What size roofing nails do you need for 1/2 inch plywood?
The math at the nailing line
At the point where a laminated architectural shingle is nailed, the nail passes through the shingle's two layers, part of the shingle below it, the underlayment, and then the 1/2 inch deck. Those layers can add up to well over half an inch before the nail reaches the wood. A 1 inch nail may stop inside the deck at the thicker spots, which holds far less than a nail that clears the underside. A 1 1/4 inch nail gives a reliable margin.
Why going through matters
A nail that passes completely through thin sheathing grips the full thickness of the wood. One that stops partway is easier for wind uplift to pull out, and it is more likely to back out as the deck swells and shrinks with Georgia humidity.
Other cases on 1/2 inch decking
- Ridge and hip caps. 1 1/2 to 2 inches, because of the extra layers.
- Three-tab shingles. Thinner, but 1 1/4 inch is still the safe standard.
- Nailing the plywood to rafters. That uses framing nails, typically 8d, on the schedule set by the code, not roofing nails.
- Roof-overs. Longer nails to clear both layers, though a full tear-off is the better practice.
Check from the attic
Shiny nail points coming through the deck are normal and a sign of proper length. If you see none at all, the nails may be short. Rusty points suggest moisture in the attic.
More on decks in OSB vs plywood roof decking and on layering in roofing over existing shingles.