Leaks do not start at the surface
The most common misconception in roof waterproofing is assuming the problem lies in the membrane itself. In practice most leaks start at chimneys, ventilation pipes, roof outlets, parapet junctions and around drains. These points require separate reinforcement and detailing.
Membrane types
Bituminous sheet
A roll material consisting of modified bitumen coated onto a polyester or glass fleece carrier. It is torch applied. It is a widespread, economical and well understood system. Its weak point is the seams; torch application depends heavily on the installer.
TPO and PVC membrane
Thermoplastic synthetic sheets joined by hot air welding. The seams become homogeneous through welding, which gives a more reliable joint than bituminous sheet. UV resistance is high, so they can be left exposed. They are preferred for green roofs and trafficable terraces.
Liquid membrane
A seamless polyurethane or polyurea based coating applied by brush, roller or spray. Having no seams, it is advantageous on roofs with complex geometry and in detail heavy areas. It is very sensitive to surface preparation and thickness control.
Detail points
- Parapet junction: the membrane must be carried over the parapet and mechanically fixed. Edges that are only bonded to the surface open up over time.
- Drain surround: the drain must sit at the lowest point of the roof and the membrane must run underneath the drain flange.
- Pipe penetrations: a sleeve must be used, with the membrane carried up to the sleeve and clamped.
- Expansion joints: resolved with a flexible tape that accommodates movement, never run straight over.
Fall and ponding water
Even surfaces described as flat roofs must have at least a 2 percent fall. Areas where water ponds shorten the life of the membrane and accelerate damage through winter freeze thaw cycles. If the fall was not formed in the screed, the surface should at least be directed towards the drains.
The relationship with thermal insulation
Whether the waterproofing goes below or above the thermal insulation depends on the roof type. In an inverted roof build up the waterproofing sits underneath and the insulation on top, protecting the membrane from UV and temperature swings. This detail extends membrane life considerably.