Roof Blistering

Blistering is the formation of raised, often rounded bumps in a roof covering. It occurs in two quite different settings: on the surface of asphalt shingles, and within or under multi-ply and single-ply low-slope membranes. In both, the cause is a pocket of gas or moisture that expands when the roof heats in the sun.

Asphalt shingle blisters

On shingles, blisters appear as small bubbles in the coating asphalt. When the thin cap breaks away, it leaves a pit or pockmark that exposes dark asphalt or even the mat. Inspectors distinguish these from hail hits because blister pits have lost their granules along with the cap, while hail typically leaves a bruise or fracture with granules partly in place.

Older shingles sometimes blistered because of mixing errors or moisture in the materials. Today, blisters usually develop within the first year as volatile compounds escape from the asphalt of a roof that runs too hot, frequently because of poor attic ventilation. Small, scattered blisters are mostly cosmetic. Widespread, large blisters can shorten shingle life by speeding granule loss.

Membrane blisters

On built-up roofing (BUR), modified bitumen and some adhered single-ply systems, blisters are areas where plies or the membrane have separated from each other or from the substrate. They often start from voids left during application, moisture trapped in insulation or between plies, or poor adhesion. Daily heating and cooling pumps air in and out, so blisters can grow. If a blister is broken by foot traffic or cracks with age, water can enter the assembly.

Assessment and repair

For shingles, the decision rests on how much of the roof is affected; isolated blisters may be ignored, extensive blistering may justify earlier replacement and a ventilation review. For membranes, small, stable blisters are often monitored rather than opened. Larger or broken ones are typically cut out, dried and patched according to the membrane manufacturer's procedures. Persistent blistering can point to wet insulation, which a moisture survey can confirm.