Robert Allison wrote:
....
They can and do back themselves out and become "nail pops". My engineer
explained the process like this; The wood will swell if there is enough
moisture in the air and will sometimes swell around a nail. When the
wood dries out, the nail is looser in the hole. The sun heats the
shingles which expand both laterally and in thickness. This causes the
shingle to "grab" the nail and the vertical expansion lifts it slightly.
Repeat this process numerous times and the nail can be lifted enough to
cause a problem.
....
I don't think the shingle has much to do with the process,
actually--they're too soft.

The heat/cooling expansion/contraction cycle combined w/ some moisture,
particularly add in perhaps a void in plywood sheathing for roofing nail
in particular and most of all time and eventually a few will work their
way back out.

In very windy areas, such as here on the High Plains, the physical
vibration/working of the material can cause it at a much higher rate
than in other areas. As noted, it's very common here on siding or even
construction joints w/ age as well as roofing to see many nails
projecting by as much as an inch or more. In the 30+ years in VA and TN
where weather isn't nearly as severe (particularly wind) I never
experienced nearly the amount as is common here.

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