Case 2

Cause of Defects

The membrane used on the roof deck is fluid-applied membrane, formed by brushing. During construction, the roof deck was not sufficiently dried when the membrane was brushed on. Preparation of the substrate is of great importance because it will affect the performance of the subsequent layer to be applied on it.

Failure to broom out the entrapped air during application of waterproofing membrane is another possible causes of blistering. After membrane is applied, inspection should be carried out to check for any entrapped air beneath the membrane.

The vapour pressures rise from the roof deck will force the membrane to rise, especially at areas without adhesive. The causes of this defect may be traced to the design stage where the designer while preparing the specification should have adopted full-bonded system instead of partial-bonded system.

Long-term exposure to high temperatures will cause vapour to rise too quickly for the trapped air and moisture to escape through the dense deck. The trapped air and moisture escaping through the dense roof deck at a rate too slow which allows blisters to form. Pressure will continue to develop, causing a displacement and stretching of the waterproofing membrane.

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.

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.