Most flat roofs don’t come with any kind of hail guarantee, but there are
some that I’ve witnessed do really well in a hail storm, and others that don’t.
So rather than spout a bunch of gibberish about different hail ratings on
different materials, and give a bunch of suppositions, I’ll tell you about a
flat roof I’ve seen and analyzed after a hail storm.
We were asked to perform a hail damage assessment on a fairly large building
located in Indiana. This particular building had three different roof types on
it (also referred to as roof “systems”), and was experiencing some pretty severe
leaks. One of roof systems was a PVC roof and was only three years old,
installed in 1999. Another roof system was an EPDM rubber roof and was
approximately 12-14 years old. The third roof system was a smooth-surfaced
built-up roof and was about 10 years old at the time of the hail storm.
The EPDM rubber roof was a 60 mil (.060” thick), mechanically-fastened roof
system. Mechanically-fastened means that screws and plated (big washers) were
used to secure it to the roof deck underneath. The roof was installed over a
foam type insulation board (polyisocyanurate insulation for those of you who
care). The roof was dirty, as roofs tend to get, and there was evidence of hail
impacts all over it ranging in size from ½” to about 1-1/2” in size; plenty big
enough to do some serious damage. No roof leaks occurred in this system.
Just over one of the firewalls lay the 3 year old white PVC roof. It suffered
through the same storm the EPDM rubber did but the damage was much worse. At
just about every impact location, the PVC single-ply material was “shattered”.
Upon very close observation, it appeared similar to a windshield impact, and
some of these impacts were hard enough to penetrate the PVC sheet and allow
water through the roof system and down into the building below. There were over
20 roof leaks on this system.
But here’s something interesting. The damage was not uniform throughout. Some
areas of the PVC suffered little or no damage at all. These areas are where ponding
occurred, and a thin film of dirt and algae covered the PVC at all times, so the
sun never directly hit the material here. From this, I assumed that UV rays can
damage this particular brand of PVC when directly exposed to the sun. I’m just
glad the storm occurred when it did because had the roof been 10 years old, the
damage might have been much worse and the leak situation even worse.
Please note that this event would not discourage this author from using PVC
in hail prone regions. Many PVC roofs have a proven performance record during
hail storms. I just would not use this particular brand and no, I will not
disclose who the manufacturer is.
The third roof system on this building was an SBS modified cap sheet roof.
SBS modified asphalt is asphalt that has “rubberizers” mixed in to give it more
of an elastomeric quality. A small piece of the roof was cut out and analyzed.
We determined that damage did occur to the fiber glass felts underneath the cap
sheet where the larger hail stones had impacted, but the SBS cap sheet did a
good job of absorbing a lot of the energy and kept any damage from causing
leaks. The owner, who was self-insured up to $100,000, opted not to reroof this
Now I’ll talk about one of my favorite types of flat roof systems, a
gravel-surfaced built-up roof. Just about any roofer who has good experience
with both built-up and single-ply roof systems will concur that this is one of,
if not the best roof system available for low slope or flat roof areas. I have
seen numerous gravel-surfaced built-up roof systems go through hail storms, and
I have seen them perform again and again and again. Not all of them, but the
gravel-surfaced built-up roofs that are done properly will outlast just about
anything out there. A key to a hail-resistant built-up roof is the amount of
gravel on it. There HAS to be a good amount of extra gravel installed and here’s
why. Ever watch those shows where some guy lies down and puts a concrete block
on his chest, and then someone shatters it with a sledge hammer and the guy then
gets up relatively unscathed? The reason he’s not seriously hurt is because the
CMU block absorbs the energy of the hammer when it stops. That energy is
basically is absorbed by the CMU block which has to disperse it, so it shatters,
leaving the guy underneath relatively unharmed (see Newton’s First Law). The
same thing occurs when a hail stone hits a gravel-surface built-up roof. The
energy from the hail stone is dispersed in the aggregate surfacing. For this
reason, a proper gravel-surfaced built-up roof can achieve a Class IV
hail-resistance rating, the highest rating available.
Other Flat Roof Materials
Metal Roofs – There are metal roofs on the market that work on slopes as low
as ½” in 12”. I don’t have any direct knowledge of how well these metal roofs
will perform in a hail storm except to say that I have never been asked to
evaluate a metal roof of this slope after a hail storm, and I don’t see many
being replaced. A severe enough hail storm will damage a metal roof, no
question. But whether the damage will necessitate replacement or not would have
to be determined on a roof-by-roof basis. All in all, I think they would perform
APP Modified Roofs – These are the torch on type roofs and like SBS roofs,
they also have plasticizers mixed in with the asphalt to give the material an
elastomeric property. These roofs, done properly, hold up well in hail storms. I
have evaluated several of them and while impact marks are evident and damage
does occur, the damage has been minimal. I’ve seen several APP roofs replaced
that I don’t think should have been replaced. Either it was from ignorance on
the part of the insurance company, or the building owner raised such a ruckus
that the insurance company caved and bought a new roof.
Foam Roofs – Believe it or not, foam roofs have been tested and received a
Class IV hail resistance rating from ASTM (the American Society of Testing &
Materials). What it takes is a higher density foam (typically 2-1/2 to 3 pounds
per cubic foot) along with a good, durable coating system such as a 4-1/2 gallon
double-granule acrylic coating system. There are numerous other coating systems
available, too, such as silicones, urethanes, polyurea, etc. Climatic conditions
and use will determine the right coating system.
TPO – I don’t have any direct knowledge of a TPO single-ply roof’s ability to
withstand hail, but I have reason to believe it would perform fairly well due to
its makeup and characteristics. I would advocate the use of a ballasted or
mechanically-fastened type TPO roof in lieu of a fully-adhered if a person’s
sole concern is hail resistance.
Ballasted Single-Ply Roofs – In a nutshell, a ballasted single-ply roof is a
rubber, TPO or PVC roof that has been loose-laid over the substrate and is held
in place using ballast such as
river rock or pavers. A ballasted roof is well-protected from hail by the
ballast holding it in place. So if the proper ballast is used and it is properly
installed, a ballasted roof should be well protected from all but the most
severe of hail storms (and sometimes even those)