Edge work. 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).

Figure 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.

	Figure 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).

Figure 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 grinder.

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).

Figure 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.