Blistering of composition shingles is not uncommon in the Houston area. It tends to occur when there’s a defect in the manufacture of the shingles, causing moisture in the layup of the shingles to vaporize after installation and causing small blisters. These blisters can break open as the roof ages, exposing the base material of the shingles to rapid deterioration due to exposure from the sunlight. Typically, these are visible from the surface of the roof. These may also occur as result of overheating of the shingles due to a lack of adequate attic ventilation. This is why inspection of the roof from its surface is far superior than using a drone or other method where these blisters are not likely to be as visible. It is not always possible to walk a roof systems; however, Professional Engineering Inspections prefers this method when it is determined safe by the inspector.