After the plywood is installed, a 1x4 pine drip edge is
fastened flat against the outside of the rim joist, flush with
the deck (Figure 3).
3. A 1x4 fastened to the rim joist will be wrapped with
fiberglass to provide a clean termination at the deck
perimeter. Fiberglass doesn’t bond well at 90-degree
corners, so the top outside edge of this trim piece is eased
with a 1/2-inch round-over router bit. A 20-degree ripped
undercut serves as a drip edge.
4. A 45-degree cant strip keeps water from
ponding in the corners and prevents cracking in the
fiberglass where it turns up the wall (top). At door
sills, a narrow trim strip serves as a backstop for the
fiberglass, which will create a seamless waterproof pan
under the door (bottom).
Any species of lumber will do so long as it’s dry, but
we prefer fir because the better quality material gives us
fewer problems. The cant strip keeps the glass, which extends
approximately 6 inches up the wall, from pulling away from the
corner. The cant strip also prevents water from ponding in the
deck-wall joint. Wood strips are also installed at the sills of
any door openings to serve as the backstop for the fiberglass,
which will create a seamless pan.
Once all the edge pieces and cant strips are nailed in place
(again, glue is not necessary), the outside corners, including
the drip edge, are rounded using a 1/2-inch rounding-over bit
in a router. Because fiberglass doesn’t adhere well to a
90-degree corner, all edges must be knocked down. Joints in the
plywood are lightly sanded with a coarse disc in a grinder, as
are the bottom few inches of the posts where they meet the
plywood and the edges of door openings (Figure 5).
5. Before applying the fiberglass mat flashing,
all edges — including the bottoms of railing
posts, the edges of door openings, and all plywood
joints — are knocked down with a coarse-disc
While one person completes the sanding, the other mixes the
"fairing" — a boat-building term for a compound used to
smooth and level a curve. Made from resin, hardener, and finely
ground silica, the fairing is spread on all joints and nail
heads, and is also used to fill any voids around posts and
deck-wall joints (Figure 6).
6. A site-mixed fiberglass and silica leveling compound
is applied to all joints and nail heads, and is used to fill in
gaps around posts and other penetrations (left) and at cant
strips and door sills (right). When dry, any lumps and high
spots are hand-sanded.
The fairing compound takes about 30 minutes to dry, after
which any lumps are hand-sanded.
Polyester resin is nasty stuff, so it’s important to
take safety precautions from the moment the fiberglass is first
mixed. Make sure the work area is well ventilated, and always
wear gloves and goggles. A respirator is standard equipment
during sanding as well.