Blistering Shingles
Raised bubbles or pockets on the surface of asphalt shingles caused by trapped moisture or volatile gases in the shingle mat expanding under heat. Blisters can be mistaken for hail damage.
What Is Shingle Blistering
Blistering shingles display raised bubbles or pockets on the shingle surface caused by trapped moisture or gas expanding within the shingle mat under heat, and the condition is frequently confused with hail damage during roof inspections. Blisters range from small pinpoint bumps to large, quarter-sized raised areas. When the blister pops, it exposes the dark asphalt mat beneath, which at a glance can look similar to a hail impact mark.
Blistering vs. Hail Damage
Distinguishing blisters from hail is one of the most contested issues in roof claims. Blisters have smooth, rounded edges with granules that are undisturbed around the perimeter. The exposed asphalt at the center of a popped blister is flat, not dimpled. Hail impacts displace granules at the point of contact, often leaving a sharp-edged crater or a bruised depression in the mat. Location patterns also differ: blisters concentrate on the hottest slopes (south and west facing), while hail damage is distributed across all slopes exposed to the storm direction.
Insurance Implications
Blistering is considered a manufacturing defect or a maintenance/ventilation issue, not storm damage. Carriers routinely deny claims where blistering is misidentified as hail. If a roof has both blistering and genuine hail damage, the two conditions must be documented separately and clearly. Photographing examples of each on the same roof with notes explaining the difference strengthens the claim for the legitimate hail damage and prevents the carrier from dismissing the entire claim as blistering.
Frequently asked questions
Blistering occurs when moisture trapped in the fiberglass mat during manufacturing, or volatile compounds in the asphalt, expand as the shingle heats up in sunlight. Poor attic ventilation accelerates blistering by increasing shingle temperature from below.
Blisters are typically circular with smooth, intact granules around the raised area. The center may show exposed asphalt where the blister popped, but the edges are not sharp or cratered. Hail impacts show granule displacement with a bruised or dimpled mat beneath. Blisters tend to cluster on south-facing slopes with the most sun exposure, while hail damage appears randomly across all slopes.
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