What causes roofing nails to back out?
Short answer. Nails back out, or pop, mainly because the wood around them moves: decking swells and shrinks with Georgia's humidity and temperature swings, and each cycle pushes a short or smooth-shank nail up a little. Nails that missed solid wood, went in at an angle, or were left underdriven also work loose until the head lifts or tears through the shingle above.
Two different nails can pop
- Deck nails hold the plywood or OSB to the rafters. When they back out, you see small bumps under the shingles, often in neat rows along the framing lines.
- Shingle nails hold the shingles to the deck. When they rise, they push up the shingle above, break its seal, and eventually poke through.
Common causes
- Moisture cycling. Attics with poor ventilation let humid air condense on the cool deck at night. Wet wood swells, dry wood shrinks, and the nails ratchet upward.
- Missed framing. A deck nail that missed the rafter has nothing to grip. In the attic you can see its point sticking out beside the framing.
- Short nails. Shingle nails that do not go through thin decking hold far less.
- Underdriven or angled nails. Nail guns set too light leave heads proud, and angled nails cut shingles and lift unevenly.
- Green or wet lumber. Wood that dries after installation shrinks around the nail.
- Smooth shanks. Ring-shank nails resist withdrawal better.
Fixing them correctly
Hammering a popped nail back down just resets the cycle. The fix is to pull it, drive a new nail into solid wood beside the old hole, seal the old hole, and replace the shingle if it is torn. If you see many pops, look at ventilation and deck fastening rather than chasing individual nails.
A raised nail head under a shingle is a common slow leak. See common roof leak causes, roof ventilation basics, and OSB vs plywood roof decking.