Built-Up Repair Techniques

One of the most common defects built-up roof systems often encounter is blistering. There are typically two types of membrane blisters that occur:

Circular blisters that occur at random throughout the membrane assembly. Longitudinal blisters that occur approximately 32 to 36 inches on center.

The circular type blisters typically indicate that when finishing felts were installed, the felt material itself had not been properly embedded into the asphalt mopping. This results in air voids within the bitumen application, which are then covered by additional felt plies. As this entrapped air becomes warm, it expands allowing the blister to form.

The longitudinal type blister typically indicates the membrane assembly was installed in a phased application. This consists of installing a base felt over the entire surface of the roof area and returning at a later time to install the finishing felts. Due to the face felt having been left exposed to the weather, moisture can be absorbed into the "felt cove" at edge lap locations. The finishing felts are then installed in moppings of hot asphalt. This naturally evaporates the moisture, which is then vented back out of the edge laps. As the membrane heats up, this entrapped moisture expands, allowing a blister to form along the path of the least resistance, or edge lap.

The following are recommendations for repairing blisters:

1. Remove all existing embedded gravel surfacing approximately 36 inches from around the affected area.
2. Cut open the blistered section, penetrating all the affected sections with an "X" type incision.
3. Using a dry cloth, torch or blower, remove any entrapped moisture from the blister section.
4. Fill the blistered section with a solid application of plastic cement and press the existing felt sections.
5. Install a repair to the blister section consisting of a 1/3, 2/3 and full section of fiberglass finishing felts, installed in a solid mopping of Type I asphalt.
6. Apply a new asphalt flood coat to the surface of the repair section, at a rate of 60 pounds per 100 square feet. While hot, a clean roofing gravel should be broadcast throughout. Note: It is recommended that felts used in the repairs be installed in an "upside-down" manner, to prevent the felt edges from curling, and as a result, not achieving embedment.