Question: I understand that having more than two layers of asphalt shingles is considered defective no matter what the age of the top layer, why?
Answer: Adding new layers of asphalt shingles over old can reduce the useful life of the shingles or even the roof itself in several ways:
- Added layers of shingles means added weight on the roof. That makes the structural roof components work harder and can lead to failure of the roof structure itself.
- Additional shingle layers may mask a leak instead of fixing the problem. For instance, a homeowner may have discovered a leak and added another layer of roofing material without ever knowing when the leak started or the extent of the damage. Adding a layer without inspecting the sheathing below may simply bandage a larger problem.
- Nails used to attach the new layer may not be long enough to penetrate through the sheathing. This weakens the attachment and makes shingles more susceptible to wind damage.
- Depending on the extent of the roof replacement, the flashings - protective coverings that direct water away from openings and seams in the roof - may not have been updated. This makes them a weak link in the new roof and a place for possible water intrusion.
NPI inspectors use several clues to determine the number of layers of shingles. Many times layers can be counted at the lower edges of the roof. A close look at the flashings or the nail pattern evident from below the sheathing in the attic may provide further clues. NPI inspectors will perform visual inspections of the roof and attic as part of a general inspection provided access is available.
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