Hail damage to thermoset (EPDM) and thermoplastic (TPO, PVC) roofing membranes appears differently and requires different inspection methods to identify. Thermoset membranes are rubber-based and show small, difficult to see tears and indentations in the underlying insulation from hail impacts. Thermoplastic membranes are plastic-based and show more visible concentric circular or star-shaped fractures on the surface from hail impacts. Proper identification of hail damage requires hands-on inspection and knowledge of each membrane type's characteristics and vulnerabilities.

1. PLRB HAIL DAMAGE TO THERMOSET AND THERMOPLASTIC ROOFING MEMBRANES
Author: Kevin Maxwell, PE, SFE, The Vertex Companies, Inc.
Edition Date: May 24, 2017
Subject: ~ TYCK; Test Your Claims Knowledge; thermoset, thermoplastic, hail damage, roof hail damage, EPDM, TPO,
PVC, rubber membrane, plastic membrane, single ply membrane, ballast, mechanically attached, fully adhered
membrane, rigid insulation, impact energy, star shaped fractures, concentric circular fractures, reinforced and non-
reinforced membranes
"Hail Damage to Thermoset and Thermoplastic Roofing Membranes"
Part 1 of 2: Recognizing Hail Damage on Single-Ply Roofing Membranes
Introduction:
When discussing single-ply roofing membranes, which
are the fastest growing new and re-roofing products in
the industry, you have probably heard the terms
"thermoset" and/or "thermoplastic" membranes. These
terms categorize much smaller sub-groups of specific
membrane types by the materials and manufacturing
process. This article will allow you to become familiar
with these terms and understand the different
characteristics of hailstone impact damage to each
category of product.
What is the difference between thermoset and
thermoplastic membranes?
When discussing thermoplastic or thermoset
membranes, they are typically referred to as the specific
type of product, which may be more familiar to your
vocabulary. The most popular thermoplastic types of
membranes are TPO and PVC which stand for
"thermoplastic polyolefin" and "polyvinyl chloride,"
respectively. TPO hardens when cooled and is more
rigid. The most common type of thermoset membrane
is EPDM which stands for "ethylene propylene diene
monomer" or is also referred to as rubber roofing.
Hypalon, another product you may be familiar with, is
manufactured as a thermoplastic, but as it cures over
time, it becomes a thermoset. During this discussion,
we will refer to thermoplastic membranes as TPO & PVC
and thermoset membranes as EPDM for ease of
discussion.
The main difference between thermoplastic and
thermoset is that thermoplastic membranes are,
essentially, plastic (compounded from plastic polymers,
and contain plasticizers) while thermoset membranes
are, essentially, rubber (compounded from rubber
polymers). Hence the reason that thermoset
membranes are sometimes referred to as "rubber
roofing." EPDM membranes are more likely to allow
impact damage to the underlying installation.
Additionally, thermoplastic membranes can be heated
and melted down for recycling or extrusion. As
thermoplastic materials cool down, they harden.
Thermoset membranes are hardened into a specific
shape using heat and pressure. This is an irreversible
chemical process which cannot be reprocessed or
melted for reusability.
To summarize, both thermoset and thermoplastic
membranes are single-ply roofing membranes.
Thermoset membranes are made of rubber compounds
and are also known as EPDM. Thermoplastic
membranes are made of plastic and are also known as
TPO or PVC (which are two different types of
thermoplastic membranes).

2. What does hail damage to a thermoplastic (TPO/PVC)
roof look like?
Due to the fact that single-ply membranes are
becoming such a large market share of new and re-
roofing products, a wide range of testing has been
conducted, including impact testing which would
correlate with hailstone impacts. These testing methods
will be discussed in more detail in a follow-up article. As
can be expected, the damage characteristics for each
type of membrane are distinctly different. Hail damage
to TPO & PVC membranes is typically evidenced by
concentric circular fractures or star-shaped fractures
within the top ply of the membrane.
Concentric fracture to thermoplastic membrane
from hailstone impact. Source: Frontier Roofing
Star shaped fracture to thermoplastic membrane
from hailstone impact. Source: Northcross Roofing
Typically, on more recent hail damage, you would
expect to find spatter marks or cleaned areas of the
membrane which correspond with the fractures. Due to
the fact that the TPO & PVC membranes are plastic-
based and hardened when cooled, the fractures within
the membrane are more visible and evident and do not
take a hands-and-knees inspection to identify, although
micro-fractures can occur. Over time, the plasticizers
evaporate and reduce the width of the membranes.
Studies have shown that a TPO membrane can lose 1
mil a year of thickness while a PVC membrane can lose
0.5 mil of thickness per year. If you think about a 45 mil
TPO membrane losing 20 mils over a 20 year period, in
some cases the reinforcement may become exposed
and therefore more acceptable to smaller hailstone
impacts. This phenomenon helps explain how TPO &
PVC membranes are fractured and damaged by
hailstones of differing size and impact energy. It should
be noted that the testing discussed in the next section
does not take into account the loss of plasticizers over
the life cycle of a membrane. Also, due to the fact that
the TPO & PVC membranes are more rigid and impact
resistant, the kinetic impact energy is dissipated
horizontally across the membrane and the underlying
insulation typically does not experience significant
indentations or facer sheet fractures.
What does hail damage to a thermoset (EPDM) roof
look like?
Hail damage to EPDM membranes is typically evidenced
by indentations within the underlying insulation boards
and small discrete tears within the membrane.
Typically, on more recent hail damage, you would
expect to find spatter marks which correspond with the
indentations and/or tears. Due to the fact that the
EPDM membranes are rubber based and are more
flexible and retract, the tears within the membrane are
difficult to see with the naked eye in most cases.
Tear to EPDM membrane from hail impact. Source: Flat Roof Doctor

3. Indentations to the underlying insulation of an
EPDM roofing system. Source: Grayco Consulting
Cleaning the membrane and then completing a hands-
and-knees inspection is the best method to conduct a
hail damage investigation on an EPDM membrane. It
may take a microscopic inspection of the membrane to
fully determine if tears occurred. Focus on finding tears
along the edges of the underlying fasteners, otherwise
known as anvil strikes. Also, due to the fact that the
EPDM membranes are more flexible, the kinetic impact
energy of the hailstone is forced downward into the
underlying insulation, often causing indentations and
tears in the facer sheet of the insulation. This can be
particularly concerning when the EPDM membrane is
fully adhered and the dents cause delamination of the
membrane from the facer sheet of the insulation;
however, this is not as concerning on a mechanically
attached EPDM membrane.
Quiz Questions
1. Which membrane type hardens when cooled and is
more rigid?
a. PVC
b. EPDM
c. Modified Bitumen
d. TPO
2. Which membrane type is more likely to allow
impact damage to the underlying installation?
a. PVC
b. BUR
c. EPDM
d. Modified Bitumen
3. What are some clues indicating hail damage to
TPO/PVC membranes? (Choose all that apply)
a. Concentric circular fractures in the top ply of
the membrane
b. Tears along edges of underlying fasteners
c. Indentations in the facer sheet of the insulation
d. Star-shaped fractures in the top ply of the
membrane
4. What are some clues indicating hail damage to
EPDM membranes? (Choose all that apply)
a. Concentric circular fractures in the top ply of
the membrane
b. Tears along edges of underlying fasteners
c. Indentations in the facer sheet of the insulation
d. Star-shaped fractures in the top ply of the
membrane
Conclusion
In this article, you learned about the difference
between thermoplastic and thermoset roofing, and how
to identify them based on the hail damage you observe.
Join us next time for Part 2 of "Hail Damage: Thermoset
versus Thermoplastic Membranes." In that article, Kevin
Maxwell will discuss hail impact testing methods and
the repair, recovery, and replacement of these roofing
membranes.
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