What size roofing nails do you need for two layers of shingles?
Short answer. When new shingles go over one existing layer, the nails must still penetrate at least 3/4 inch into the deck or all the way through it, which usually means 1 3/4 inch to 2 inch roofing nails instead of the 1 1/4 inch nails common on a single layer. The code does not allow a third layer, and many roofers discourage overlays even when they are permitted.
The rule behind the number
The residential code calls for corrosion-resistant roofing nails with at least a 12 gauge shank and a 3/8 inch head, long enough to go 3/4 inch into the sheathing or through it, whichever is less. Add up what the nail has to pass through: the new shingle at its thickest, where laminated layers and courses overlap, the old shingles, the old underlayment, and then the deck.
- Single layer on 7/16 or 1/2 inch decking: 1 1/4 inch nails typically reach through the deck.
- Over one existing layer: 1 3/4 inch is common, and 2 inch nails may be needed when both layers are thick laminated shingles.
- Check from the attic: nail points should show through the underside of the deck.
Why overlays are discouraged
- The old layer hides rotted decking and lets lumps and curls telegraph through.
- Nails driven through two uneven layers are more likely to go in crooked or overdriven.
- Heat builds up in the double layer, which can shorten shingle life.
- Some warranties are reduced or unavailable on overlays, and the next replacement costs more because two layers must come off.
- A roof that already has two layers cannot get a third; it must be torn off.
See roofing over existing shingles and OSB vs plywood roof decking.