At the beginning of developing a roof preventative maintenance plan you’ll see various issues; one common issue is roof blisters. Blisters form when there are pockets of air or moisture trapped between layers of your roof’s membrane or between the membrane and the roof deck. As the sun heats the roof, those pockets expand and stretch the membrane. In severe cases, the membrane will crack when the pressure becomes too great. Even if the roof membrane doesn’t crack, blisters are usually a cyclical problem. That is, once they form, the membrane stretches permanently, which allows for even more air or moisture to infiltrate the membrane layers. This causes even more expansion. Each time the sun heats your roof, there is the potential for the blisters to grow a little larger. All blisters do not require immediate attention, however, if left unattended for too long it can pose a huge problem for your roof system, it can lead to water infiltrations and moisture damage to insulation and building interior.

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

Should it be determined that blister removal is necessary, the following steps should be followed:

  1. To promote thorough adhesion of a patch, it is essential to begin by preparing the surface. Remove debris, contaminants, aggregate or loose surfacing from the surface of the membrane or flashing to be repaired. The area to be repaired should extend a minimum of 18 inches beyond the perimeter of the defect to provide an ample, clean area on which to install the patch and tie it into the existing roof membrane.
  2. If the membrane surface has been flood-coated and aggregate-embedded, carefully spud the aggregate free from the surface and sweep clean.
  3. Carefully cut and remove the blistered material until good adhesion of the membrane is reached.
  4. 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.
  5. Prime the surface of the membrane (asphalt BUR only) with asphalt primer and allow to dry. The primer contains solvents and is used to enhance adhesion; however, overuse of primer can harm the membrane and inhibit adhesion of newly applied material.
  6. Install the same number of plies as were damaged or removed (a minimum of two plies) in hot bitumen or cold-applied adhesive. Extend the bottom ply at least 6 inches beyond the area to be repaired and each succeeding ply at least 3 inches beyond the previous ply. When using hot bitumen as the method of application, take care to maintain the bitumen at EVT. As an alternative, on asphalt BUR membranes only, two plies of smooth-surfaced modified bitumen sheets may be installed by torch application, in hot asphalt or suitable cold adhesive, whichever is appropriate for the modified sheet being used. When using hot asphalt as the method of application, use Type III or Type IV asphalt and take care to maintain the asphalt at EVT.
  7. Reapply the surfacing. Either embed aggregate in hot bitumen or roof cement or apply a surface coating compatible with the existing surfacing. Liquid-applied coatings should be installed per the manufacturer’s instructions, which may include a waiting period. For small repair areas, it is a common practice to apply the coating immediately after the membrane repair is made.

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. Some characteristics that may determine the need to repair blisters on modified systems are:

  • Loss of granules or other surfacings
  • 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

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. To promote thorough adhesion of a patch, it is essential to begin by preparing the surface. Remove debris, contaminants, surfacing, ballast or loose granules from the surface of the membrane or flashing to be repaired. The area to be prepared should extend beyond the perimeter of the patch to provide an ample clean work area on which to install the patch.
  4. Clean the surface of the membrane. If the membrane surface has been flood-coated and aggregate-embedded, carefully spud the aggregate free from the surface and sweep clean. The exposed asphalt flood coat may need to be heated with a torch in order to smooth out irregularities, then allowed to cool.
  5. Prime the surface of the membrane with asphalt primer and allow to dry. The primer contains solvents and is used to enhance adhesion; however, overuse of primer can harm the membrane.
  6. Cut a patch of like material 8 inches larger in all dimensions than the defect to be repaired. Round the corners of the patch to a minimum radius of 3 inches.
  7. Install the patch in hot asphalt, cold adhesive or by heat welding (in accordance with the manufacturer’s recommendation) over the repair area, extending 8 inches in all directions from any part of the defect. When using hot asphalt as the method of application, use Type IV asphalt and take care to maintain the asphalt at a minimum of 400 degrees F at the point of application. When torching, work gradually, applying heat only sufficient to achieve adhesion without damaging the membrane reinforcement or scorching surrounding membrane.
  8. Apply moderate pressure to the patch to ensure adhesion to the existing membrane.

Repair of Blisters on Adhered Single-ply Membrane Systems

Blisters on adhered single-ply membrane systems are typically caused by lack of adhesion at the point of application. If the membrane is not weathered or cracked, repairs are typically not required. Bonding of the membrane to the substrate is of concern at perimeter areas, particularly at low-profile edges, to eliminate the probability of roof blow-offs. If unbonded areas are at these points, or if they are of considerable size or frequency, repairs should be completed. Refer to the membrane manufacturer’s proper repair procedures in these cases.

 Conclusion

The decision to repair blisters on a conventional low-slope membrane system is largely a judgment call. Hopefully, the criteria established in this article will help you when you are faced with this decision on a roof. The size and the frequency of blisters will also have a bearing on your decision. One other fact to consider is that blisters are defects to the membrane system. They have the potential to open up, creating leaks and moisture infiltration into the system. They also have the ability to diminish the service life of the roof system. When repairs are completed in the context of a maintenance program, where the intent is to add service life to the system, it may be best to complete the repairs. If you would like to know more about solving common roof issues or if you have any questions about developing a roof preventative maintenance plan, please give us a call at (786) 860-2063 or email us at [email protected]