Blisters on Asphalt Roof Shingles

Cause

• Blisters occur when trapped gases inside the shingle, such as air or water vapor, expand due to the intense heat of sunlight. Poor ventilation of an attic can result in excessively high temperatures on the roof and promote blistering. Leaks in the roof can also lead to blisters, as moisture enters the shingle and expands in the heat. Also, overhanging trees can drop resin on the roof, damaging the shingles and making them vulnerable to blistering.

Types

• Blisters have the potential to cause serious damage to a roof. Small blisters might not pose a serious threat as long as blister remains closed, but an open blister that exposes the interior of the shingle to the elements presents problems. Moisture and ultraviolet radiation can enter the roofing material through the opening and degrade the shingle, hastening the aging of the roof and making premature failure more likely.

Age

• Blisters on shingles can occur as a normal part of a roof's aging process, though there may be other causes. Blisters may start to form a few years after the roof installation. A few closed small blisters that occur as the shingles get older should not affect the lifespan of your roof. Inspect your roof regularly as it ages, looking for larger blisters, such as those more than 1/4 inch in diameter, and blisters that have come open.

Hail or Blister

• Some homeowners mistake hail damage for blistering. Hail damage might produce a noticeable indentation on the shingle, but it does not create a bump on the shingle surface like a blister. Hail damage opens up the shingle and can lead to reduced effectiveness of the roof and decrease its lifespan. When you determine you have hail damage rather than small blisters that pose no threat, contact a roofing professional.