What causes nail pops on roof shingles?
Short answer. Nail pops happen when roofing nails back out of the decking and push up under the shingles. The usual causes are wood decking that swells and shrinks with heat and humidity, nails driven crooked, too high or not fully seated, nails that missed solid wood or are too short to grip, and attic heat and moisture cycles that work them loose over the years.
The main causes
- Decking movement: plywood and OSB swell as they absorb moisture and shrink as they dry, and every cycle lifts nails slightly. Georgia's humid summers and a poorly ventilated attic make this worse.
- Installation errors: nails driven at an angle, left proud of the surface, or placed above the nailing line grip less and work loose sooner. Overdriven nails cut into the shingle and lose holding power.
- Short nails: the code calls for shingle nails to go through the sheathing or at least three quarters of an inch into it, whichever is less. Nails that fall short do not hold.
- Nailing into a void: a nail that lands in the gap between sheets or misses the deck edge has nothing to grip.
- Old plank decking: boards with gaps between them, common on older homes, are especially prone.
- Nailing through two layers: a second layer of shingles leaves less nail in the deck.
Why they matter
A popped nail lifts the shingle above it, which breaks the seal strip and lets wind get under the shingle. Eventually the nail head wears a hole through the shingle above, and water follows the nail shank into the deck. Pops often show up as small bumps in rows, easiest to see in low morning or evening light.
What to do
A handful of pops can be fixed by pulling the nail, renailing beside it into solid wood and sealing the old hole. Pops spread across the whole roof point to decking or ventilation problems that spot repairs will not cure, and better attic ventilation reduces the moisture swings behind them. See roof ventilation basics and OSB vs plywood roof decking.