Case 1

Cause of Defects

Occurs “from pockets of trapped air and moisture” which expand under the sun and displaces the membrane to form a small blister. Air and moisture passing through dense substrate materials at slow rate of flow allows blisters to form (Figure 1). Under high surrounding temperatures, the vapour pressure rises too quickly for the trapped air and moisture to escape through the dense deck. Pressures will then develop, causing a displacement and stretching of the waterproofing membrane. Air pocket increases in size (Figure 2).

If the waterproofing has suffered an irreversible stretch, subsequent cooling will not cause the air pocket to return to the original size and a partial vacuum may be drawn through the substrate to refill the original pocket of air. Another cycle of development of the blister will occur when the sun appears again (Figure 3).

A blistering occurs at areas where there are voids or unadhered areas in the bonding coat of bitumen. This can be either between the piles or between they substrate and the membrane. Blistering is a means of failure because it represents a weakening and breakdown of the membrane, leading to the intrusion of water.

The water vapour transmission property of some common sheet membranes and liquid applied waterproofing materials are:

1. Self adhesive membranes (thickness abt. 1.5mm) WVT 5-15 g/m2.24h
2. Bituminous membrane (torch-on, thickness 3-4mm) WVT 5-15 g/m2.24h
3. Liquid applied roof waterproofing (thickness 0.8-1mm) WVT 40-80 g/m2.24h
4. Flexible cementitious membrane (thickness 0.8-1mm) WVT <30 g/m2.24h
5. Elastomeric Wall Coating (thickness about 0.2mm) WVT >80 g/m2.24

When the WVT is low, water vapour can pass through the film into the concrete substrate. When the WVTis high, it may cause moisture related problems. However, a high WVT is desirable for the “breathability” of the film. If the amount of water vapour in the substrate is high and it has to permeate the film of low WVT, there will be a pressure on the film which can cause debonding from the substrate, resulting in blistering which is common among liquid-applied roof coatings. Whether a waterproofing membrane has a high or low WVT, the liquid water will still be repelled as liquid water has a high surface tension which prevents it from passing through the membrane unlike water vapour.

Blistering usually will not cause any problem, except that they appear unsightly. However if there is frequent human traffic on the roof top, blisters may tear off and this increases the likelihood that of water penetrating the roofing system to result in roof leakage (Figure 6).