Most of the single-story bump-out additions we build have
pitched roofs, though under the right circumstances it’s
not hard to add a flat rooftop deck to the design. That was the
case with the job shown here — a 6-foot-by-20-foot
kitchen expansion with a master bedroom roof terrace above. In
the past we’d used asphalt-based or single-ply rubber
roofing membranes for flat-roof jobs, then covered the roofing
with deck boards. But for this project, the architect specified
Duradek, a PVC thermoplastic waterproof deck membrane
What Is Duradek?
Originally developed in the 1960s as a slip-resistant flooring
for small boats, Duradek was later introduced as a roofing
material, first in Canada and then in the United States. In the
manufacturing process, PVC is pressed through rollers into a
thin film that is then laminated to a woven polyester fabric
for stability and multidirectional strength. Because the
membrane serves as both a walking surface and a roof covering,
it’s textured for slip resistance, and the vinyl
formulation includes mildew and ultraviolet inhibitors and heat
stabilizers for better long-term performance. Duradek comes
with a 10-year manufacturer’s warranty.
The 60-mil-thick vinyl traffic membrane — called Duradek
Ultra — that was installed on this project is Class C
fire-rated (Class A when installed over 5/16-inch HardieBacker
or concrete) and approved for use over conditioned living
space. (The membrane is also sold in 40-and 45-mil thicknesses
for decking applications with no living space below.) Test
results show that Duradek exceeds IBC requirements for
resistance to wind uplift, even in special wind regions.
Our membrane was delivered in a 72-inch-wide roll (slightly
narrower than our roof deck); 54-inch rolls are also available.
According to Duradek, the product comes in two textures and 18
colors and patterns. While colors can vary slightly, production
orders are tagged so that all the rolls for larger projects are
shipped from the same production run.
Like other sheet goods, Duradek is subject to wear and tear.
But punctures, cuts, and burn marks are relatively easy to
repair with glued and welded patches, and won’t
compromise the membrane’s watertightness.
For cost information and a list of authorized installers, I
went to the company’s Web site, where I found the names
of three local contractors. Two returned my calls and provided
bids for the project. For 130 square feet of deck area with 22
linear feet of drip-edge flashing, 25 feet of wall flashing,
and four rail posts, the bids were $1,350 and $1,458. Although
we’d never worked with Duradek before, the installation
— which was done by factory-trained installers —
was straightforward and quick. The installed cost of about $12
per square foot was less than that of a conventional roof with
protective deck boards, and we were able to start working on
the railing immediately after the membrane was glued in
Flat Roof Framing
Although the PVC membrane itself can be installed only by an
authorized Duradek dealer, we needed to construct the deck
properly to receive it. For proper drainage, the manufacturer
requires a deck slope of 2 percent, or 1/4 inch every 12 inches
(see illustration). For our 6-foot-wide deck, we pitched the
roof 11/2 inches by ripping tapered wedges from framing lumber
and gluing and nailing the wedges to the ceiling joists (Figure
Duradek Ultra is intended for use as both a roof covering
and a traffic surface, making it a good choice for residential
decks built above living spaces.
Figure 1. Tapered strips of framing lumber
glued and screwed to the deck joists supply the necessary 2
percent slope for drainage (top). Four-by-four PT railing posts
are bolted to the framing (left) before the plywood deck
sheathing is installed (right).
After bolting the 4x4 rail posts in place, we installed
3/4-inch T&G plywood, per the manufacturer’s
recommendations, gluing the plywood to the joists and leaving
the proper gap at the ends and edges of panels. Because of
compatibility problems between the PVC membrane and the
compounds used with treated plywood and polymer-enhanced OSB
underlayments like AdvanTech, those sheet goods aren’t
We next installed the fascia boards, then covered the deck with
15-pound felt paper and installed temporary flashing at the
deck-wall intersection to keep the plywood dry until the
Duradek sub arrived approximately three weeks later. The
manufacturer recommends that the decking be at no more than 12
percent to 14 percent moisture content at the time of
installation. In hindsight, getting estimates earlier in the
process would have tightened the schedule.
On the day of installation, we removed the felt and flashing
and swept and vacuumed the deck. When the installers arrived,
we made ourselves available, but stayed out of their way.
Prepping the Deck
The two installers started by nailing a galvanized metal drip
edge around the outside perimeter. Duradek offers several
edge-finishing options; the one used here is designed so that
the membrane laps over the metal and is secured with continuous
PVC clips that slip over the metal’s flared edge.
Next, the installers filled the plywood joints, voids, knots,
and low areas with a patching compound they mixed up on site.
Duradek — like vinyl — will telegraph even small
imperfections in the deck sheathing, so they inspected
carefully as they smoothed the compound with their sanders
(Figure 2). After filling the joint where the terrace plywood
deck meets the second floor walls with a flexible caulk, they
carefully swept and vacuumed again.
Figure 2. After installing the metal
perimeter drip edge, the Duradek installers patch joints, nail
heads, and voids in the plywood deck with a silica-based floor
filler (top left), then sand the deck smooth (top right).
Sealant fills the joint between the deck and wall sheathing,
preventing the membrane from forming a water-collecting crease
Cutting the Membrane
To cut the membrane, the installers rolled it out on the ground
and measured its length so that it would fold up the walls
surrounding the deck by at least 6 inches (Figure 3).
Figure 3. After cutting the first section
of membrane to length (top), the installation crew dry-fits it
to the deck. Once it’s in position, they roll back half
the membrane lengthwise (left) and apply adhesive to the
exposed part of the deck before laying the membrane back in
Planning for seams. Because the deck was slightly
wider than the membrane, there would have to be one seam; it
fell under the railing where it was barely visible. The
9-inch-wide strip of membrane that finishes the deck is
overlapped by the first piece and folds down over the drip
Duradek seams are heat-welded rather than glued together, and
require a minimum 3/4-inch overlap. Making these seams properly
is the most critical part of the installation: The weld will
fail if any of the adhesive used to fasten the membrane to the
deck contaminates the seam.
Installing the Membrane
After dry-fitting the full-width piece, the installers pulled
the material back away from the wall and folded it in half
lengthwise onto itself. They then spread the water-based
Duradek adhesive on the exposed plywood and folded the first
half of the membrane back into place, taking care to eliminate
air pockets as they smoothed out the material and pressed it
into the adhesive.
Once the first half of the Duradek was glued down, the
installers carefully cut openings for the four railing posts
(Figure 4), marked the position of the outside edge of the
unglued half of the membrane, and then folded this half back
over the glued section. Next, they glued down the narrow
outside strip of membrane, then glued down the second half of
the large piece. They used heat guns to heat the overlap,
blowing hot air into the seam and then pressing the two halves
together with a roller (Figure 5). Every few feet, they checked
the seam strength.
Figure 4. A crew member carefully marks and
cuts openings for the rail posts (left). Then, after gluing
down the strip of membrane along the edge of the deck, he glues
down the main section of membrane, overlapping the perimeter
strip 1 inch (right).
Figure 5. Watertight joints are made by
heating overlapping edges of the membrane with hot air from a
heat gun and then applying pressure to the seam with a hand
roller (left). The collars around the posts are also
heat-welded to the deck membrane (right).
Around the base of each post, the installers wrapped a
6-inch-wide PVC collar; L-shaped flaps cut at the bottom of the
collar overlap the deck membrane and were heat-welded in place.
To create a watertight joint, small corner patches were slipped
behind each slit in the membrane before it was welded together.
When the railing was assembled, boxed newels that slide down
over each post would cover the PVC collars, making the joint
At the parapet walls, the outside corners were treated in a
similar way. At inside corners, the installers used a folded
“pig’s ear” detail to create a watertight
At the new doorway into the master bedroom, the membrane wraps
up and over the rough sill. To make this critical area
watertight, all of the joints were welded. After the membrane
was in place, we installed the new door, which sits on top of
this waterproof pan.
In other areas where the membrane folds up onto a wall, the
installers used Duradek’s solvent-based adhesive, which
is applied like contact cement to both surfaces. Later, we
patched in the housewrap and shingles, lapping them over the
Around the perimeter of the deck, the membrane folds down over
the drip edge. The installers heat-welded the corner seams and
fused the overlapping membrane to the metal. After trimming the
edge of the membrane flush with the bottom of the metal, they
finished the bottom edge with continuous PVC clips.
As soon as the membrane was installed, we were able to get to
work on the railing. This is when we really began to appreciate
Duradek: Working on top of an asphalt-based membrane in the
summer heat typically means scuffing and uncomfortable
conditions, even if we plan our work around the hottest parts
of the day. But the light-colored Duradek surface doesn’t
radiate heat to nearly the same extent as asphalt roofing, and
we were able to finish the project comfortably and without
damaging the roof.
We first wrapped the 4x4 posts with boxed newels, treating the
ends with a penetrating sealer and holding the newels up off
the roofing membrane (Figure 6). At the inside corners, we
lag-bolted short 4x4 PT posts to the wall, then wrapped these
posts with three-sided newels.
Figure 6. Boxed newels slide down over each
post, covering the collar at the bottom.
We used stock cedar handrail and balusters and ripped the rail
inserts — which hide the upper baluster fasteners —
from clear 2x4 cedar. We gave the lower balustrade rail a
water-shedding beveled profile and put a corresponding V-shaped
notch cut into the bottom of each baluster (Figure 7).
Figure 7. A simple painted cedar railing
finishes the deck (top). Balusters are attached with screws
through a rail insert at the top (middle). A V-shaped notch at
the base of each baluster (bottom) matches the beveled top of
the shoe rail.
At the sides of the deck, the railing follows the profile of
the sloped roof. We gave the rail a paint finish and topped the
posts with copper caps.Rob Corbo is a building contractor in