Blisters in Built Up Roof

  Blistering is a common problem found in many commercial built-up or EPDM roofs.  A blister is a raised surface area over voids which contain entrapped air and/or moisture. As the surface temperature rises, the pressure inside the blister rises and the adhesion of asphalt weakens.  Blisters occur in all sizes/shapes from very small to very large. Blisters develop from voids in the roof caused by skips in bitumen mopping, entrapped debris, curled felts, uneven gases and bitumen bubbling.  Blisters have two primary forms:               *  Blisters between the roof membrane and the substrate:  Develops when pockets of air and moisture which are trapped between the membrane and substrate expand with heat and displace the membrane to form a blister.  The sudden rise in temperature from the sun causes expansion of the entrained moisture/gases faster than it can escape through the substrate.  As the moisture/gases expand, it causes the membrane to expand/stretch to accommodate the internal expansion of the moisture/gases.  As the temperature cools, the moisture/gases contract and the overall size of the blister will also contract – to a point. Over time, the roof membrane will lose its elasticity and is will suffer an irreversible stretch and the blister will have a vacuum inside.  This increases the likelihood of damage to the membrane.  Further, the constant swelling of the membrane will lead to cracks in the asphalt and loss of protective granule and eventual failure of the roof. Also noteworthy, is the common occurrence of blisters in membrane roofs which have been directly applied to cellular foam insulation.  Many believe the release of gases from the insulation contribute to blistering, although this is not entirely proven.  Personally, I believe that due to the nature of the product, it is simply more difficult to adhere the membrane to this product and therefore, it is susceptible to voids.  The CRCA strongly recommends the addition of an overlay of wood or glass fibers, perlite or other substrate be laid over the cellular foam insulation panel prior to installing the membrane.  Few roofers recommend this primarily due to the competitive nature of the industry.                *  Blisters between the membrane plies:  Entrainment of air/moisture during the topping up process are almost a given.  Blisters exhale moisture/gases during the heat of the day and inhale during the cooler night air.  Membranes are not elastic – they do not shrink back to their original shape.  This deformity will create a vacuum which will absorb additional air/moisture through microscopic cracks in the bitumen or in the felt underlayments.  As the level of moisture/air increases, the increased pressure will defeat the peripheral bond of the membrane and the blister will grow in size. Treatment of Blisters:  A few small blisters rarely cause problems (although they should be monitored).  If bare spots/cracking develop, they can be treated with a cold process asphalt coating and sprinkled with gravel.  Blisters exceeding 18” should be marked and monitored to determine if it is getting larger.  Blisters which are worn, cracked, in high traffic areas or large in size (over 3 feet) should be properly repaired by a competent roofer. Roofs which have numerous large blisters existing throughout the roof should be considered for replacement.  

William Chandler

The Building Inspector