Last year, my remodeling company worked on a classic two-story
brick Colonial Revival home in one of Washington, D.C.’s
historic districts. Part of the job involved replacing an aging
rooftop deck located at the rear of the house over the garage,
which is separated from the house by a 3-foot-wide breezeway.
The deck faces south, so it gets lots of sun; accessed via a
short bridge from the den, it’s a nice adjunct to the
living space on this small lot.
The existing deck, with a surface of 2x6 pressure-treated
lumber, rested upon wood sleepers placed directly on a poorly
installed and leaking rubber membrane roof. The roof framing
was recessed inside a level parapet topping the 8-inch-thick
brick walls, with single 2x10 Doug fir joists set in the wall
at 12 inches on-center. A shallow 1/2-inch slope drained the
roof to the rear, with a scupper and downspout directing runoff
to the garden below.
Though most of the joists were in decent shape, a few required
replacement. Many years’ worth of wet leaves were trapped
and rotting under the deck surface, and once the roof membrane
failed, portions of the joists — as well as areas of the
1x6 sheathing — rotted too.
To begin, we demolished the old deck and stripped the roofing
membrane and corroded parapet flashing, then removed the
affected roof sheathing and rotted joists. The original joists
had been mortared solidly into the wall and showed no signs of
decay there at the bearing points, so we simply widened the
pockets to accommodate a second member sistered on. We used
pressure-treated 2x10 southern pine for the sistered joists,
nailed them thoroughly to the originals, then patched the
pockets with mortar (see Figure 1).
Figure 1. The existing pressure-treated deck (left)
covered a leaking roof membrane and rotted sheathing. To
reinforce the existing roof framing, the author sistered new
joists alongside the old, supporting them in expanded masonry
pockets (right). The old decking was replaced and a new EPDM
roofing membrane installed.
The joists spanned 17 feet, and even though they were now
doubled, our engineer thought that the roof might still be too
bouncy when loaded with people. Since we were also converting
the garage to living space — with drywall and a
decorative beam treatment on the ceiling — we
didn’t want problems with nail pops or cracking. To help
stiffen the roof, we added two 9 1/4-inch LVL beams at
one-third intervals along the roof’s 22-foot length. The
beams are set in new beam pockets, which we made by breaking
out some of the interior face bricks, then patching with
mortar. Once the joist repairs and reinforcement were
completed, we replaced the 1x6 sheathing and called in the
For the new roofing membrane, I specified .060 mil EPDM over
1/2-inch medium-density fiberboard underlayment. That’s
heavier than the standard .045 EPDM, but it seemed like a good
idea to provide some extra wear resistance to offset eventual
maintenance under the new deck. The brick parapet received new,
flat-seamed copper flashing (Figure 2). An
existing iron railing surrounded the rooftop, with its posts
embedded in the masonry. The railing was in good condition, so
we simply cleaned it and protected it with a fresh coat of
rust-inhibiting paint. The roofing crew cut the flashing around
the iron posts, applying Fillgard pourable sealer
to seal the penetrations.
Figure 2. New copper flashing fitted around an iron
deck railing serves as a waterproof cap over the
As with the original surface, we planned for the new decking to
sit a few inches below the top of the parapet. The parapet was
generally level but somewhat irregular from point to point; by
recessing the new deck below the top, we could avoid
highlighting the discrepancies. Instead of shimming or tapering
a conventional deck frame to sit level on the sloped roof, we
opted for an innovative system that our production manager,
Steven Barnard, had recently seen demonstrated at a
JLC Live show.
The Versadjust deck-support system (800/ 333-4324,
bisonus.com) is based on a
height-adjustable pedestal made of high-density polypropylene
(Figure 3). The pedestals are designed to
support a stone, masonry, composite lumber, or wood
“tile” surface. A rotatable eccentric disk built
into their base allows you to dial in slope compensation
— handy for holding plumb on roof slopes up to 1/2/12.
Adapters are also available to adjust for slopes up to 1 inch
per foot. Four pedestal models are available, with heights
ranging from 2 1/4 inches to 9 3/4 inches. Pedestal height can
be precisely adjusted by twisting the threaded shaft over a 4
3/4-inch range. If needed, the tallest unit can be extended in
4-inch increments up to a maximum height of 24 inches using
optional snap-on couplers. Each pedestal is rated for a
1,500-pound working load. (However, pedestals must not be
installed over roofing underlayments with a density of less
than 20 psi.)
Figure 3. A series of adjustable plastic pedestals,
each rated for a 1,500- pound load, hold up the deck. This
support system makes it possible to clean leaves and debris
from under the deck, or make adjustments.
The pedestals are not just for rooftops. They can also be
installed over an existing patio or directly on grade using an
optional base support. The decking does require some kind of
site-specific edge restraint to prevent the surface from
shifting laterally, and nothing but gravity holds it down. If
wind uplift is a concern, you’ll want to consider another
We contacted our local dealer and found that it offered a
package including both the deck pedestals and wood decking
“tiles” made of tropical ipe. After viewing the
pedestals and wood samples, our client gave us an enthusiastic
Because the roof had an end-to-end slope of about 11 inches
overall, we required all four pedestal heights for this job.
The nominal 24-by-24-inch wood tiles sit directly on the
pedestals, with one pedestal installed beneath each four-way
tile junction. To establish an overall level line for the deck,
we shot a few reference marks with a laser. After that, we
simply shifted a bubble level across the tops of the pedestals
as we set them, fine-tuning their height as needed.
The tiles are preassembled using stainless steel screws through
cleats from the underside, presenting a fastener-free surface
and saving considerable on-site labor. Exposed end grain comes
factory-sealed with wax to repel water; field cuts should be
similarly treated. We used Panel-Seal edge sealer
uccoatings.com), a thin
wax emulsion applied with a brush.
The tiles are undersized by 1/8 inch in both directions to
create a 1/4-inch installed gap between them, controlled by
four upright spacer tabs on the pedestal surface
(Figure 4). There’s a 3/16-inch groove
cut diagonally across each corner between the 3/4-inch-thick
surface boards and the cleats on the underside. These grooves
snugly accommodate a plastic disk that’s secured with a
single screw into the center of the pedestal, creating a
four-way hold-down for the tiles. There’s a pie-shaped
cutout in the disk; with the disk rotated to engage four
corners at once, you install three adjoining tiles and insert
the screw. You then drop the fourth tile in, fitting it onto
the disk. For maintenance access beneath the deck surface, the
cutout can be realigned under a tile corner to allow removal.
That’s a handy feature; the decades of rotting leaves
trapped beneath the old deck no doubt contributed to the
failure of the membrane. The feature also will make it easier
to adjust pedestals should any shrinkage or settling occur in
the supporting structure or grade.
Figure 4. A plastic disk and a single screw secure the
corners of four ipe tiles at every pedestal (left). Spacer tabs
maintain a uniform 1/4-inch gap between adjacent tiles; a
wedge-shaped cutout in the disk can be rotated to allow a tile
to be removed. Tiles and pedestals are installed and leveled
one at a time (right).
We edged the deck between the field tile and the parapet with a
border of ipe planks (Figure 5). It’s
important not to leave a finished gap wider than 1/4 inch
around the perimeter. A wider gap could allow unsafe lateral
deck movement. To help stiffen the border planks against
deflection between the 2-foot-on-center pedestals, Steven made
custom ipe biscuits, which we inserted on 12-inch
Figure 5. To prevent lateral shifting, a maximum
clearance of 1/4 inch is allowed around the deck’s
perimeter. The author installed an ipe border in the gap
between the parapet and field tiles (top), supported between
pedestals with custom-made ipe biscuits (bottom).
The entire system of pedestals and tiles for this
374-square-foot deck cost a total of $6,300, excluding labor.
Ipe is heavy; shipping costs alone for the 87 tiles ran to
$770, and then we had to muscle them up onto the roof,
something we’ll keep in mind for the next job.
Because the pedestal support system varies in height by model,
it’s a good idea to have a product sales representative
review the plans. Our local rep came to the site, measured the
existing conditions, and provided shop drawings. The new roof
deck combines functional beauty with low maintenance —
and makes our company look smart.
Bruce Wentworth, AIA, is a registered architect and
licensed contractor in Chevy Chase, Md.