Preparing a deck for synthetic slate involves ensuring the roof’s surface is strong, flat, and properly layered to support the installation and maintain warranty conditions. Start with a solid sheathing base, preferably plywood or oriented strand board (OSB) rated for roofing use, free of warps or damage. The deck needs to be flat; any dips or uneven spots over 1/4 inch in 10 feet should be corrected with shims or replaced boards to prevent stress on the synthetic slate panels. Fasteners are critical—they should be corrosion-resistant, like stainless steel or hot-dipped galvanized nails or screws, suitable for the sheathing type and the weight of synthetic slate. Ensure the deck is thoroughly clean and dry before installation to avoid trapping moisture under the panels, which can lead to rot or degradation over time.
To diagnose flatness and fastening, homeowners can walk the deck or visually inspect for soft spots and unevenness, but avoid heavy walking that could cause damage. Using a straightedge or level across the surface can reveal imperfections. If you suspect the deck isn’t flat or strong enough, or if there are soft spots, it’s best to call a professional roofer to assess the condition. When talking to your contractor, ask if they recommend a specific type or thickness of sheathing for synthetic slate and if they use fasteners that meet the roofing manufacturer’s warranty standards. Also, inquire about how they handle uneven decks—whether they use shims or replace sheathing—and if additional underlayment is necessary for your climate and roof pitch.