Nail Pops

A nail pop occurs when a nail that holds a shingle, or the sheathing below it, backs out of the wood far enough to raise the shingle above or push through it. On an asphalt shingle roof it often appears as a small bump or a tented spot in an otherwise flat course. Sometimes the nail head breaks through and is exposed to rain.

Causes

• Wood movement. Rafters, trusses and sheathing swell and shrink with seasonal moisture. Each cycle can push a smooth-shank nail slightly outward.
• Nails that missed framing. Sheathing nails that missed the rafter, or shingle nails driven into a gap between boards, have little holding power.
• Under-driven nails left standing above the surface by an incorrectly set nail gun.
• Short or wrong nails that do not penetrate the deck properly.
• Attic moisture that makes the deck wood move more than normal.
• Thermal and wind loads that tug on shingles over time.

Why it matters

A lifted shingle can lose its seal and be more vulnerable to wind. A nail head that has punched through the shingle above can let water run down the shank into the deck, causing localised rot and leaks. Inspectors also note that pops cluster where moisture problems exist below.

Repair

Typical repairs involve carefully lifting the shingle, pulling or driving the popped nail, refastening with a new nail beside the old hole (into solid wood where possible), sealing the old hole, and replacing any damaged shingle. Where many sheathing nails are popping, the cause is often in the deck or attic, for example excessive moisture or nails that missed the framing, and screws or ring-shank nails may be used when the roof is next replaced. A related issue on metal roofs is fastener back-out.