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Q.What causes nail pops in asphalt roofing shingles? Where I live, in the South, this seems to be a common occurrence. Is this an expansion/contraction problem? Are the wrong kinds of nails being used?

A.Harrison McCampbell, a consulting architect in Nashville, Tenn., who specializes in construction defects, responds: A nail pop — a tent-shaped blister on a shingle roof caused by a nail pushing up from below — can result from a number of factors. If the pop is caused by a shingle nail, it's often one inadvertently left in place from the last money-saving "roof-over." The pop could also be caused by a sheathing nail that was used to fasten the plywood or OSB roof deck. If coated or ring-shanked or annular-shanked nails weren't used to fasten the roof deck to the framing, a few nails could be incrementally migrating out on a daily or seasonal basis from an unstable deck.

Unfortunately, roof decks are rarely properly installed or vented, which makes pops more likely. For example, I often see decking panels butted right next to each other, rather than installed with the APA-recommended 1/8-inch gap at both ends and edges. Soffit venting is often blocked off by batt insulation in the attic, while ridge venting is often inadequate for exhausting hot, humid air from the attic.

And even if a properly sized ridge vent is used, I frequently see the opening underneath covered over with felt paper, presumably to avoid callbacks after driving rains. Panels with no room to swell and no opportunity to dry out tend to buckle against the fasteners holding them in place. A Southern climate — with its double-edged sword of high humidity and high temperatures in the summer months contrasting with the colder, drier winter months — compounds the problem.

There are no industry standards for roofing-nail length, just common sense. Some roofers will try to use the shortest nail they think they can get by with, thinking they'll save time and money and increase profits. Others use the longest nails they can, particularly when they're roofing over an existing roof and aren't sure what they're nailing into. Both situations can contribute to nail pops.

Pneumatic nailers are another piece of the puzzle. Often, gun nails are driven in at an angle, or the gun isn't set properly and either overdrives or underdrives the nail. And when they have a smooth shank, these nails don't grip the sheathing very well.

The prescription for nail pops is simple: Install and ventilate roof decking properly, use the right fasteners for both sheathing and shingles, and don't shingle blindly over an existing roof covering.