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...
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