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:
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.
