Blisters: Repair or Re-Cover?

This article provides some guidance on whether or not to repair troubled areas on various types of low-slope membrane systems.

Some of your existing roof systems may have gone well beyond their warranty period, while others are suffering from premature failure.

One of the oldest low slope roofing types is a multi-ply bituminous roof system. The bitumen could be asphalt or coal-tar pitch, both for adhesion to the substrate and to laminate together the multiple plies of organic, asbestos, or glass-fiber mats. A flood coat of bitumen would then be poured onto the membrane to protect the felts and to adhere the roof aggregate. This was designated as a gravel-surfaced built-up roof (BUR).

When organic felts were in vogue, blistering was a major problem. Walking the roof surface would reveal many blisters, usually between the base sheet and ply sheets. With the greater demand for energy-efficient roofing, you may be planning to superimpose a new roof system directly over the existing BUR.

Some of the blisters may be open, but most of them may not. The question is, should we repair all of the blisters that your maintenance crew has found or only the blisters that have opened?

The industry does not provide a concise answer to this question, though everybody appears to be in agreement that if a blister has opened it must be repaired. This is due to the probability that moisture will enter the system at these locations. The decision to repair unopened blisters has typically been left up to the judgment of the contractor. This article provides some guidance on whether or not to repair blistered areas on different types of conventional low-slope membrane systems.

Causes of Blisters
All blisters originate with the formation of a void, or unadhered area, in the mopping bitumen, either between the felt plies, or between the substrate and the membrane.

All conventional low-slope roof systems can experience blisters in some form. Membrane systems are most susceptible to blistering because blisters are formed by voids between the plies or at the point between the substrate and the membrane. In bituminous systems, voids can be created by improper adhesion during application, improper mopping viscosity, moisture in or on the felts, unfilled insulation joints, or coverage of loose or trapped particles of debris.

In bitumen-based systems, the dynamics of blister expansion is a cyclic process. Blister growth is predicated by the absorption of moisture and the expansion of the water vapor that is contained within the void. This is less of a problem with glass-fiber mats.

The formation of blisters in all conventional low-slope membrane systems can be avoided with proper adhesion of the membrane to the substrate. One hundred percent adhesion is required during application.

Repair of Blisters on BUR Systems
Deciding whether to cut out and remove, patch over to reinforce, or simply monitor blisters is a judgment call. In general, it may be preferable to leave them undisturbed. Some characteristics that may determine the need to repair blisters are:

• Excessive loss of gravel or surfacing
• Membrane deterioration
• Blisters in laps that have reduced lap coverage
• Blisters that have breaks
• Blisters that have fatigue cracking
• Blisters that occur in areas of high traffic such as the installation of solar panels.

Repair of Blisters on Modified Systems
In general, blisters that are keeping air in will keep water out, and unless the blisters have certain characteristics, it may be preferable to leave them undisturbed. You may need to repair blisters on a modified system if you see:

• Loss of granules or other surfacing
• Membrane deterioration
• Blisters in seams which have reduced lap coverage
• Blisters that have breaks that can admit moisture
• Blisters that have fatigue cracking around the circumference
• Blisters that occur in areas of high traffic

MB systems are less susceptible to hail or impact damage than BUR systems.

If it is determined that blister removal is required, the following procedure should be followed:

1. Remove the membrane from the blistered area down to the existing substrate. Inspect the membrane for possible moisture infiltration.
2. If water infiltration is suspected, open the membrane and inspect the insulation and deck for damage. Remove wet or damaged insulation and repair or replace the deck as required. Properly attach new, dry insulation consistent with the thickness of the existing insulation and compatible with the other roof system components.
3. Torch application of the MB may help remove surface moisture.

Repair of Blisters on Adhered Single-Ply Membrane Systems