Elastomeric Roof Coatings
Film Thickness in Roof Coatings
Proper film thickness for elastomeric roof coatings has been a topic of much discussion and study in the roofing industry.
In general, acrylic elastomeric roof coatings should be applied at a minimum of 20 dry mils, (which equals 0.020"), to
achieve the reported flexibility and longevity. Here, we will examine how our building science experts arrived at this
number and why proper film thickness is important in roof coatings.
1. Mechanical Properties
Elongation at
Break
Tensile Strength
(psi)
The graph below demonstrates the relationships among tensile strength, elongation and installed film thickness. The
trend clearly shows that elongation will increase and tensile strength will decrease as the film thickness is increased.
This flexibility is important because roofs tend to expand and contract with building movement, changes in temperature
and thermal shocks caused by severe weather. In these situations, it is important for the coating to move with the roof
and not provide any routes for the sun to attack the
surface underneath. Also, if a crack were to form
ARM-91-1 Mechanical Properties
in the substrate, the coating would be expected to
300
220%
bridge the crack and continue to provide a seamless
280
200%
surface. While a coating at 20 dry mils will stretch
260
180%
by almost 200% in this case, it is important to
240
160%
note that at 5 dry mils, the same coating will
220
140%
only have about 140% elongation. Keep in mind
200
120%
that ASTM D6083 requires conducting all
180
100%
mechanical testing with films that have a dry
0
10
20
30
40
film thickness of 20 +/- 2 mils. This provides a
Film Thickness (mils)
level playing field for the evaluation of different
Elongation at Break
coatings, but is not always consistent with what
Tensile Strength
will be installed on a roof.
2. Proper Hiding
Another key reason for proper film thickness is to effectively protect the roof substrate from harmful solar ultraviolet (UV)
rays. These rays, which make up only about 10% of the energy from the sun, cause most of the degradation in the roofing
membranes coatings are designed to protect. The main opacifier in an elastomeric roof coating is titanium dioxide (TiO2),
which works by efficiently scattering the sun’s energy and blocking it from reaching the substrate. However, this can only
be accomplished by having enough UV blockers present to prevent the UV energy from reaching the substrate. A quick
test of this theory can be run by making drawdowns of various thicknesses over black surfaces. You will see that at lower
film thicknesses, the black surface will show through the coating and make the white coating appear gray. At higher film
thicknesses, this effect will disappear as the coating completely hides the surface below it.
TECHNICAL BULLETIN
TECHNICAL PAPER: CASE STUDY I 1
2. Proper Hiding
Another key reason for proper film thickness is to protect the roof substrate properly
3.
Weathering
from harmful solar Effects
UV rays. These rays which make up only about 10% of the energy from the
Asun
factcause
of lifemost
in the of
roofing
business is thatinallthe
roofs
will failmembranes
over time duethat
to weather
effects
and exposure
the degradation
roofing
coatings
are designed
to to the
sun’s
UV
rays.
In
this
respect,
acrylic
elastomeric
roof
coatings
are
no
different
than
EPDM,
asphalt
or
thermoplastic
protect. The main opacifier in an elastomeric roof coating is titanium dioxide (TiO2) and it
sheets
– over time,
theby
elements
will take
their toll the
by breaking
down the
chemical
components.
elastomeric
accomplishes
this
efficiently
scattering
sun’s energy
and
blocking
it from In
reaching
theroof
coatings
specifically,
the UV
from the
sun will slowly break
down theenough
acrylic resin
time and
microscopic
substrate.
However,
thisenergy
can only
be accomplished
by having
UV over
blockers
present
to
layers
of
the
coating
will
slowly
wash
away.
prevent the ultraviolet energy from reaching the substrate. A quick test of this theory can be run
Coatings
applied
at greater thicknesses
proven to over
perform
bettersurfaces.
over the long
term.
Thewill
photo
below
shows
by making
drawdowns
of varioushave
thicknesses
black
What
you
see
is that
at a
“ladder”
of film
thicknesses the
at 5,black
10, 15will
and 30
mils through
for a standard
ARM-91-1and
coating
polyurethane
foam.
lower film
thicknesses
show
the coating
add over
a level
of gray to
the
As
you can
see, theAt
coating
applied
5 dry mils didthis
not adequately
the foam
fromcoating
the sun, is
andcompletely
consequently
white
coating.
higher
film at
thicknesses,
effect willprotect
disappear
as the
the
foamthe
broke
down into
a powder.
hiding
surface
below
it. This process had begun in the 10 mil and 15 mil samples, as is evidenced by
the holes forming in the samples. On the other hand, after 12 years, the 30 dry mil sample on the far right was still
unmarred
and providing
protection to the foam below. These examples illustrate why a thicker membrane or coating is
3. Weathering
Effects
expected to withstand the elements for a longer period of time, and why thickness is often the main driver in warranty
A fact of life in the roofing business is that all roofs will fail over time due to exposure to
length for many different types of roofing systems.
the sun’s energy and the effects of weather. Acrylic elastomeric roof coatings are no different
than EPDM, asphalt, or
thermoplastic sheets,
5 mils
10 mils
15 mils
30 mils
over time the elements
will take their toll by
breaking down the
chemical components.
Specifically in
elastomeric roof
coatings, the ultraviolet
energy from the sun
will slowly break down
12 Year Polyurethane Foam Exposure
the acrylic resin over
Panels
on horizontal
exposure
deck (Spring House, PA)
12
Yearmounted
Polyurethane
Foam
Exposure
time and microscopic
Spring House Technical Center – Spring House, PA
layers of the coating
will slowly wash away. The obvious effect over the long term is that the more coating that is
installed, the longer the coating will perform. The photo above shows a ladder of film
thicknesses at 5, 10, 15, and 30 mils for a standard ARM-91-1 coating over polyurethane foam.
As you can see, the coating that was applied at 5 dry mils did not adequately protect the foam
from the sun, and as a consequence the foam broke down into a powder. This process has begun
in the 10 mil sample, as is evidenced by the holes that are formingWant
in thetosample.
On the other
Learn More?
Start Here.
hand, after 12 years, the 30 dry mil sample on the right is still unmarred and providing protection
to the foam below. As this example shows, a thicker membrane or DowConstructionChemicals.com
coating is expected to
withstand the elements for a longer period of time, and that is why thickness is often the main
driver in warranty length for many different types of roofing systems. The panels in the photo
above were exposed at our Spring House, PA exposure station and were mounted on a horizontal
exposure deck.
RHOPLEX is a registered trademarks of the Rohm and Haas Company or of its subsidiaries or affiliates and are intended to designate goods
marketed in North and South America; the same goods may be marketed in other countries, generally under other company trademark designations.
The information contained herein is offered in good faith, but without guarantee or warranty, as analytical conditions and methods of use of the
materials described herein may vary. The data and assessments contained herein are based on information we believe to be reliable and accurate,
however, Rohm and Haas does not intend for you to use, and you therefore should not construe, the contents of this report as business, technical,
legal, or any other form of advice. Any use of this information is strictly at the user’s risk.
Suggestions for uses of our products or the inclusion of descriptive material from patents in this publication should not be understood as
recommending the use of our products in violation of any patent or as permission or license to use any patents of the Rohm and Haas Company.
®
™Trademark of The Dow Chemical Company (“Dow”) or an affiliated company of Dow
Form No. 832-00212-0911 BBI