Influences of a Variety of Reinforcements on the Durability of Reinforced Bitumen Sheets Operating at Variable Temperatures

Abstract

This manuscript provides an overview of the most commonly-produced bitumen roofing sheets, focusing on the types of reinforcements used for their production and the reinforcements’ effects on the durability of tensile mechanical properties of roofing sheets under thermal loads. The paper includes the analysis of working conditions of roof coverings in the mid-European transitional climate, i.e., exposed to temperatures passing through 0 °C for three seasons in a year, periodic exposure to negative temperatures reaching −15 °C and positive temperatures up to +70 °C, justifying the above-mentioned emphasis on thermal load. It draws attention to technical problems related to the cooperation of roofing sheets with roofing substrates, with particular emphasis on concrete substrates. For the purposes of the work, the analyses were carried out with regard to the assessment of the service life of roof coverings made of various reinforcements working in conditions of variable temperatures and thus exposed to the transfer of thermal movements of substrate plates. The analyses also included the impact of different coefficients of thermal expansion of the materials in contact with other materials within roof coverings on the incidence of damage to cover layers. Particular attention was paid to the conditions resulting from the production process of roofing sheets effect on the durability of roof coverings made of these materials. Additionally, there were set directions for further work to calculate the impact of stresses, arising in layers of roof coverings during their operation in changeable negative and positive temperatures, on the incidence of mechanical damage to these coverings.

Full text

Citation: Francke,B.; Szymczak-Graczyk,
A.; Ksit, B.; Szulc, J.; Sieczkowski, J.
Influences of a Variety of
Reinforcements on the Durability of
Reinforced Bitumen Sheets Operating
at Variable Temperatures. Energies
2023,16, 3647. https://doi.org/
10.3390/en16093647
Academic Editor: Chi-Ming Lai
Received: 25 February 2023
Revised: 16 April 2023
Accepted: 21 April 2023
Published: 24 April 2023
Copyright: © 2023 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
4.0/).
energies
Brief Report
Influences of a Variety of Reinforcements on the Durability of
Reinforced Bitumen Sheets Operating at Variable Temperatures
Barbara Francke 1, Anna Szymczak-Graczyk 2, * , Barbara Ksit 3, Jarosław Szulc 4and Jan Sieczkowski 4
1Institute of Civil Engineering, Department of Mechanics and Building Constructions, Warsaw University of
Life Sciences-SGGW, Nowoursynowska 159, 02-787 Warsaw, Poland; barbara_francke@sggw.edu.pl
2Department of Construction and Geoengineering, Faculty of Environmental and Mechanical Engineering,
Poznan University of Life Sciences, Pi ˛atkowska 94E, 60-649 Pozna´n, Poland
3Institute of Building Engineering, Faculty of Civil and Transport Engineering, Poznan University of Technology,
Piotrowo 5, 60-965 Pozna´n, Poland; barbara.ksit@put.poznan.pl
4
Building Research Institute, Filtrowa 1, 00-611 Warsaw, Poland; j.szulc@itb.pl (J.S.); j.sieczkowski@itb.pl (J.S.)
*Correspondence: anna.szymczak-graczyk@up.poznan.pl
Abstract:
This manuscript provides an overview of the most commonly-produced bitumen roofing
sheets, focusing on the types of reinforcements used for their production and the reinforcements’
effects on the durability of tensile mechanical properties of roofing sheets under thermal loads. The
paper includes the analysis of working conditions of roof coverings in the mid-European transitional
climate, i.e., exposed to temperatures passing through 0
◦
C for three seasons in a year, periodic
exposure to negative temperatures reaching
−
15
◦
C and positive temperatures up to +70
◦
C, justifying
the above-mentioned emphasis on thermal load. It draws attention to technical problems related
to the cooperation of roofing sheets with roofing substrates, with particular emphasis on concrete
substrates. For the purposes of the work, the analyses were carried out with regard to the assessment
of the service life of roof coverings made of various reinforcements working in conditions of variable
temperatures and thus exposed to the transfer of thermal movements of substrate plates. The analyses
also included the impact of different coefficients of thermal expansion of the materials in contact
with other materials within roof coverings on the incidence of damage to cover layers. Particular
attention was paid to the conditions resulting from the production process of roofing sheets effect
on the durability of roof coverings made of these materials. Additionally, there were set directions
for further work to calculate the impact of stresses, arising in layers of roof coverings during their
operation in changeable negative and positive temperatures, on the incidence of mechanical damage
to these coverings.
Keywords:
reinforced bitumen sheets; durability of sheet coverings; variable operating temperatures;
variety of reinforcements
1. Introduction
Roof coverings made of reinforced bitumen sheets are used in small- and large-
cubature buildings. These coverings have been used for many years and over the last
few decades they have undergone a complete evolution.
Reinforced bitumen sheet is a factory-made flexible sheet, including any carri-
ers/reinforcements, facings, surface texture and/or backing. The top surface is covered by
a finishing layer which protects the sheet against weathering, for example, fine or coarse
mineral granules. The underside is protected by an anti-sticking substance for transport
and/or storage purposes. Bitumen sheets are supplied in roll-form ready for use. The
reinforcement plays one of the key roles in the structure of roofing bitumen sheet, since it
provides at least 90% of tensile strength of the finished products [
1
–
3
]. manufacturers must
ensure the stability of bitumen sheets, and especially their tensile mechanical properties.
Energies 2023,16, 3647. https://doi.org/10.3390/en16093647 https://www.mdpi.com/journal/energies

Energies 2023,16, 3647 2 of 14
Coatings are used to create a tight and continuous waterproofing layer on the reinforce-
ment, thanks to which it has no direct contact with the external environment. Absorbent
reinforcements (cardboard or non-woven fabric) are impregnated with asphalts with a low
softening point prior to the application of the coating mass. During the manufacturing
process, the reinforcement is highly stretched, which, in the case of tension-prone materials,
contributes to the development of unwanted stresses within the membrane, which are then
stabilised in the final product by the cooled coating compound. This phenomenon becomes
apparent after such a membrane has been incorporated into the roofing, when the coating
compound softens after being heated by the sun on hot days. This allows the reinforcement
to relax, and consequently, for the longitudinal shrinkage of the laid membrane strips,
which is particularly visible in transverse overlaps [2,4,5].
It should also be noted that, despite the similar geometrical and material proper-
ties of the reinforcements, their mechanical properties are very much dependent on the
microstructure and direction of the reinforcing fibres [6].
The main component of coating masses is bitumen supplemented with the addition of
fillers and modifiers. Initially, tar as well was used to produce coating masses. In the 1980s,
the use of tar [
7
] was discontinued due to its carcinogenic effect, and modified bitumen
additives were introduced on a large scale to improve the rheological properties of coatings.
These techniques have been continued to this day. Modifications of coatings [
8
,
9
] have
been successively introduced, extending their viscoelasticity, which in turn has resulted
in increases in the temperature-resistance of finished products, both for high and low
temperatures. The modifications primarily use SBS [
10
–
12
], i.e., styrene–butadiene–styrene,
obtained in the process of copolymerisation of styrene and butadiene, increasing the range
of viscoelasticity of coating masses in temperatures from
−
15
◦
C to +110
◦
C. At ambient
temperature, two phases are formed in the bitumen mixture with the SBS copolymer:
−
Macro polymers saturated with low-molecular components of bitumen (paraffins,
maltenes); and
−“Bitumen” rich in asphaltenes, practically free of polymers.
The polymer phase consists of saturated polybutadiene microphase and pure polystyrene.
This composition allows for achieving a cross-linked three-dimensional structure, thanks
to which the composites act similarly to thermoplastic rubbers, showing similar elasticity,
i.e., a practically
complete elastic recovery after deformation [
13
,
14
]. The effects of the
coatings on the final properties of the material depends on the percentage content of these
additives. For modified bitumen roofing sheets, the percentage content of SBS in relation
to bitumen is within the range of 10–15%, but attempts have been made to increase this
additive up to 30%. An increase in the modifier content of up to 30% leads to significant
increases in the costs of finished products, and therefore this solution has not found wide
interest among producers. On the other hand, a 5–7% additive level of SBS is commonly
used to produce so-called “low-modified” roofing sheets.
In mid-European transitional climatic conditions, i.e., in Poland, the second modifier,
i.e., APP (atactic polypropylene), an amorphous substance obtained as a by-product in the
process of propylene polymerisation, is used less frequently. When mixed with bitumen,
APP penetrates deeply into the maltene phase, creating a stabilised network and dispersing
asphaltenes [
2
,
13
]. The addition of this ingredient increases the viscoelastic range of
coatings within the temperature range of
−
5
◦
C to +130
◦
C and is therefore more useful in
hot climates. Polymers from the polyolefins group (PO) are used as modifiers in bitumen
masses, as well. These polymers are primarily manufactured to produce modified bitumen.
Such modified bitumen is used for the construction of roofs or roads. The strength of the
mechanical parameters and the quality of bitumen mass made of PO are both higher [15].
As mentioned above, the bitumen used for roofing applications is, as a standard prac-
tice, modified by polymers. Austrian scientists [
16
] proposed to re-use recycled polymer
during the production of polymer-modified bitumen (PmB), reducing the amount of poly-
meric waste. The cited research results confirmed that recycling processes may degrade or
contaminate polymers, leading to reduced crystallinity and lower melting temperatures.

Energies 2023,16, 3647 3 of 14
Such changes may even be taken as beneficial, resulting in improved mixing behaviours
and a more homogeneous distribution of the polymer within the bitumen.
The search for methods to increase the viscoelasticity range of roofing sheet coating
masses is accompanied by works on modifying the textures of the undersides of roofing
sheets, efforts aimed at widening the functionality of finished products used as ventilation
layers [
2
]. The underside of roofing sheet features strip-profiled channels, which, together
with the properly selected composition of coating mass (bitumen-resin or bitumen-sand),
after gluing strips to the substrate, enable the distribution of gases under the waterproofing
layer. These solutions are mainly used in roof coverings, but they can also be effective
for repairs of damp horizontal insulation of ceilings located directly above rooms in un-
derground parts of buildings. These methods of preparing the underside of roofing sheet
strips allow for the gluing of them to the substrate in strips, leaving ventilation ducts
unglued and under the surface of previously made layers. Depending on the method used
to modify coating masses within ventilation ducts, gluing processes consist of either hot-air
welding or thermal activation of the bitumen adhesive applied in strips. In the process of
this activation, which takes place at up to 1000
◦
C, coating masses applied on the underlay
or intermediate layers of roofing sheet strips do not melt and bitumen does not liquefy,
but they both stick to the area of profiled ridges. For several years, manufacturers have
been increasing the thickness of roofing sheets, offering single-layer solutions instead of
multi-layer ones, which requires applying several layers of roofing sheet (at least two)
within installed waterproofing systems. In addition to lower costs, in comparison to multi-
layer systems, single-layer roofing reduces the emissions of harmful gases generated in the
process of gluing.
Roofing sheets are used for waterproofing roof coverings and insulating terraces
and balconies, as well as waterproofing underground parts of buildings [
1
,
2
]. In each
of the above areas, the scope of their operation depends on the operational loads that
affect them. The effects of these actions are different in different parts of buildings and
structures, and each of the mentioned areas, due to their operational specificity, should be
considered individually. For many years, attempts have been made to assess the durability
of the mechanical properties of roofing sheets under tension [
6
,
17
]. Article [
6
] analyses the
mechanism of damage to roofing sheets under tension in different directions to the point of
breakage. Article [
18
] assesses the effect of the relative stiffness of a roofing sheet reinforcing
insert on the type of damage that it causes, in both single- and multi-layer systems.
The durability of roofing sheet coverings has a considerable impact on the durability
of buildings, since most processes that destroy the substance of buildings occur in the
presence of water and moisture. For this reason, it is vital to ensure the longest possible
period of failure-free operation of roofing materials. Monitoring of roof coverings and
proper diagnostics of their damage is a key issue from the point of view of all users of
buildings [
19
]. However, to meet this requirement, it is necessary to know the processes
that significantly affect the proper selection of roofing solutions for specific service loads.
Although the possible destruction of the roof covering at the time of its creation is not
a construction disaster, its destructive character lasting for a long time may consequently
lead to a failure or a construction disaster [20].
Roofing sheets can be laid on rigid substrates, i.e., concrete and cement plaster, as
well as on flexible substrates, for instance, ones made of thermal insulation materials. This
manuscript focuses on the durability analysis of one of the applications, i.e., roof coverings
made of bitumen sheets welded to the substrate using propane-butane gas torches, and
laid on rigid substrates. Such solutions are encountered in the following cases:
−
In non-ventilated flat roofs when the covering is laid on additional layer made of
cement mortar, which separates the thermal insulation from the roofing layer;
−
In ventilated flat roofs when the thermal insulation is laid on the ceiling above the top
storey and the roof covering lies on top of the roof situated above the air void made
above this thermal insulation.

Energies 2023,16, 3647 4 of 14
The purpose of these analyses is to draw attention to usefulness of making the ap-
propriate selection of roofing sheet types for specific substrates, with special attention
paid to the material’s ability to carry service loads by the reinforcements of bitumen
sheets. This manuscript attempts to clarify whether the movements of the substrate slabs
caused by temperature changes have an effect on the breaks in the reinforcements of bitu-
men sheets bonded to these substrates, and which types of reinforcements best transmit
such interactions.
2. Analysis of Failure Characteristics of Reinforced Bitumen Sheets under Service
Loads—Discussion of the Problem
2.1. Analysis of the Effect of Bitumen Covering on the Durability of Bitumen Roofing Sheets
In this section, the effect of the tape of bitumen coating on the occurrence of mechan-
ical damage to the entire section of the roofing bitumen sheet during use under natural
conditions is analysed. In the bitumen sheet, the thickness of the bitumen compound is
small, up to a maximum of 5 mm, and it protects the surface of the layer against ageing
factors associated with the surface effects of varying positive and negative temperatures,
and UV effects in the presence of water and moisture. According to the literature, the
influence of bitumen coating on the transfer of tensile stresses acting on the substrate side
is small [
3
]. In the case of bitumen, when a constant amplitude of stress is applied, the
crack-propagation time of the specimen is short, so the end of the test is strictly defined
by the rupture of the bitumen film. During fatigue testing of viscoelastic materials, the
value of the modulus of stiffness changes due to the occurrence of creep or relaxation. The
reason for these changes is the heating of the specimen during the test. As the mechanical
properties of bitumen change significantly as a function of temperature, fatigue tests should
be conducted at a minimum of two temperatures. As a result of the above-mentioned
factors, in most cases a parallel Wohler fatigue curve is not observed for bitumen in the
stress
σ
, log N system, or for a larger number of fatigue cycles N. Fatigue tests on bitumen
are usually carried out using a viscometer to apply a sinusoidally varying stress with a
constant amplitude [21].
The available technical literature mainly discusses the results of tests on bitumen
sheets subjected to accelerated ageing under laboratory conditions involving both high
and low temperatures with simultaneous exposure to UV radiation, water and moisture,
although changes in physical and chemical properties due to accelerated weathering might
not directly correlate to those occurring naturally [
12
]. Heat aging has been reported to
be effective in evaluating the performance of modified bituminous membrane [
22
] and
simulating the effects of natural aging [
23
]. For example, Puterman et al. [
24
] measured the
tensile properties, cold flex temperature and the water-pressure resistance of various types
of roofing membrane which were naturally exposed under normal service conditions and
showed that SBS-modified membranes retained better low-temperature flexibility than did
the APP-modified membranes, after comparable periods of exposure under similar service
conditions. Data on the SBS- and APP-modified membranes showed that the exposure
hardly affected the properties determined by the reinforcement but had a strong effect on
the properties that are governed by the bitumen/polymer material. They reported that
the cold flex temperature of an SBS-modified bituminous membrane increased by about
12
◦
C after five years of natural exposure, but the tensile strength and elongation were
hardly affected. The data also showed that SBS-modified membranes retained better low-
temperature flexibility than did the APP-modified membranes, after comparable periods of
exposure under similar service conditions. Baxter et al. [
25
] showed that the tensile strength
of five SBS-modified bituminous membranes increased and the cold flex temperature of
two SBS-modified bituminous membranes rose by about 15
◦
C after heat aging at 80
◦
C for
28 days. Rodriguez et al. [
26
] studied the effects of heat aging and test temperature on the
tensile strength and elongation of two APP-modified and two SBS-modified bituminous
membranes. For the two SBS-modified membranes, heat aging (80
◦
C for 168 days) did not
significantly affect the tensile strength at 23
◦
C but it reduced the elongation by
20–40%.

Energies 2023,16, 3647 5 of 14
However, heat aging reduced the tensile strength of these membranes at –30
◦
C by about
20% and increased the elongation by about 60–330%. The results of these studies are
mainly related to the modification of bitumem coverings of the sheets [
27
,
28
] and not to
the influence of ageing factors on the reinforcements. Liu et al. [
12
] showed that heat
aging affected matrix-controlled properties such as elongation and strain energy but hardly
affected the reinforcement-controlled properties such as breaking strength.
Although accelerated testing can be useful in ranking different materials, it has many
shortcomings [
29
] without giving an answer as to how these bitumen sheets will behave
under service conditions as a result of their interaction with the substrate.
A major problem with roofing bitumen sheets is damage to the coating compound
during welding, which leads to a loss of bitumen viscosity and deformation of the roofing
sheet, as shown in Figure 1.
Energies 2023, 16, x FOR PEER REVIEW 5 of 14
retained better low-temperature flexibility than did the APP-modified membranes, after
comparable periods of exposure under similar service conditions. Baxter et al. [25] showed
that the tensile strength of five SBS-modified bituminous membranes increased and the
cold flex temperature of two SBS-modified bituminous membranes rose by about 15 °C
after heat aging at 80 °C for 28 days. Rodriguez et al. [26] studied the effects of heat aging
and test temperature on the tensile strength and elongation of two APP-modified and two
SBS-modified bituminous membranes. For the two SBS-modified membranes, heat aging
(80 °C for 168 days) did not significantly affect the tensile strength at 23 °C but it reduced
the elongation by 20–40%. However, heat aging reduced the tensile strength of these mem-
branes at –30 °C by about 20% and increased the elongation by about 60–330%. The results
of these studies are mainly related to the modification of bitumem coverings of the sheets
[27,28] and not to the influence of ageing factors on the reinforcements. Liu et al. [12]
showed that heat aging affected matrix-controlled properties such as elongation and
strain energy but hardly affected the reinforcement-controlled properties such as breaking
strength.
Although accelerated testing can be useful in ranking different materials, it has many
shortcomings [29] without giving an answer as to how these bitumen sheets will behave
under service conditions as a result of their interaction with the substrate.
A major problem with roofing bitumen sheets is damage to the coating compound
during welding, which leads to a loss of bitumen viscosity and deformation of the roofing
sheet, as shown in Figure 1.
Figure 1. Roofing sheets damaged during the welding of the cover.
2.2. Analysis of the Effect of Roofing Reinforcements on Mechanical Properties of Roofing Sheets
The main component of a roofing sheet that transfers deformations in a structure is
its reinforcement, and from this point of view the mechanical strength of the latter is crit-
ical [1,2]. Based on market analysis carried out for the leading European roofing sheet
manufacturers, a study compared how this characteristic changed in different products
by setting the limit values of roofing sheet tensile strength as a function of the type and
mass per unit area of the reinforcement used. The values are summarised in Table 1. The
literature reports showed a clear difference in the mechanical response depending on the
material and tested sample [6]. The quoted values of the tensile mechanical properties of
Figure 1. Roofing sheets damaged during the welding of the cover.
2.2. Analysis of the Effect of Roofing Reinforcements on Mechanical Properties of Roofing Sheets
The main component of a roofing sheet that transfers deformations in a structure
is its reinforcement, and from this point of view the mechanical strength of the latter is
critical [
1
,
2
]. Based on market analysis carried out for the leading European roofing sheet
manufacturers, a study compared how this characteristic changed in different products
by setting the limit values of roofing sheet tensile strength as a function of the type and
mass per unit area of the reinforcement used. The values are summarised in Table 1. The
literature reports showed a clear difference in the mechanical response depending on the
material and tested sample [
6
]. The quoted values of the tensile mechanical properties
of the roofing sheet, i.e., maximum tensile force and elongation at maximum force, were
determined based on the analysis of the records provided in the declaration of performance
specified in roofing sheet tests according to the research methodology included in EN
12311-1:1999 [
2
,
30
], and carried out on strips 5 cm wide and 200 cm long, with a distance
between measuring points of 100 mm and between jaws of 120 mm, under tension, and
at a testing speed of 100 mm/min. The above analysis also showed that at present the
most common groups of materials used to produce roofing reinforcements are polyester
non-woven fabrics, glass fabrics, mixed polyester-glass fibres, and glass veil. For bitumen
sheets, there are no requirements in the European standard EN 13,707 [
31
] for tensile
properties, i.e., elongation and maximum tensile force, they should be in accordance with

Energies 2023,16, 3647 6 of 14
the values declared by the manufacturer, and with the declared tolerance. This lack of
requirements has contributed to the fact that the values given in Table 1, determined on
the basis of an analysis of the European market, are generally regarded in Poland as the
requirements for bitumen sheets produced on the mentioned types of reinforcements.
Table 1.
Summary of tensile mechanical properties of roofing sheets and mass per unit area of the
most common roofing reinforcements [2].
Properties
Roofing Reinforcement
Polyester
Non-Woven Fabric
Glass
Fabric
Mixed
Polyester-Glass
Fibres
Veil and/or
Glass Fleece
Mass per unit area of roofing reinforcement, g/m2≥180 ≥
200
≥160 ≥60
Max. tensile force, N/50 mm:
−longitudinal
−transverse
≥800
≥600
≥
900
≥
900
≥600
≥500
≥300
≥200
Elongation at max. tensile force, %
−longitudinal
−transverse
≥40
≥40
≥2
≥2
≥2
≥2
≥2
≥2
All types of roofing sheets listed in Table 1are used in roof coverings, with the underlay
or intermediate layers of the roofing made of underlayment sheets, and the top layers of
top-covering sheets, i.e., with the upper side of the strip protected against solar radiation,
e.g., using coarse mineral roofing granules. The given summary shows that the production
of sheet reinforcements uses materials with the following mass per unit area: polyester
non-woven fabrics—at least 180 g/m
2
, glass fabrics—min 200 g/m
2
, glass veil also known
as glass fleece—at least 60 g/m
2
. Reinforcements made of mixed polyester-glass fibres with
a mass per unit area of at least 160 g/m
2
are most often made of polyester non-woven fabric
reinforced with glass threads. The images of the reinforcements mentioned are shown
in Figure 2.
Energies 2023, 16, x FOR PEER REVIEW 6 of 14
the roofing sheet, i.e., maximum tensile force and elongation at maximum force, were de-
termined based on the analysis of the records provided in the declaration of performance
specified in roofing sheet tests according to the research methodology included in EN
12311-1:1999 [2,30], and carried out on strips 5 cm wide and 200 cm long, with a distance
between measuring points of 100 mm and between jaws of 120 mm, under tension, and at
a testing speed of 100 mm/min. The above analysis also showed that at present the most
common groups of materials used to produce roofing reinforcements are polyester non-
woven fabrics, glass fabrics, mixed polyester-glass fibres, and glass veil. For bitumen
sheets, there are no requirements in the European standard EN 13,707 [31] for tensile prop-
erties, i.e., elongation and maximum tensile force, they should be in accordance with the
values declared by the manufacturer, and with the declared tolerance. This lack of require-
ments has contributed to the fact that the values given in Table 1, determined on the basis
of an analysis of the European market, are generally regarded in Poland as the require-
ments for bitumen sheets produced on the mentioned types of reinforcements.
Table 1. Summary of tensile mechanical properties of roofing sheets and mass per unit area of the
most common roofing reinforcements [2].
Properties
Roofing Reinforcement
Polyester
Non-Woven
Fabric
Glass
Fabric
Mixed
Polyester-Glass
Fibres
Veil and/or
Glass Fleece
Mass per unit area of roofing reinforce-
ment, g/m
2
≥180 ≥200 ≥160 ≥60
Max. tensile force, N/50 mm:
− longitudinal
− transverse
≥800
≥600
≥900
≥900
≥600
≥500
≥300
≥200
Elongation at max. tensile force, %
− longitudinal
− transverse
≥40
≥40
≥2
≥2
≥2
≥2
≥2
≥2
All types of roofing sheets listed in Table 1 are used in roof coverings, with the un-
derlay or intermediate layers of the roofing made of underlayment sheets, and the top
layers of top-covering sheets, i.e., with the upper side of the strip protected against solar
radiation, e.g., using coarse mineral roofing granules. The given summary shows that the
production of sheet reinforcements uses materials with the following mass per unit area:
polyester non-woven fabrics—at least 180 g/m
2
, glass fabrics—min 200 g/m
2
, glass veil
also known as glass fleece—at least 60 g/m
2
. Reinforcements made of mixed polyester-
glass fibres with a mass per unit area of at least 160 g/m
2
are most often made of polyester
non-woven fabric reinforced with glass threads. The images of the reinforcements men-
tioned are shown in Figure 2.
(a) (b) (c) (d)
Figure 2. Images of the various most common roofing reinforcements, respectively: (a) reinforce-
ment of polyester non-woven fabric, (b) reinforcement of glass fabric, (c) reinforcement of polyester
non-woven fabric with glass threads, and (d) glass veil.
Figure 2.
Images of the various most common roofing reinforcements, respectively: (
a
) reinforcement
of polyester non-woven fabric, (
b
) reinforcement of glass fabric, (
c
) reinforcement of polyester non-
woven fabric with glass threads, and (d) glass veil.
By analysing the mechanical properties of roofing sheets under tension, it can be
unquestionably stated that only roofing reinforcements of polyester non-woven fabric
elongate at maximum tensile force (above 40%), simultaneously maintaining their high me-
chanical strength at least 600 N/50 mm (for transverse samples) and at least
800 N/50 mm
(for longitudinal samples). Considering the above values, these roofing sheets can be used
on roofs whose structures work within a large range of service loads, and even with con-
currence of major deformations. This statement finds confirmation in structural tests which
indicate the increased strength and durability of this group of products [
6
]. Admittedly,
roofing reinforcements of glass fabric, due to the high values of maximum tensile forces,

Energies 2023,16, 3647 7 of 14
min. 900 N/50 mm, transfer even higher service loads occurring in roof coverings than do
roofing reinforcements of polyester non-woven fabric, with, however, a much smaller range
of deformations, i.e., at least 2%. The recorded maximum elongation values at maximum
tensile force for roofing reinforcements of glass fabric usually do not exceed 5%. Roofing
reinforcements of mixed polyester–glass and glass fleece are characterised by even lower
strength parameters, in both cases with elongation values at maximum strength slightly
exceeding 2%. Although, with these low elongations, roofing reinforcements of mixed
polyester–glass are characterised by relatively high values of maximum tensile force: at
least 500 N/50 mm (for transverse samples) and at least 600 N/50 mm (for longitudinal
samples). In the case of roofing reinforcements of glass fleece, these values are two times
lower: at least 200 N/50 mm and 300 N/50 mm. Considering the above, roofing reinforce-
ments of glass veil, according to the present authors, can be used as one layer in multi-layer
roof coverings and should not be folded onto the vertical planes of over-roof elements.
2.3. Service Loads Acting on Roofing Sheet Coverings
In roof coverings, roofing sheets are exposed to numerous actions, such as climatic
factors—different negative and positive temperatures in the presence of precipitation and
wind suction [
32
–
35
]. There are also actions related to snow removal from roofs and other
maintenance and repair works that often cause mechanical damage from both static and
dynamic loads [
36
]. These additional actions are difficult to predict and therefore cannot be
classified as cyclic loads, which interferes with analysing this phenomenon as a function of
durability. This manuscript focuses on assessing the effect of variable temperatures acting
on roofing sheets in operating conditions, excluding the action of wind suction, which is
such a broad phenomenon that it would require a separate analysis.
Roofs with a traditional layer system are subject to deformations due to temperature
differences and uneven subsidence of buildings. Temperatures acting on roofing layers
vary depending on the location of buildings and climate variability, including the action
of solar radiation energy throughout the year, as well as the colour of topcoat layers. In
an urbanized area, there is a phenomenon of “urban heat zone” consisting in a significant
increase in temperatures inside urban centres in comparison to surrounding peripheral
areas. The action can be equated to an island (or sometimes to an archipelago of islands)
surrounded by an “ocean of relative coolness” [
37
]. This phenomenon occurs even in
towns with a population not exceeding 3,500 inhabitants [
37
]. The key factor contributing
to the heating of roof covering materials is solar energy. The action of solar energy on
buildings depends on numerous elements, such as: architectural form, shading probability
by neighbouring buildings, type of external surfaces and reflectivity of the environment. It
should also be remembered that solar radiation is a source of energy with different values.
The level of solar energy reaching the border of the atmosphere is only a small part of
the calculation of energy emitted by the sun. This value is known as solar constant. As
it passes through the atmosphere, the value of the solar constant decreases due to the
processes of scattering and absorption. This change in the intensity of solar radiation causes
fluctuations in the temperatures of the outside air. The relationship between solar radiation
and air temperature was defined by Mackey and Wright as the solar value of outside air
temperature. The hypothetical value of outside air temperature at which thermal power
would be encountered by non-sunlit surfaces of outer partitions is equal to the thermal
power encountered by sunlit partitions at a given value of outside air temperature, defined
as solar temperature [
38
,
39
]. One of the formulas used to calculate solar temperature is
presented below (1) [40].
toe =tao +Rso ·α·IG(1)
where:
toe—Solar temperature, ◦K;
tao—Outdoor air temperature, ◦K;
Rso—Heat transfer resistance on the outer surface, m2K/W;
α—Coefficient of solar radiation absorption;

Energies 2023,16, 3647 8 of 14
IG—Total intensity of solar radiation, W/m2.
Poland is located in the mid-European transitional climate, i.e., exposed to tempera-
tures passing through 0
◦
C for three seasons in a year. The average air temperature in the
summer of 2022 (June-August) in Poland was 19.3
◦
C, i.e., 1.3
◦
C higher than the multi-year
average temperature value for that month (climatological normal period 1991–2020). The
anomaly index, i.e., deviations from the multi-year monthly averages over the period
1991–2020, ranged from 1.0 ◦C to 2.0 ◦C [41].
In winter, roof coverings are exposed to negative temperatures. The anomaly rate
in 2022, i.e., deviations from the long-term monthly averages from the period 1991–2020,
ranged from
−
1.0
◦
C to 3.0
◦
C [
42
]. It was found that in the central and eastern part of
Poland, extreme temperature values even drop below
−
17
◦
C. The layers of roof coverings
made of bitumen roofing sheets may therefore be exposed to the extreme temperatures
quoted above.
The studies on the correlation between ambient temperature and temperature mea-
sured on the surface of roofing conducted in North America [
43
] confirm that the colours of
roofing membranes have a significant effect on the temperatures occurring at their surface.
The maximum temperatures attainable on the surface of a black membrane can reach up
to 70
◦
C in summertime, with an ambient temperatures of up to 24
◦
C. In winter, the
values reach
−
20
◦
C at an ambient temperature of
−
15
◦
C. Studies conducted in the Czech
Republic have confirmed the above observations and indicated that the colour of roofing
granules on the roofing sheet surface has, in practical terms, a considerable effect on the
surface temperature of bitumen sheets and thus, on their aging rate. From the selection of
the colour range of roofing granules, light colours should be preferred to dark colours such
as red. At the extreme, combined with the reflection of the sunlight, the temperature can ex-
ceed, in the long run, approx. 80
◦
C [
44
]. However, it cannot be denied that, in the climatic
conditions of Europe, deviations from the above values are confirmed [
45
]. For example, in
the northern Alps, for the purposes of testing waterproofing systems [
46
], minimum daily
temperatures in winter were set at 0
◦
C and maximum average temperatures in summer at
18
◦
C, with an average 65 mm/month precipitation in winter and up to 150 mm/month in
summer. At the above-mentioned outside temperatures, the temperature determined on
the surface of the topcoat made of dark ceramic tiles reached 45–50 ◦C on sunny days.
Considering the above, the following maximum temperatures at the surface of roofing
membranes can be assumed for assessing the change in mechanical properties of roofing
products in a mid-European transitional climate:
−+70 ◦C in summer, with outside air temperatures of +24 ◦C;
−In winter, −15 ◦C at outside air temperatures of −20 ◦C.
2.4. The Effect of Variable Service Temperatures on the Durability of the Substrate—Roofing
Sheet System
Roof coverings are laid on, among other surfaces, rigid substrates, i.e., reinforced
concrete elements of flat roofs. The substrate and the roof covering are subject to mutual
interactions, which may result in damage to the roofing sheet layers, longitudinal or
transverse overlaps or joints at the roofing sheet-substrate interface. These situations
can result from the differences in deformations in the cross-sections of roofing layers
caused by temperature gradients, whether daily, seasonal, or annual, as determined for
specific locations of buildings, and different for substrates and covering materials. In
summer, deformations of roof coverings with the traditional arrangement of layers affect
predominantly roof-supporting structures, whereas in winter—roof coverings, contributing
to their cracking.
In Poland, the following temperature ranges are customarily adopted [47]:
−∆
T = 20
◦
C—Most common range of changes of day temperatures (in winter—from
–5 ◦C to +15 ◦C, in summer—from +10 ◦C to +30 ◦C);
−∆T = 40 ◦C—Frequent range of changes of daily and annual temperatures;
−∆
T = 70
◦
C—Maximum annual range of changes of temperatures occurring in Poland.

Energies 2023,16, 3647 9 of 14
With reservation of the above, the extreme elongation of the concrete substrate (under
the roofing sheet layer), assuming the maximum value of temperature gradient and linear
expansion coefficient of concrete to be
αt
= 1.0
×
10
−5◦
C, is approx. 0.07%. This value is
many times lower than the value of maximum elongation of roofing sheets with other types
of reinforcements, such as those obtained, for instance, during tensile tests, as provided in
Table 1. However, such a comparison may lead to erroneous conclusions, such as failing
to consider the effects of the roofing sheet’s production process affecting its performance
properties and further, the specificity of work in the roof covering.
Roofing sheets are anisotropic materials. Therefore, the coefficient of linear expansion
may also show anisotropy in this case. In addition, the value of the coefficient of linear
expansion of a roofing sheet is the resultant stemming from the type of reinforcement
used and the type of coating mass. Hence, it is difficult to determine it unequivocally.
For example, the value for pure bitumen is assumed to be 1.9
×
10
−4
/K. In addition,
in the first summer season after the roof covering is completed, the effects of possible
linear expansion of the roofing sheet resulting from the structure of raw materials used
for its production are eliminated by reinforcement shrinkage due to relaxation after the
bitumen mass’ exposure to sunlight. These stresses are mainly found in the roofing sheets’
polyester reinforcement, which tends to stretch during the manufacturing process, and this
elongation is temporarily fixed by the cured coating mass, but only until it relaxes, which
may occur after reheating the strip laid in the roofing material. When this is applied, the
shrinkage of the roofing sheet can reach up to 0.5% of the strip length. In roofing sheets
with glass reinforcement, this phenomenon usually does not occur. A different coefficient
of thermal expansion of both contact materials, i.e., the concrete/cement mortar substrate
and the roofing sheet, may be an additional reason for reducing the durability of roofing
coverings during their operation.
Additionally, in accordance with good construction practices, in rigid substrate under
roofing sheets (e.g., those made of cement concrete), thermal expansion joints are made, the
purpose of which is to eliminate the impact of thermal deformations on the deformation of
structural elements. Such expansion joints are usually made by cutting wet cement mortar,
creating a grid of squares of 2.0 m to 3.0 m. Assuming a distance between the expansion
joints of 3.0 m and temperature differences
∆
T = 70
◦
C, the change in the length of the
substrate element (under the roof covering) under thermal load is max. approx. 2.1 mm,
which is compensated for by the width of the expansion joint. With greater elongation
of the roofing sheet covering (due to a different coefficient of thermal expansion) and the
resulting difference in elongation, damage to the roofing sheet may occur due to stresses
exceeding tensile strength or insufficient adhesion of the roofing covering to the substrate.
When analysing the durability of roofing sheet in roofing coverings, the cyclical nature
of thermal loads should also be taken into account. This fact should be considered when
assessing the effect of temperatures on the technical condition of bitumen roofing sheets
and their possible damage. The research results regarding this phenomenon that have been
published so far in the technical literature confirm that, as a result of repetitive cycles of
opening and closing, the gap formed between the slabs of the concrete substrate horizontal
plate movements, from 0 to 2 mm, on the surface of which waterproofing layers are laid,
when simulated at variable temperatures:
−
15
◦
C and +70
◦
C, damage to the roofing layer
can occur, even if it is made of roofing sheet with high values of maximum tensile force
and/or elongation at maximum force [
17
]. For example, after 100 cycles of opening and
closing the gap between the substrate plates at
−
15
◦
C and another 100 cycles as above at
+70 ◦C, occurring in both cases at a testing speed of 16 mm/h:
−
Bitumen sheet with reinforcement of polyester non-woven fabric and SBS-modified
coating mass, at average maximum tensile force: 1034 N/50 mm and relative elon-
gation at maximum tensile force on average: 44.9%, was not damaged above the
test joint;

Energies 2023,16, 3647 10 of 14
whereas:
−
Bitumen sheet with reinforcement of glass fabric and SBS-modified coating mass, of
average maximum tensile force 1498 N/50 mm and relative elongation at maximum
force 5.0%, cracked locally over the test joint in the entire cross-section, while in the
case of bitumen sheet with the same kind of reinforcement, but with oxidized bitumen
coating mass, of maximum tensile force on average: 1750 N/50 mm and relative
elongation at maximum force on average: 7.5%, the crack occurred only in the top
layer of the coating mass.
These results were obtained in the test carried out in the apparatus shown in Figure 3[
17
].
Energies 2023, 16, x FOR PEER REVIEW 10 of 14
When analysing the durability of roofing sheet in roofing coverings, the cyclical na-
ture of thermal loads should also be taken into account. This fact should be considered
when assessing the effect of temperatures on the technical condition of bitumen roofing
sheets and their possible damage. The research results regarding this phenomenon that
have been published so far in the technical literature confirm that, as a result of repetitive
cycles of opening and closing, the gap formed between the slabs of the concrete substrate
horizontal plate movements, from 0 to 2 mm, on the surface of which waterproofing layers
are laid, when simulated at variable temperatures: −15 °C and +70 °C, damage to the roof-
ing layer can occur, even if it is made of roofing sheet with high values of maximum tensile
force and/or elongation at maximum force [17]. For example, after 100 cycles of opening
and closing the gap between the substrate plates at −15 °C and another 100 cycles as above
at +70 °C, occurring in both cases at a testing speed of 16 mm/h:
− Bitumen sheet with reinforcement of polyester non-woven fabric and SBS-modified
coating mass, at average maximum tensile force: 1034 N/50 mm and relative elonga-
tion at maximum tensile force on average: 44.9%, was not damaged above the test
joint;
whereas:
− Bitumen sheet with reinforcement of glass fabric and SBS-modified coating mass, of
average maximum tensile force 1498 N/50 mm and relative elongation at maximum
force 5.0%, cracked locally over the test joint in the entire cross-section, while in the
case of bitumen sheet with the same kind of reinforcement, but with oxidized bitu-
men coating mass, of maximum tensile force on average: 1750 N/50 mm and relative
elongation at maximum force on average: 7.5%, the crack occurred only in the top
layer of the coating mass.
These results were obtained in the test carried out in the apparatus shown in Figure
3 [17].
Figure 3. For testing resistance to fatigue—block diagram of the device: 1—base, 2—concrete slab of
the substrate with the possibility of horizontal movement, 2’—immobilised concrete slab, 3—tested
waterproofing layer, 4—electric actuator, 5—thermocouples, 6—temperature chamber, 7—heating
lamps/variable cooling device, 7—temperature recorder, 8—gap between two concrete substrates,
and 9—temperature recorder.
However, the cited publication lacks data on the stresses that occur when the roof
covering is damaged. Only the velocity of movement and information on the absence or
detection of cracks after completed test cycles are known. It is undeniable that the men-
tioned observations are very important from the operational point of view and should be
treated as an important signal indicating the need for further studies on the durability of
roofing sheet coverings in terms of mechanical damage caused by cyclical thermal defor-
mations of the substrate at variable negative and positive temperatures. The presented
results indicate that the effects of these actions are significantly affected by both the type
Figure 3.
For testing resistance to fatigue—block diagram of the device: 1—base, 2—concrete slab of
the substrate with the possibility of horizontal movement, 2
0
—immobilised concrete slab, 3—tested
waterproofing layer, 4—electric actuator, 5—thermocouples, 6—temperature chamber, 7—heating
lamps/variable cooling device, 7—temperature recorder, 8—gap between two concrete substrates,
and 9—temperature recorder.
However, the cited publication lacks data on the stresses that occur when the roof
covering is damaged. Only the velocity of movement and information on the absence
or detection of cracks after completed test cycles are known. It is undeniable that the
mentioned observations are very important from the operational point of view and should
be treated as an important signal indicating the need for further studies on the durability
of roofing sheet coverings in terms of mechanical damage caused by cyclical thermal
deformations of the substrate at variable negative and positive temperatures. The presented
results indicate that the effects of these actions are significantly affected by both the type of
roofing sheet coating mass modifications and the tensile mechanical properties of roofing
sheet, with particular emphasis on elongation at maximum tensile force. Naturally, it
should be assumed that the damage process progresses successively over time, starting
with cracks in the coating mass. The cracking in roofing sheet in the entire section does not
occur immediately. However, bearing in mind that it is the continuity of coating masses
covering roofing sheet strips that guarantees tightness to water and moisture, it can be
concluded that such damage indicates the prior loss of serviceability of the assessed roof
covering. Figure 4shows examples of such failures.

Energies 2023,16, 3647 11 of 14
Energies 2023, 16, x FOR PEER REVIEW 11 of 14
of roofing sheet coating mass modifications and the tensile mechanical properties of roof-
ing sheet, with particular emphasis on elongation at maximum tensile force. Naturally, it
should be assumed that the damage process progresses successively over time, starting
with cracks in the coating mass. The cracking in roofing sheet in the entire section does
not occur immediately. However, bearing in mind that it is the continuity of coating
masses covering roofing sheet strips that guarantees tightness to water and moisture, it
can be concluded that such damage indicates the prior loss of serviceability of the assessed
roof covering. Figure 4 shows examples of such failures.
(a)
(b)
Figure 4. Surface damage to the roofing sheet: (a) hairline cracks in the coating mass, and (b) cracks
in the roofing sheet, with visible traces of biological corrosion in the place of damage.
3. Conclusions
Although roofing sheet coverings have been in use since the 18th century and have
undergone a major metamorphosis since then, to this day, not all mechanisms affecting
their durability are known. An important, though not fully-explored scientific problem is
the mechanism behind the way in which variable positive and negative temperatures af-
fect roofing coverings during their operation and their impact on the resistance of roofing
sheets to mechanical damage. Furthermore, the effect of interaction within the roof cover-
ing of two contact materials with different coefficients of thermal expansion, i.e., the con-
crete substrate and the roofing sheet, is still unexplained, which may also be a potential
cause of damage to roofing coverings during their operation.
The analysed examples permit the conclusion that there is no direct correlation be-
tween the unit value of the linear expansion coefficient of the substrate, the durability of
the roof coating resulting from repeated actions simulated in the quoted tests of resistance
and the fatigue caused by the work of substrate panels at variable positive and negative
temperatures. It is also recommended that this problem should be analysed periodically.
Paper [48] provides alternative methods of roof covering, the use of which, however, did
not ensure the durability and tightness of the roofing at excessive wind gusts. Numerical
analyses of the effect of moisture on roof coverings were cited in [49,50], where it was
clearly stated that the thermal conductivity coefficients of materials depend on moisture
content of the material used.
The durability of roofing sheet coverings has a considerable impact on the durability
of buildings, since most processes that destroy the substance of buildings occur in the
presence of water and moisture. For this reason, it is vital to ensure the longest possible
period of failure-free operation of roofing materials. Monitoring of roof coverings and
proper diagnostics of their damage is a key issue from the point of view of all users of
buildings [34]. However, to meet this requirement, it is necessary to know the processes
that significantly affect the proper selection of roofing solutions for specific service loads.
Although the possible destruction of the roof covering at the time of its creation is not a
construction disaster, its destructive character lasting for a long time may consequently
lead to a failure or a construction disaster [35].
Figure 4.
Surface damage to the roofing sheet: (
a
) hairline cracks in the coating mass, and (
b
) cracks
in the roofing sheet, with visible traces of biological corrosion in the place of damage.
3. Conclusions
Although roofing sheet coverings have been in use since the 18th century and have
undergone a major metamorphosis since then, to this day, not all mechanisms affecting
their durability are known. An important, though not fully-explored scientific problem is
the mechanism behind the way in which variable positive and negative temperatures affect
roofing coverings during their operation and their impact on the resistance of roofing sheets
to mechanical damage. Furthermore, the effect of interaction within the roof covering of
two contact materials with different coefficients of thermal expansion, i.e., the concrete
substrate and the roofing sheet, is still unexplained, which may also be a potential cause of
damage to roofing coverings during their operation.
The analysed examples permit the conclusion that there is no direct correlation be-
tween the unit value of the linear expansion coefficient of the substrate, the durability of
the roof coating resulting from repeated actions simulated in the quoted tests of resistance
and the fatigue caused by the work of substrate panels at variable positive and negative
temperatures. It is also recommended that this problem should be analysed periodically.
Paper [
48
] provides alternative methods of roof covering, the use of which, however, did
not ensure the durability and tightness of the roofing at excessive wind gusts. Numerical
analyses of the effect of moisture on roof coverings were cited in [
49
,
50
], where it was
clearly stated that the thermal conductivity coefficients of materials depend on moisture
content of the material used.
The durability of roofing sheet coverings has a considerable impact on the durability
of buildings, since most processes that destroy the substance of buildings occur in the
presence of water and moisture. For this reason, it is vital to ensure the longest possible
period of failure-free operation of roofing materials. Monitoring of roof coverings and
proper diagnostics of their damage is a key issue from the point of view of all users of
buildings [
34
]. However, to meet this requirement, it is necessary to know the processes
that significantly affect the proper selection of roofing solutions for specific service loads.
Although the possible destruction of the roof covering at the time of its creation is not a
construction disaster, its destructive character lasting for a long time may consequently
lead to a failure or a construction disaster [35].
Summing up, as part of their further work, the authors foresee the need to examine
the mechanism of stress formation in roofing sheet layers, arising from variable positive
and negative thermal loads acting on roof coverings, with the simultaneous need for
cooperation in these conditions between two materials with different coefficients of thermal
expansion. The following research tools are planned to be used to clarify this problem:
−
Tests on the level of stresses that occur in the roofing-substrate system due to
thermal loads;
−
Numerical simulation of the system as above with different types of roofing sheets
and real physico-chemical parameters of materials (experimentally verified).

Energies 2023,16, 3647 12 of 14
Author Contributions:
Conceptualization, B.F., J.S. (Jan Sieczkowski) and J.S. (Jarosław Szulc);
methodology, B.F.; validation, B.K. and J.S. (Jarosław Szulc); formal analysis, A.S.-G.; resources, B.F.
and A.S.-G.; writing—original draft preparation, B.F., J.S. (Jan Sieczkowski) and J.S. (Jarosław Szulc);
visualization, B.F. and B.K.; supervision, B.K. and A.S.-G. All authors have read and agreed to the
published version of the manuscript.
Funding:
The publication was co-financed within the framework of Ministry of Science and Higher Edu-
cation programme as “Regional Initiative Excellence” in years 2019–2023, Project No. 005/RID/2018/19.
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: Not applicable.
Conflicts of Interest: The authors declare no conflict of interest.
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