One of the most common defects I find with asphalt shingles is improper nailing. The manufacturers of asphalt shingles give specific instructions on how to fasten shingles, and they're all pretty much the same. Unfortunately, following those instructions seems to be a difficult thing to do for a lot of roofers in Minnesota. The two most common nailing defects that I find with shingles are overdriven nails and improperly located nails.
Most roofers use pneumatic nail guns to nail down shingles. When the pressure is set too high, the nails get driven in to the shingles too far, plain and simple. When the nails are overdriven, the heads of the nails punch right through the mat of the shingle. This voids the shingle manufacturers warranty and greatly increases the potential for shingles to come loose and blow off, possibly in sheets.
The diagram below shows what proper and improperly driven nails look like.
The photos below show several examples of overdriven nails.
Improperly Located Nails
The other most common nailing defect that I find with asphalt shingles is improperly located nails; specifically, high nails. When shingles are located too low on a shingle, it's an obvious defect that anyone with a good eye can usually spot from the ground. This is probably why I don't find this defect all that often; it's just too obvious of a defect for most roofers to leave uncovered.
The more common and problematic nailing problem is to have nails located too high on the shingle. There is a fairly narrow strip on every shingle where it's acceptable to place the nail. When properly placed, the nail will actually catch the top edge of the shingle below it. When nails are located too high on the shingle, they never catch the shingle below, which effectively cuts the amount of nails going in to each shingle in half. Hopefully my diagram below will help to illustrate this; the blue dots are supposed to be nail heads.
The photos below show examples of high nailing. Yes, I know what I did there.
When shingles are improperly fastened, they have a tendency to slide and rip out of the nails holes, and this won't be covered by the manufacturers warranty. When the entire roof covering is installed like this, there is no simple fix. Either the new buyers need to accept the fact that their roof will be prone to having shingles blow off, or the roof covering will need to be repaired or replaced.
These installation defects can't be seen from the ground, even with a very expensive pair of binoculars. For this reason, you shouldn't expect most municipal inspectors to identify these issues; it's outside the scope of their inspection. I wrote about this topic here - Who Inspected Your Roof? If you want a thorough roof inspection, you would do well to hire a home inspector who can will access the roof.
Reuben Saltzman, Structure Tech Home Inspections