Roof Blisters- What are they?

July 20, 2012

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Blistering is a common problem with both field applied exterior coatings such as paints and built-up roofing systems.  In built-up roofs blisters are raised surface areas over voids in a roof system which contain entrapped air and/or moisture.  As the  roof surface temperature rises, the pressure inside the blisters rises and the adhesion of asphalt weakens.  Blisters are usually noticeable on hot, sunny days and range from small spongy spots to large pronounced areas.  Blisters develop from voids built into the roof. Voids can result from skips in bitumen mopping, entrapped debris, and uneven gases and bitumen bubbling.  Even in perfect conditions, a few small voids are inevitable in built-up roofing applications.  A few small voids do not pose a problem with respect to membrane blistering as membrane elasticity and adhesion can accommodate small movements.  However, numerous large voids will result in membrane blistering problems.








Blisters grow when:

  1. the volume of air sucked into them at night exceeds the volume of air forced out of them by day;
  2. the increased pressure overcomes the peripheral bond of the blister

Treatment of Blisters

If a few small blisters develop, they often do not result in problems, provided they remain watertight and do not become too large.  Eroded bare spots on small blisters can be coated with a cold-process asphalt coating and sprinkled with gravel. If a blister approaches a dimension of about 2 ft, it is recommended that its outer boundary be marked so that it can be checked periodically to determine if its getting larger. When blisters grow out to about 5 ft., or if they occur in a high-traffic area, blisters should be repaired.  To avoid breaking blisters, you should not step on  them, particularly when the membrane is cold.

If a blister is broken or otherwise capable of taking in water, the usual repair process is to remove the entire raised portion of the blister and patch the remaining void with alternate layers of asphalt and successively larger pieces of felt.

The Wrap Up

  • Prevention is the best deterrent: When getting a new roof, try to make sure that it is installed according to manufacturer’s specifications as well as industry best practices. It’s difficult to have a roof installed perfectly, but by doing your homework and choosing a quality and qualified roofer your chances are much better.
  • Limit Foot Traffic: If you know that you have blisters on your roof, it’s important to keep walking on your roof to a minimum, especially in warmer weather. Walking over blisters can break the blisters open, allowing water and air to damage them even more.
  • Keep your roof cleaned and maintained.: Allowing moss or algae to grow on your roof only adds to the problems of blisters.