P 9.4 HAIL DAMAGE TO ASPHALT ROOF SHINGLES
Timothy P. Marshall*, Richard F. Herzog, and Scott J. Morrison
Haag Engineering Co.
Controversies can arise with regard to how asphalt
shingles are damaged by hail, and what hail damage
actually looks like. More specifically, questions have
been raised as to whether granules removed from
asphalt shingles during a hailstorm will reduce the
expected life or water shedding ability of the roof
shingles. In this paper, the authors will review the
definition of hail damage to asphalt shingles and
explain the characteristics of such damage. We also
will present the results of our ten-year study on
granule loss to asphalt shingles as well as review the
methodology to assess hail damage to an asphalt
The authors have inspected thousands of asphalt
shingle roofs and found that damage inspectors
frequently mistake various shingle anomalies such as
foot scuffs, adhesive spots, etc., as hail damage. We
have also inspected numerous roofs where people
have tried to simulate hail damage by using a variety
of tools or other objects in order to defraud an
insurance carrier. Therefore, the last part of this paper
will focus on various shingle anomalies that are
frequently misidentified as hail damage and explain
how to differentiate between intentional and
unintentional roof damage.
2. ASPHALT ROOF SHINGLE COMPONENTS
Asphalt roof shingles are one of the most common
and affordable roof coverings on the market today.
Base mat materials are either paper (organic) or
glass-fiber (inorganic). The mats are coated with an
asphaltic mixture composed of asphalt, limestone
powders, and other mineral stabilizers (fillers).
Granules are applied to the shingle surfaces to give
them color, add weight, and to block the underlying
asphalt from deleterious effects of the sun. Most
granules are crushed stone coated with a ceramic
material. The ceramic gives color to the granules.
Generally, one third of the shingle weight is granules,
one-third asphalt, and one-third filler. The mat is a
small fraction of the total weight.
Asphalt roof shingles come in various sizes,
shapes, and thicknesses. Generally, the thicker or
heavier the asphalt shingle, the more it costs. The
most common asphalt shingles are three-tab and
*Corresponding author address: Timothy P. Marshall,
2455 McIver Ln., Carrollton, TX 75006. Email:
Three tab shingles contain slots or joints that give
the appearance of a common brick pattern when
installed on the roof. Laminated type shingles are
comprised of one full shingle and half a shingle
bonded together with asphalt to give them thicker
look, similar to that of wood shingles or slate.
2. HAIL DAMAGE DEFINITION
Morrison (1999) defined damage to roofing as a
diminution of water-shedding capability or a reduction
in the expected long-term life of the roofing material.
Marshall and Herzog (1999) more specifically defined
functional hail-caused damage to asphalt shingles as
punctures, tears, or fractures (bruises) in the shingle
mats (Figure 1). Shingle bruises are an indentation
with fracture in the mat that feels soft like that of an
apple bruise. The bruise is usually obvious as
granules are also dislodged from the impact area.
Marshall et al. (2002) presented their ice impact
test results that employed a mechanical launching
device. Ice stones were launched at standard
velocities against roofing products that included
various 11-year-old, naturally aged, asphalt shingles.
Impacts were oriented perpendicular to the shingles.
The study concluded that aged organic mat-based
asphalt shingles were damaged half of the time by
one-inch diameter ice stones, whereas it took 1.25 in.
(3.1 cm) diameter ice stones to damage the aged
glass-fiber mat asphalt shingles. Thicker, aged
laminated type shingles were damaged by 1.5 in. (3.8
cm) ice stones. Greenfeld (1969) and Koontz (1991)
have presented similar results in conducting ice ball
impact tests on asphalt shingles.
Figure 1: Hail damage to asphalt shingles: a) broken
edges, b) bruise, c) puncture, d) torn edge.
However, there remains a controversy whether
granules removed by hail, without visible asphalt
exposure, constitutes hail damage. Many asphalt
shingle manufacturers have issued "technical
bulletins" about hail and granule loss, stating that if
granules are lost from the shingle due to hail, the
shingle has lost life. However, there are no published
scientific studies to validate this statement.
3. GRANULE LOSS STUDY
In order to determine how many granules, if any,
must be removed in order to affect the service life or
water shedding ability of the shingle, the authors' firm
conducted a granule loss study on asphalt shingles.
Varying quantities of granules were removed with a
wire brush from new, three-tab, glass-fiber mat
shingles. The shingles then were exposed naturally to
the weather for a period of ten years. The quantities
of granules removed were none (control), and
approximately 6, 15, 45, and 70 percent of the total
granules on the shingles. Another shingle was
installed upside down such that the asphalt-coated
mat was exposed to the weather. The shingles were
installed conventionally over a plywood deck on a
4:12 pitch that faced south. The shingles were
examined at intervals throughout the ten year period
as well as at the conclusion of the study (Figures 2
Figure 2. Test panel at the beginning of the study
showing percentage of granules removed. "C" is the
Figure 3. Same view as Figure 2 only at the
conclusion of the ten-year study.
In one year, the exposed asphalt had oxidized grey
but there was no visible evidence of surface cracks or
erosion. After five years, areas of exposed asphalt
had oxidized but this did not affect the function of the
shingles to shed water. Surface erosion was visible
on the shingle without granules, and some of the
glass fibers had become exposed. After ten years, no
significant change was noted in the shingles except
for the shingle without granules. More glass fibers
were exposed on this shingle due to erosion; but, the
shingle continued to shed water (Figure 4).
Figure 4. Close up views of new shingle and ten-year
weathering with both 70 percent of the granules
removed and no granules, respectively.
The quantity of granules lost from the roof shingles
during a hailstorm is a relatively small amount.
Generally, about one-third the weight of an asphalt
shingle is granules such that a 25 square roof covered
with three-tab shingles would have about one ton of
granules. Granule loss is expected from the moment
shingles are manufactured, shipped, installed, and
during the weathering process. Granules are part of
the wearing surface on the shingle and exposure to
hail is part of the wearing process that is actually built
into the design. Thus, more granules are initially
placed on the shingles than needed to cover the mat.
In our study, we found between 12 to 15 percent of
the surface granules had to be removed from new
shingles before the asphalt-coated mat was exposed.
The amount of "excess" granules on new shingles
varied by plus or minus ten percent. We would expect
that the quantity of granules lost during a hailstorm
generally would fall within the normal variation of
granules placed on a shingle.
Therefore, it seems logical to conclude that the
small quantities of granules removed from shingles
during a hailstorm does not shorten the life of the roof
or adversely affect its water shedding ability as long
as the impacted areas are not bruised or punctured,
and remain covered with granules. This conclusion
agrees with the work done by Morrison (1999).
4. ASPHALT SHINGLE ANOMALIES
There are usually a number of anomalies on an
asphalt shingle roof not related to hailstone impact.
Some of these anomalies may take rounded forms
that can be mistaken as hail damage. Understanding
how shingles are manufactured, installed, and
weather is important when properly differentiating
between non-hail conditions and hail damage.
Asphalt shingles are manufactured in a high
speed, fully automated process. Occasionally, certain
defects involve insufficient granule or asphalt
coverage, or the use of poor quality asphalt. Shingle
manufacturers should "cull" or remove such defects
before the shingles are shipped. However, the level
of quality control of shingle products varies. Thus, it is
not unusual to find shingle defects on a roof that
involves bands or spots of missing asphalt and/or
granules (Figure 5).
Figure 5. Various shingle manufacturing defects: a)
lack of granule adhesion on a three-tab, b) blotchy
appliques, c) asphalt exposed in lower laminate, and
d) lines of missing asphalt and granules.
As asphalt shingles age, their components break
down. The extent of aging depends upon many
factors including the quality of the asphalt, shingle
color, roof pitch, slope direction, and attic ventilation.
Common deficiencies inherent with aged asphalt
shingles are blistering, splitting, cupping, clawing,
crazing, and flaking. In many instances, these
anomalies are not discovered until after a hailstorm;
however, this does not mean they were created or
aggravated by the storm (Figures 6 and 7).
Figure 6. Various shingle anomalies not caused by
hail: a) closed blisters, b) open blisters including
close-up view in the inset photograph, c) diagonal
splitting, and d) horizontal splitting.
Shingle blisters occur from a combination of poor
quality asphalt combined with heat. They appear as
small bubbles in the shingle surfaces where a portion
of the granule surface is raised. Eventually, the
shingle bubbles rupture exposing steep-sided voids in
the shingle surfaces that frequently extends down to
the shingle mat. Shingle blisters are usually 1/4 in. (.6
cm) in diameter or less and are not caused by
Diagonal and horizontal splitting of asphalt
shingles involves a combination of asphalt shrinkage,
deck movement, and low tensile strength in the mat.
Ribble et al. (1993) further explain such problems with
Figure 7. Various shingle anomalies not caused by
hail: a) cupping, b) clawing, c) crazing, and d) flaking.
Cupping and clawing results from asphalt
shrinkage on the top and bottom surfaces of the
shingles, respectively. The corners and edges of the
shingles are prone to curling or cupping as the mat
shrinks. Crazing of the shingle surfaces also results
from asphalt shrinkage. Eventually, chunks of
granules flake away from the mat leaving the asphalt-
coated mat exposed to the weather.
Additional shingle anomalies can be created during
installation. The most common shingle installation
deficiencies are marring, edge scuffing, elevated
staples, and adhesive spots (Figure 8).
Figure 8. Shingle installation deficiencies: a) marring,
b) edge scuffing, c) elevated fasteners, and d)
Shingle marring occurs when people walk across
the roof on a day when the shingles are hot, soft, and
pliable. The asphalt in the shingle surface softens to
the point where it is pushed aside along with the
granules and typically forms a ridge on the outside
edge of the mark. Persons walking on the roof can
also remove granules along the bottom edges of the
Elevated fasteners can occur during the installation
of the shingles and can protrude through or buckle the
overlying shingles. The fasteners are either not
driven flush to the shingle or are driven into joints
between the roof decking. Elevated fasteners are not
caused by hail striking the roof.
Adhesive can drip off the shingles onto the other
shingles leaving a round, dark spot that can be
mistaken by some as hail damage. If the adhesive
from one shingle contacts another shingle and bonds
to it, a portion of the shingle surface can be removed
when the shingles are separated leaving a rounded
area of missing granules that can also be mistaken by
some as hail damage.
5. ASSESSING HAIL DAMAGE
Accurate assessment of hail damage is a step by
step process that involves an examination of the roof
shingles as well as other objects on and around the
roof. Marshall and Herzog (1999) presented a
methodology on how to quantify hail damage to a roof
through the use of test squares. The number of hail
damaged shingles are counted in each test square on
each directional roof slope and that number
determines whether the roof slope is repaired or
replaced through the use of the DURA formula.
Shingles are particularly susceptible to hail
damage if they have little or no underlying support,
especially along ridges, rakes, eaves, and valleys.
Shingle edges also are vulnerable to being chipped or
broken. Therefore, the entire roof must be examined.
Recently exposed asphalt appears black or
unweathered, whereas asphalt exposed for several
months oxidizes forming a surface film that is a grey
color. This color difference is one way to tell new hail
damage from old hail damage (Figure 9).
Figure 9. Example of 9-year-old hail damage to a
glass-fiber mat asphalt shingle.
6. INTENTIONAL MECHANICAL DAMAGE
On occasion, some people have utilized various
tools or other objects in an attempt to simulate hail
damage on a roof. Popular items have included: 1)
ball peen hammers, 2) claw hammers, 3) coins, and
4) screwdrivers. The authors have recognized a
number of factors that distinguish intentional damage
from hail damage. For example, intentional damage
is not randomly distributed on the roof but usually
occurs in groups or lines concentrated in upper
portions of the roof, away from roof edges. Impact
angles of the tool or object are nearly perpendicular to
the affected roof slope, therefore indicating multiple
impact directions (Figure 10). In contrast, hail would
leave a random distribution of damage on the roof.
The windward slope typically sustains the most
concentrated and direct hail impacts whereas the
leeward slopes have fewer, glancing hail impacts.
Figure 10. Intentional roof damage examples: a) line
of impact marks, b) circular arrangement of impact
marks, and c) impact marks perpendicular to each
affected slope with no marks on the ridge.
Intentional damage is concentrated frequently in
the interior or center portions of the shingles, away
from shingle edges, as it is human nature to hit the
center of an object. Such centered impacts usually
are found on each affected slope, regardless of slope
direction (Figure 11). The impacts tend to be singular,
Figure 11. Attempts to simulate hail damage with a
ball peen hammer.
occurring once per shingle. In contrast, hail does not
prefer the centers of the shingles nor strike shingles
In our inspections of suspicious roof damage, we
utilize a series of magnification rings to closely
photograph the impact marks. Typically, shingles
struck by metal objects will have broken or shattered
the ceramic coating on the granules. This will leave a
"powder" residue containing shattered ceramic
material within the impact mark. Any side-to-side or
"rounding out" motions will tend to leave swirl marks
within the powder residue and/or leave smudges in
the exposed asphalt surface (Figure 12).
When claw hammers are utilized, the metal peen
frequently does not strike the roof slopes exactly
perpendicular but tends to tilt forward slightly leaving a
characteristic curving fracture in the shingles that
opens towards the direction of impact. The concave
fracture in the shingle resembles a "frowny" face when
looking upslope. Granules closest to the concave
side of the fracture are frequently compressed
uniformly into the shingle mat.
Sometimes coins are utilized to leave small divots
in the shingle surface. Quarter coins have small
ridges around their perimeters that can leave a series
of ridges in the asphalt under magnification.
Figure 12. Close-up views of mechanically caused
impacts to shingles using: a) a ball-peen hammer, b)
claw hammer, c) screwdriver, and d) quarter coin.
7. DOCUMENTING INTENTIONAL DAMAGE
The authors have developed a methodology to
better document intentional damage to a roof. The
procedure involves examining objects around and on
the house similar to the hail damage inspection
protocol as explained by Marshall and Herzog (1999).
Hail-caused "spatter" marks are usually found on
faded metal surfaces such as air conditioners,
electrical junction boxes, and metal window frames.
Hail-caused "scuff" marks are recorded on wooden
fences and dents occur in aluminum fins on air
conditioners. Such items provide good estimates of
hail size and direction of hailfall. The same can be
said with the examination of metal items on the roof.
Thus, items on and around the house can provide
evidence of hail size and direction that should match
the size and direction of any alleged hail damage to
the roof shingles.
After a general examination of the building
surroundings is performed, a roof plan diagram is
drawn and shingle marks are plotted. Any pattern or
grouping of shingle marks quickly becomes apparent
in the diagram (Figure 13). The diagram will indicate
those roof slopes or ridges that are notably without
shingle marks as well as any grouping of marks.
Usually, large areas of the affected roof slopes are
without shingle marks as are other, perhaps smaller,
roof slopes that face the same direction. Shingle
marks closest to the roof edge are measured.
Figure 13. Roof plan diagram showing the distribution
of mechanically-caused impact marks in an alleged
hail damage claim.
In this paper, we have explored certain issues with
regard to hail damage on asphalt roof shingles. The
results of our ten-year granule loss study were
presented where it was found that there was no loss
of life or reduction of water shedding ability even with
70% of the granules removed from the glass-fiber mat
shingles. The shingle with 100% of the granules
removed did exhibit more erosion than the other
shingles. Therefore, asphalt roof shingles that lose
some granules during a hailstorm are not considered
damaged as long as the shingles remain covered with
granules. Functional damage to asphalt roof shingles
includes punctures, tears, or fractures (bruises) in the
We also have shown there are a number of
anomalies on asphalt shingles that occur during
manufacturing, installation, and weathering. Some of
these anomalies take on rounded forms that can be
mistaken by some as hail damage. We also have
discussed how to recognize intentional damage to
asphalt roof shingles where someone attempts to
simulate hail-caused damage. A methodology was
presented to better document intentional damage to a
The authors would like to thank our reviewers: C. S.
Kirkpatrick, P. Lawler, J. Stewart, and D. Teasdale.
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products, Building Science Series #23, National
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Marshall, T. P., and R. F. Herzog, 1999: Protocol for
Assessment of Hail-Damaged Roofing, Proc. of the
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