blister

A blister is a raised, bubble-like defect in a roofing membrane, waterproofing layer, or coating where air or vapor has become trapped between the membrane and its substrate, or within the membrane itself. The membrane typically consists of bitumen, synthetic rubber, or PVC; the substrate is usually concrete, plywood, or an insulation board. The trapped pocket lifts the membrane away from the surface below, creating a hollow dome that is vulnerable to puncture, water infiltration, and thermal stress.

Blisters form when moisture or solvent vapor beneath an impermeable layer cannot escape during the curing or installation process. In built-up roofing, blistering occurs when residual moisture in the substrate heats up and expands faster than it can migrate through the bitumen layers. In single-ply membranes, improper adhesive application or substrate preparation traps air pockets. Liquid-applied coatings blister if the substrate is wet or contains trapped solvent vapor when the topcoat is applied. The size ranges from small pinprick blisters, barely visible until they fail, to large domes several feet across that are obvious from ground level.

Severity and consequences

A blister itself does not immediately leak. However, the membrane over the blister is thinner and more prone to puncture from foot traffic, weather exposure, or thermal cycling. Once punctured, water enters the cavity, and the blister may rupture further. In roofing, blistering also accelerates deterioration because the trapped air reduces the thermal mass of the system and causes temperature extremes within the blister.

Prevention requires controlling moisture before membrane installation: allowing concrete to cure fully, drying out insulation boards, and ensuring the substrate is clean and dry. During installation, technique matters: proper heat or solvent application in bituminous systems, correct adhesive coverage in single-ply work, and adequate drying time between coats in liquid-applied systems. Specification of breathable underlays or vapor retarders appropriate to the climate and substrate can reduce blister risk significantly. Once blisters appear, repair involves cutting out and re-adhering the affected membrane section, or replacing it entirely if the damage is widespread.