The main purpose of under-tile plastic membrane is to:
- prevent the ingress of wind-driven rain and dust in roofs that are not 100% waterproof;
- prevent tiles from being dislodged by assisting with the containment of upward pressure forces in a roof that can build up during storms.
SABS certified under-tile waterproofing membranes are made of virgin polyethylene. The minimum thickness of SABS certified under-tile membranes is 250 micron. A mixture of virgin polyethylene, UV (Ultra-Violet) and heat stabilizers renders high quality products that can withstand:
- High levels of UV radiation;
- High levels of heat exposure (typically found under a tile roof);
- High temperature variance (from below freezing point in winter to 50/60 degrees Celcius on summer days).
In addition the products have excellent tear, break and tensile strengths, high, puncture, aging and weather resistance. In general terms, a product is rendered that can be guaranteed for at least twenty years when installed correctly.
Under-tile waterproofing membranes should be installed by approved contractors and roof erectors in line with good building practice. The membrane is to be installed across rafters, under the purlins or tiling battens with at least 150mm overlaps. The membrane should be taken over the last tiling batten at the eaves to prevent the formation of ponds on the membrane (see figure 1). The side on which the product identification mark is printed should face upwards. The membrane is affixed to the trusses by using galvanised clout nails. Sufficient slack should be allowed between rafters to allow the membrane to form a trough for water runoff if rainwater should penetrate or leak through the tiles (see figure 3).