Preparing a deck for fiberglass shingles starts with ensuring the underlying sheathing is solid, flat, and properly installed. The roof deck should be made of plywood or oriented strand board (OSB) with a minimum thickness of 1/2 inch to provide a stable base. Over time, wood can warp or develop soft spots, so walk the deck carefully and look for any areas that feel spongy or uneven. These spots should be replaced or reinforced before proceeding. Fasteners play a key role in securing the sheathing; nails or screws should be spaced no more than 6 to 8 inches apart along the edges and 12 inches in the field. Proper fastening prevents the sheathing from shifting, which could lead to a poor shingle installation and potential leaks. Using galvanized or stainless steel fasteners helps avoid rust stains and extends the roof’s lifespan. Flatness is equally important; the deck should not have dips or bumps greater than 1/4 inch over a 10-foot span. Uneven surfaces can cause the shingles to crack or not lay flat, weakening their protective barrier.
A simple DIY check for flatness involves using a long, straight board or level laid across the deck in several directions. Any noticeable gaps or variations should be addressed by sanding down high spots or adding a plywood shim to low spots. It’s also wise to look for any nails driven too deep or missing fasteners, as these can compromise the deck’s strength. If you spot extensive warping, water damage, or need to replace large sections of sheathing, it’s time to call a professional roofer. When consulting a contractor, ask specifically about the type and spacing of fasteners they use and whether the sheathing meets local building codes for fiberglass shingle installations. Another good question is how they will ensure the deck’s flatness before installing shingles, as these details help protect your roof’s warranty and long-term durability.