would have practical benets in protecting covered bridges
from re. It should be noted that the borate formulations
evaluated in this MSU study were commercial wood pre-
servative formulations, and not commercial re-retardant
formulations. The results of this portion of the study indicate
that if re retardancy is the primary goal, then in-place ap-
plication of treatment intended for this purpose, such as an
intumescent coating, may provide greater benet than ap-
plication of borates.
The overall conclusions of the authors are that (1) replace-
ment bridge members should be pressure-treated with re
retardant, and (2) spraying the interior of covered bridges
with borates solutions is worthwhile. They based the lat-
ter recommendation on the ease of application, possible
increase in re retardancy, and additional benets such as
preservative protection and possible protection of metal
fasteners from corrosion.
Fire Protection Technology
For more integrated protection, FRT of bridge members may
be combined with other forms of protection such as lights,
alarms, sprinklers and monitoring systems.
Installation of lighting is the least expensive deterrent to
vandalism. However, lighting may not be a great deterrent
in remote locations, and could increase the use of the bridge
as a gathering place.
Cameras can be deterrents to vandalism, especially with
accompanying warning signs. However, they can also be
targets of vandalism, and only serve as a re detection tool
if the camera is being actively monitored.
Alarms based on smoke or heat detection can be congured
to alert a local re department and/or activate warning si-
rens. Smoke detectors may require frequent maintenance
to remove dust. Heat detectors must be broadly dispersed
to ensure that heat is detected before substantial damage
has occurred. Heat-detecting cables (linear heat detectors)
can be used for this purpose. The value derived from the
sensor-alarm combination depends largely on the potential
responses to the alarm. Major damage can only be prevented
if re suppression crews can reach the bridge quickly.
Sprinkler Systems
Sprinkler systems that are automatically activated by heat or
smoke sensors provide the most immediate re suppression,
but are also costly to install. Typically, pumping stations
must be installed to service the sprinkler system.
Remote Monitoring
In another study funded by the FHWA, researchers at the
FPL and Iowa State University evaluated the use of remote
monitoring based on newer technologies using ame detec-
tors, ber optic sensors, and infra-red cameras. The type of
ame detector evaluated was designed to detect re based
on the light wavelength spectra of a burning ame. Field
tests found that the ame detector detected ame within a
bridge within 5–7 s. Detection did not appear to be limited
by distance between the ame and detector, but was limited
by line of sight interference (i.e., a bridge member between
the ame and detector). The ber optic sensors also detected
ame within 5–8 s, but only if the ame source was located
within a few feet of the detector. The infra-red camera was
mounted on a pole approximately 300 ft from the bridge
and could only monitor one end of the bridge and the ap-
proach. It detected re in those locations within 10 s, and
most cases in less than 5 s. It could also be programmed to
detect the heat signal of the human body, which may have
value in alerting authorities to other types of vandalism. As
was noted with other types of alarms, these new monitoring
technologies will not prevent damage unless coupled with a
sprinkler system or rapid response from a local re station.
Wooden bridges, whether historic covered bridges or current
highway timber bridges, can be vulnerable to damage from
biodegradation and re. This manual describes procedures
for selecting and applying in-place treatments to bridges to
prevent or arrest degradation. Although the guide focuses on
preservative treatments to protect against biodeterioration,
approaches to minimizing damage caused by re are also
Efciently protecting bridges requires some understand-
ing of the causes of deterioration. Decay fungi are the most
common cause of deterioration and are commonly grouped
into brown-rot, white-rot, and soft-rot fungi. Although these
groups of fungi differ in their preferences for wood species
and environmental conditions, they all require moisture to
colonize wood. Insects, especially subterranean termites,
can also be important causes of deterioration in warmer cli-
mates. Termites prefer moist wood, but can also degrade
dry wood if a source of moisture is available. In general,
General Technical Report FPL–GTR–205
emphasis should be placed on protecting wood from mois-
ture, with use of preservative treatments focused on those
areas where moisture cannot be controlled. Vulnerable areas
in covered bridges include members contacting abutments,
members near the ends of bridges subject to wetting from
splashing, and members below windows or other openings
that allow entry of wind-blown precipitation. Pressure-
treated timber bridge members can be vulnerable when
untreated wood beneath the treated zone is exposed by eld
fabrication or by the development of large drying checks. In
older structures, the external ground contact area of treated
members may need supplemental treatment.
The objective of an in-place treatment is to distribute preser-
vative into areas of a structure that are vulnerable to mois-
ture accumulation and/or not protected by the original pres-
sure treatment. Types of eld treatments range from nishes
(coatings), to boron rods, to fumigants. A major limitation
of in-place treatments is that they cannot be forced deep into
the wood under pressure as is done in pressure-treatment
processes. However, some can be applied into the center of
large members via treatment holes. These preservatives may
be available as liquids, rods, or pastes.
Surface-applied liquid treatments should not be expected
to penetrate more than a few millimeters across the grain
of the wood, although those containing boron can diffuse
more deeply under certain moisture conditions. Liquid sur-
face treatments are most efciently used to ood checks,
exposed end-grain, and bolt holes. They may move several
centimeters parallel to the grain of the wood if the member
is allowed to soak in the solution. Surface treatments with
diffusible components will be washed away by precipita-
tion if used in exposed members. However, their loss can be
slowed if a water-repellent nish is applied after the diffus-
ible treatment has dried. Surface treatments will not effec-
tively protect the interior of large piles or timbers.
Paste surface treatments can provide a greater reservoir of
active ingredients than liquids. When used in conjunction
with a wrap or similar surface barrier, these treatments can
result in several centimeters of diffusion across the grain
into moist wood over time. Pastes are typically used for the
groundline area of posts or piles that are not usually exposed
to standing water but can also be applied to end-grain of
connections or pile tops.
Internal treatments are typically applied to the interior of
larger members where trapped moisture is thought to be a
current or future concern. Treatments can also be applied
to smaller members in some situations. Water-diffusible
internal treatments move through moisture in the wood.
They are relatively easy to handle but do not move for as
great a distance as do fumigants, nor do they move in dry
wood. Diffusible treatments may be best suited for focusing
on specic problem areas such as near exposed end-grain,
connections, or fasteners. In contrast, fumigant internal
treatments move as a gas through the wood. They have the
potential to move several feet along the grain of the wood,
but have greater handling and application concerns.
Fire is another serious threat for covered bridges and is
a leading cause of loss and damage. All covered bridges
are potentially vulnerable to re, but several factors can
increase the risk for damage. Bridges that are in isolated
areas can be more vulnerable to re deliberately caused by
arsonists. Bridges in dry climates are more vulnerable to
wildre and accumulation of dry organic matter, such as
leaves, within the bridge. Dry vegetation near the bridge can
increase re vulnerability.
Bridge members can be treated with re retardants to im-
prove their re performance. Fire-retardant treatment (FRT)
results in delayed ignition, reduced heat release rate, and
slower spread of ames, but it does not make the wood
noncombustible. Fire-retardant treatment of wood generally
improves the re performance by reducing the amount of
ammable volatiles released during re exposure or by re-
ducing the effective heat of combustion, or both. The wood
may then self-extinguish when the primary heat source is
To be most effective, covered bridge replacement members
should be pressure-impregnated with re retardants in a
manner similar to preservative-treated wood. Negatives
associated with re-retardant pressure treatment include in-
creased cost as well as some reduction in mechanical prop-
erties. Fire-retardant-treated wood is often more brash than
untreated wood. For structural applications, information on
mechanical properties of the FRT wood product needs to be
obtained from the treater or chemical supplier.
In-place coating products are available to reduce the sur-
face ammability characteristics of wood. The two types of
coatings are intumescent and nonintumescent. The widely
used intumescent coatings intumesce (expand abnormally)
to form an expanded low-density lm upon exposure to
re. This multicellular carbonaceous lm insulates the
wood surface below from high temperatures. Intumescent
formulations include a dehydrating agent, a char former,
and a blowing agent. Clear intumescent coatings are avail-
able. Such coatings allow the exposed appearance of old
structural wood members to be maintained while providing
improved re performance. However, studies have indicated
that these systems would need to be reapplied on a regular
basis if exposed to weathering.
Even pressure-applied re-retardant treatments should not
be considered an absolute solution for the threat of re to
covered bridges. A determined arsonist can cause substantial
damage to a bridge treated with re retardants. For more
integrated protection, FRT of bridge members may be com-
bined with other forms of protection such as lights, alarms,
sprinklers, and monitoring systems.
Guide for In-Place Treatment of Covered and Timber Bridges
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Guide for In-Place Treatment of Covered and Timber Bridges
Department of Agriculture
Division of Plant Protection and
PO BOX 3336
Montgomery, AL 36109-0336
(334) 240-7171
Alaska Department of
Environmental Conservation
Pesticide Program
1700 E. Bogard Rd. Building B
Suite 202
Wasilla, AK 99654
(907) 376-1870 or 1-800-478-
2577 (In-State Only)
Arizona Department of
Environmental Services
1688 W Adams
Phoenix, AZ 85007
(800) 223-0618 (602) 255-3664
(602) 542-3579
Arizona Structural Pest Control
9545 East Double Tree Ranch
Scottsdale, AZ 85258
Arkansas State Plant Board
Division of Feeds, Fertilizers
and Pesticides
#1 Natural Resource Dr
PO BOX 1069
Little Rock, AR 72205
(501) 225-1598
California Department of
Pesticide Regulation
1001 I Street
Sacramento, CA 95812
Contact: County Agric.
Colorado Department of
Division Plant Industry
700 Kipling St Suite 4000
Lakewood, CO 80215-5894
(303) 239-4140
Connecticut Department
Environmental Protection
Pesticide Division
79 Elm St
Hartford, CT 06106
(860) 424-3369
Delaware Department of
2320 South Dupont Hwy
Dover, DE 19901
(800) 282-8685 (302) 739-4811
Florida Department of
Agriculture & Consumer
Bureau of Entomology and
644 Cesery Boulevard, Suite
Jacksonville, FL 32211
(904) 727-6592
Georgia Department Agriculture
Pesticide Division
19 Martin Luther King Dr SW
Atlanta, GA 30334
(404) 656-9378
AppendixContact Information for State Ofces Conducting the U.S. PA’s
Certied Pesticide Applicator (CPA) Program
General Technical Report FPL–GTR–205
Hawaii Department of
Division of Plant Industry
Pesticides Branch
1428 S King St
Honolulu, HI 96814-2512
(808) 973-9401
Idaho Department of Agriculture
Division of Agricultural
PO BOX 7723, Boise, ID 83707
2270 Old Penitentiary Rd.
Boise, ID 83712
(208) 332-8590
Illinois Department of
Bureau of Environmental
PO BOX 19281
Springeld, IL 62794-9281
(217) 785-2427 (800) 641-3934
Illinois Department of Public
Division of Environmental
Structural Pest Control Program
525 West Jefferson
Springeld, IL 62761
Ofce of Indiana State Chemist
Purdue University
175 S. University St.
West Lafayette, IN 47907-1154
(765) 494-1594
Iowa Department of Agriculture
Pesticide Bureau
Wallace State Ofce Bldg.
502 E. 9th Street
Des Moines, IA 50319
(515) 281-5321
Kansas State Board of
Pesticide and Fertilizer Program
109 SW 9th Street, 3rd Floor
Topeka, KS 66612 (785) 296-
Kentucky Department of
Division of Pesticides
107 Corporate Dr.
Frankfort, KY 40601
(502) 573-0282
Louisiana Department of
Pesticide & Environmental
PO BOX 3596
Baton Rouge, LA 70821-3596
(225) 925-3763
Maine Department of
Pesticides Control
State House Station 28
Augusta, ME 04333
(207) 287-2731
Maryland Department of
Pesticide Regulation Section
50 Harry S Truman Parkway
Annapolis, MD 21401
(410) 841-5700
Massachusetts Department of
Pesticides Bureau
251 Causeway St. Suite 500
Boston, MA 02114
(617) 626-1720
Guide for In-Place Treatment of Covered and Timber Bridges
Michigan Department of
Pesticide and Plant Pest
Management Division
PO BOX 30017
Lansing, MI 48909
(800) 292-3939 (517)-241-6666
Minnesota Department of
Pesticide and Fertilizer
Management Division
625 Robert St. N
St. Paul, MN 55155
(612) 201-6615 (800) 627-3529
http://www.mdac. http://
Mississippi Department of
Bureau of Plant Industry
PO BOX 5207
Mississippi State, MS 39762
(662) 325-7763
Missouri Department of
Bureau of Pesticide Control
PO BOX 630, 1616 Missouri
Jefferson City, MO 65102
(573) 751-5504
Montana Department of
Agricultural Sciences Division
302 North Roberts
Helena, MT 59620-0201
(406) 444-5400
Nebraska Department of
Bureau of Plant Industry
301 Centennial Mall
Lincoln, NE 68509
(402) 471-2394
(800) 831-0550
Nevada Department of
Plant Industry Division
405 South 21st Street
Sparks, NV 89431
(775) 353-3600
New Hampshire
New Hampshire Department of
Agriculture, Markets and Food
Division of Pesticide Control
PO BOX 2042, 25 Capitol St.
2nd Floor
Concord, NH 03302-2042
(603) 271-3550
New Jersey
New Jersey Department of
Environmental Protection
Bureau of Pesticide Operations
22 South Clinton Ave.
PO BOX 402
Trenton, NJ 08625-0411
(609) 530-4070
New Mexico
New Mexico Department of
Pesticide Bureau
MSC 3189 BOX 30005
Las Cruces, NM 88003-8005
(575) 646-2134 (800) 222-1222
New York
New York Department of
Environmental Conservation
Bureau of Pest Management
625 Broadway
Albany, NY 12233-7254
(518) 402-8748
North Carolina
North Carolina Department
of Agriculture & Consumer
Structural Pest Control &
Pesticide Divistion
1090 Mail Service Center
Raleigh, NC 27699-1090
(919) 733-3933
General Technical Report FPL–GTR–205
North Dakota
North Dakota Department of
Pesticide, Feed and Fertilizer
600 E Boulevard Ave. Dept. 602
Bismark, ND 58505-0020
(701) 328-4922
Ohio Department of Agriculture
Pesticide and Fertilizer
Regulation Section
8995 E Main St
Reynoldsburg, OH 43068
(800) 282-1955 (614) 728-6987
Oklahoma Department of
Division Plant Industry
2800 N Lincoln Blvd
Oklahoma City, OK 73105-4298
(405) 521-3864
Oregon Department of
Pesticides Division
635 Capitol St NE
Salem, OR 97310-0110
(503) 986-4635
Pennsylvania Department of
Bureau of Plant Industry
2301 N Cameron St
Harrisburg, PA 17110-9408
(717) 787-4843
Rhode Island
Rhode Island Department of
Environmental Mgmt
Division of Agriculture
235 Promenade St.
Providence, RI 02908
(401) 222-2781
South Carolina
South Carolina Department of
Pesticide Regulation
Clemson University
511 Westinghouse Rd.
Pendleton, SC 29670 (864) 646-
South Dakota
South Dakota Department of
Division of Agricultural
523 E Capitol, Foss Bldg
Pierre, SD 57501
(800) 228-5254 (605) 773-4432
Tennessee Department of
Pesticide and Agricultural Inputs
440 Hogan Rd, Bruer Bldg
Nashville, TN 37204
(615) 837-5148
Texas Department of
Agriculture - Pesticide Division
PO BOX 12847
Austin, TX 78711
(512) 463-7622 (800) 835-5832
Utah Department of Agriculture
and Food
350 N Redwood Rd
PO BOX 146500
Salt Lake City, UT 84114-6500
(801) 538-7188
Vermont Agency of Agriculture
Agrichemical Management
116 State St
Montpelier, VT 05620
(802) 828-3482
Guide for In-Place Treatment of Covered and Timber Bridges
Virginia Department of
Ofce of Pesticide Services
102 Governor St. 1st Floor
Richmond, VA 23219
(804) 371-6558 (800) 552-9963
Washington Department of
Pesticide Management Division
PO BOX 42560
Olympia, WA 98504-2560
(360) 902-2010 (877) 301-4555
Environmental Regulation
Department of Consumer and
Regulatory Affairs
1104 4th St. SW
Washington, DC 20024
(202) 442-4307
West Virginia
West Virginia Department of
Pesticide Regulatory Program
1900 Kanawha Blvd E
Charleston, WV 25305
(304) 558-2209
Wisconsin Department of
Agriculture, Trade and
Consumer Protection
Agricultural Resources Mgmt
PO BOX 8911, 2811 Agriculture
Madison, WI 53708-8911
(608) 224-4500
Wyoming Department of
Technical Services Division
2219 Carey Ave
Cheyenne, WY 82002
(307) 777-7321
Technical Report
Full-text available
These guidelines are designed for decision makers (selectmen, county commissioners, city planners, preservation officers, etc.) that have responsibility for repairing and maintaining existing covered bridges to help them understand what goes into making effective decisions about how, and when, to repair a covered bridge. The purpose of these guidelines is to present the steps necessary for decision makers to identify effective rehabilitation techniques for restoring the structural integrity of covered bridge members. The intent is to retain the maximum amount of historic fabric while ensuring public safety and minimizing future maintenance requirements. To make informed repair decisions about existing covered bridges, it is important to (1) start with a basic understanding of the type of bridge, (2) know the current condition, (3) be informed of what to consider when conducting an engineering analysis, and (4) be aware of various repair options that can meet the long-term goals for the bridge. Only after the decision maker knows the type of bridge, its condition, and to what loads it is subjected can a repair strategy be developed. How to support the bridge during the repair phase, what repairs are appropriate, and how to maintain the bridge after repairs are implemented are all critical to ensuring a long service life for the covered bridge.
Full-text available
The Forest Products Laboratory uses the cone calorimeter for the initial evaluation of the flammability of untreated and fire retardant treated wood products. The results of various studies are reviewed using a model presented at the 12th Annual BBC Conference on Flame Retardancy. The model uses data from the cone calorimeter to provide measures of fire growth propensity based on material bulk properties and surface fire growth propensity and to provide an estimate of the ASTM E 84 flame spread index. In a study on the fire performance of treated wood exposed to elevated temperature, wood treated with borax/boric acid did not maintain fire retardancy after being exposed to 66°C (150°F) for 1 to 3 months.
Sections of Douglas-fir poles were used to assess the ability of boron and fluoride to diffuse from sodium fluoride and fluoride/boron rods. In general, chemical levels in the wood over the test period were below the threshold for inhibiting fungal attack regardless of the dosage, although acceptable levels were achieved in isolated locations in the pole sections. Higher dosages may be required to develop adequate chemical loadings in the wood, but the potential for weakening the poles by drilling additional treatment holes may outweigh any possible benefits of increasing the dosage.
The potential for moisture sorption by boron and fluoride rods following application of rods to wood to affect subsequent chemical diffusion was investigated in small Douglasfir blocks conditioned to 30, 60, or 90 percent target moisture content (MC). MCs tended to decline over the 180-day test period, but there was no evidence that the rods acted to draw moisture away from the wood. As expected, chemical movement tended to increase with increasing MC. Threshold levels were reached within 180 days for boron, even in blocks at 30 percent target MC. Fluoride levels tended to be much lower, reflecting the much lower dosages applied. There was no evidence that rods sorbed enough water to reduce moisture availability for subsequent diffusion.
Borate penetration relies on diffusion when borate and glycol-borate preservatives are applied to the surface of wood. This study evaluated the extent of borate penetration in framing lumber as a function of preservative formulation, wood moisture content, and diffusion time after treatment. In Phase I of the study, end-matched specimens were conditioned to target average moisture contents of 15, 25, or 35 percent, briefly immersed in borate formulations, and then placed into wooden frames to minimize air exchange during diffusion. Penetration in these specimens was generally less than 5 mm (or 35% of the cross section) regardless of treatment solution, target moisture content at time of treatment, or diffusion period (2,4, or 8 wk). Assay of boron concentrations after 8 weeks of diffusion also indicated that the boron was concentrated in the outer 5 mm of the wood. Diffusion appeared to have been limited by the relatively rapid drying of the specimens, even with the restricted air movement within the wooden frames. In Phase II of the study, specimens were conditioned to a target average moisture content of 20 percent prior to dip immersion and then placed in a room that maintained an equilibrium moisture content of 19 to 21 percent. Penetration in these specimens was assessed after 6, 13, and 26 weeks of diffusion. After 6 weeks of diffusion, average boron penetration exceeded 5 mm, and after 26 weeks of diffusion, penetration exceeded 11 mm, or over 70 percent of the cross section. Little difference in diffusion was observed between the types of borate formulations evaluated in either phase of this study. The results of this study indicate that rapid drying conditions may limit penetration of boron from spray applications; however, in situations where high humidity is maintained in a structure, substantial diffusion is possible.