How long should shingle nails be?
Short answer. Shingle nails should be at least 1 1/4 inches long on a typical roof, because the nail has to pass through about three layers of shingle at the nailing line and still reach 3/4 inch into the deck or go completely through it. Thicker shingles, caps and roof-overs need longer nails.
The math behind 1 1/4 inch
At the nailing line of a laminated shingle, the nail passes through the two layers of the shingle being installed plus the top edge of the course below it. Together that is roughly 3/8 inch of material, though it varies by product. Subtract that from the nail length to see what is left for the deck:
- 1 1/4 inch nail minus 3/8 inch of shingle leaves about 7/8 inch for the deck.
- 7/16 inch OSB: the nail goes through with plenty to spare.
- 1/2 inch or 5/8 inch plywood: still through, which meets the rule for decks under 3/4 inch.
- 3/4 inch board decking: about 7/8 inch available, so it just makes the 3/4 inch minimum.
When to go longer
- Ridge and hip caps: the cap folds over the peak and sits on top of field shingles, so there can be four or more layers. Use 1 3/4 to 2 inch nails.
- Thick designer or impact rated shingles: add material at the nailing zone; follow the wrapper.
- A roof-over, where local code allows one, adds the full thickness of the old shingles.
Why not just use long nails everywhere
Longer nails cost more, jam some guns, and can poke through finished overhangs. The goal is the shortest nail that meets the penetration rule and the manufacturer's spec. If you are not sure of your deck thickness, check from the attic at a vent or hatch, or pull a shingle at the edge.
See roofing over existing shingles and roof anatomy explained.