The traditional way to waterproof a flat roof is to cover it with a thick membrane from a roll and weld or glue the sections together. The simplest materials are bitumen based roofing felts with the sections joined together by melting with a flame torch. This method often leads to fire breaking out during the night, from scorched wood smouldering underneath. The bitumen has a poor resistance to sunlight and starts to break up within a few years, with cracks appearing that allow water to pass through. Synthetic coverings are available that have a much better light resistance, but these still have to be fitted to the roof and glued together.
The alternative is to spray a thick synthetic roof coating which eliminates the time required to fit and glue a manufactured film. Spraying a roof coating is therefore much faster than using a film and ensures a perfect seal with a long life, depending on the nature of the material. Acrylic based roof coatings can have a working life exceeding 20 years, compared with bitumen felt that has only a few years depending on the exposure to UV light.
The roof requires preparation before spraying. If it is an old roof, attention is given to neutralising any rust and ensuring that the substrate is well attached. The manufacturer of the roof coating will probably have a range of products for the prep work.
Roof coatings are applied by high pressure airless spray equipment or "Roof Rigs" These are typically powered by a 16 HP petrol engine. Pneumatic airless pumps can also be used for this work which has the advantage of reducing the weight of the equipment on the roof.
Prior to spraying the roof coating, thermal insulation may be spray applied, usually as a polyurethane foam. This requires specialist 2-component airless equipment (eg Graco Foamcat) to pump and mix the components in the correct ratio and feed them separately to a special spray gun where they are mixed as they are sprayed.