I can’t count the number of times a homeowner has
called me to fix a leaking flat or low-slope roof. If
you’re a remodeler faced with this situation and you go
to the lumberyard to buy materials, they’ll probably sell
you 17-inch selvage roll roofing and a few cans of cold patch
cement and send you on your way. But if you install a half-lap
roof in a climate that gets much weather, chances are in a few
years the homeowner will be calling you again.
The world of commercial buildings is full of flat roofs, and
obviously commercial roofers know how to make a flat roof last
for longer than a few years, in any climate. But many of the
roofs used on commercial buildings — torch-applied
modified bitumen or ballasted systems, where the roof membrane
is held in place with stone — are just not practical on
residential jobs. Most roofs aren’t built for the extra
weight of ballast, and it’s insanity to fire up a torch
on the roof of a wood-frame building.
As a roofing contractor with a reputation to protect, I
wasn’t about to resort to roll roofing for residential
flat roof repairs. So out of necessity, I began using EPDM for
these jobs: It’s light, durable, and easy to install. And
if installed correctly, the product is virtually
What Is EPDM?
EPDM is a
black, single-ply rubber roofing membrane. The letters stand
for ethylene propylene diene monomer, which describes its
chemical makeup. EPDM stands apart from — and above
— other single-ply rubber membranes. You may see ads for
"rubberized one-ply" roofing or other such names. These are
typically types of modified bitumen, which, though less
expensive, are not as good as EPDM. With EPDM, you don’t
have to worry about different formulations or compatibility
issues: It’s all the same stuff, with the same chemical
makeup, regardless of where you get it.
EPDM was developed as a pond liner. Then, during the early
1970s, someone had the bright idea that if it worked under
water, why not use it as a roof? When I started using it on
homes, it was still a commercial-only product. In recent years,
though, because of its amazing qualities, EPDM is finding its
way into more and more residential projects.
primarily use EPDM made by Roofing Products International (RPI,
5120 Beck Dr., Elkhart, IN 46516; 800/628-2957). It’s
available in two thicknesses, .045 inch and .060 inch, and
comes in a variety of widths from 71/2 to 50 feet and lengths
from 25 to 200 feet. I usually buy 100-foot-long rolls of
20-foot-wide material. For residential work, I always use the
.060-inch EPDM, because it lies flatter with less wrinkling and
offers better durability.
As a certified installer, I can offer a 10-year residential
warranty. But in over 20 years of installing EPDM roofs,
I’ve never had a product failure. If an EPDM roof fails,
I’m willing to bet that eleven times out of ten it has to
do with installation error.
Durability. EPDM has
excellent UV resistance, so it requires no further covering for
protection from sunlight. It deteriorates at a rate of about 1
mil per year, so the 60-mil product ought to perform well for
at least five decades.
EPDM will stretch without tearing, and holds up well to the
rigors of installation. It also performs well in extreme
temperatures: The product I use will stay flexible down to
–49°F and resist heat up to 300°F without
cracking or deforming.
EPDM should not be exposed to grease, solvents, oils, and
petroleum products; fortunately, these are not usually a factor
on residential roofs. (If you use EPDM on a rooftop deck, you
should warn the homeowner about grease spills from a grill.)
EPDM also has a good fire rating, and is available in a
Where to Use EPDM
hasn’t replaced asphalt shingles on all my jobs, but I
use it for flat or low-slope roofs, and primarily on roofs that
can’t be seen from the ground. I tell customers that EPDM
makes the ugliest roof they’ll ever see — but that
they won’t see it. Typically, a flat roof on a one-story
addition can be seen from a second-floor window, but
On rooftop decks, EPDM can be covered with a wood deck on
pressure-treated sleepers, with outdoor carpet or Astroturf,
and even with concrete pavers. EPDM can also be painted with
Hypalon paint, but the paint will have to be recoated every
three or four years.
Most of my
residential EPDM jobs are replacements for roofs that
don’t drain. Often the framing has sagged under the
weight of ponding water, so the deck has to be given a slight
slope before reroofing with EPDM. In cases where the old
half-lap or built-up roof has leaked, I check the framing to
make sure there’s no rot, and replace or reinforce
members as necessary.
The first phase of a replacement job is to completely strip
the old roof and replace the sheathing where necessary. Next,
we lay down a covering of polyisocyanurate insulation board
(see Figure 1).
1. EPDM is installed over a special felt-covered
insulation board. Screws and large metal fastener
plates secure the insulation board to the roof
This board, called "iso board" by roofers, is different from
ordinary rigid foam board in that it has a special
glass-fiber-reinforced felt skin that is compatible with the
EPDM adhesive. You’ll have to buy this board through a
roofing supply house. On a residential job, I typically use
1-inch board which has an R-value of 6.
The foam board is attached to the roof deck with screws and
large metal plates, which are also available through roofing
suppliers. You need one fastener per every 2 square feet. The
fastener plates sit slightly proud of the surface, so when the
EPDM is installed, the plates are easy to see through the
rubber. Each manufacturer has its own dimple pattern on the
fastener plate. That way, for warranty work, the
manufacturer’s inspector can identify and count the
plates to be sure that enough were used.
In most cases, with a 20-foot-wide roll, we can cover a
residential roof area without creating seams. First, we spread
the rough-sized sheet out on the deck, smoothing out the
wrinkles and getting it into final position.
When we’re sure the sheet is where we want it, we roll
it back onto itself halfway (see photo).
2. After the adhesive has tacked up, workers
roll the EPDM into place (top). Wrinkles are smoothed
out with a stiff bristle broom (bottom).
Brushing the surface with a stiff deck broom ensures good
adhesion. The process is repeated for the other half of the
they’re not usually needed on residential jobs, seams are
not difficult with EPDM, but you must make them carefully. I
used to use splicing adhesive until seam tape became available;
now I use only seam tape: It makes a stronger seam, and the
seam is completely waterproof — an important feature on a
flat roof where snow sits or where water might pond.
With splicing adhesive, you allow for a 6-inch overlap. The
bottom sheet (the downhill sheet in a sloped roof) is first
fully adhered to the deck. When gluing down the top sheet, be
careful not to get any adhesive on the overlap area. To make
the seam, carefully clean the overlapping surfaces with white
gas or a cleaning solvent recommended by the EPDM manufacturer.
Then brush on a special splicing adhesive to both mating
surfaces and allow it to tack up. When the adhesive is dry to
the touch, carefully roll the top piece into place, making sure
not to wrinkle or stretch the material. (If the rubber is
stretched, this puts a built-in stress on the seam.) Finally,
roll the seam with a hand roller to get 100% adhesion, then
apply a bead of a special lap caulk to the edge of the seam to
keep water from getting between the layers.
Seam tape is a very sticky two-sided tape that comes in a
6-inch roll with a release paper on one side. When using seam
tape, the manufacturer recommends both cleaning and priming the
mating surfaces. Then unroll the seam tape, sticking it to the
bottom sheet as you go. Leave the release paper in place and
hand-roll the seam tape to get good adhesion to the bottom
sheet. Finally, fold the lapping piece over on top of the seam
tape and pull the release paper off as you press the top piece
into place. Make sure you carefully trim the top sheet before
sticking it down: Its edge should be parallel to and 1/8 inch
back from the edge of the seam tape. The edge can then be
finished with lap caulk or just left as is, since the exposed
edge of the seam tape acts as a water stop.