Flat roofs have slopes less than 15 degrees, and ideally they shouldn’t be flat. In order to stop water pooling they should have a slope of at least 1 in 80 so that the rain runs off.
There are several different kinds of flat roof construction, and while modern flat roof constructions can be trouble free for more than 30 years, many older flat roofs develop problems much faster and start to leak which can lead to serious problems such as rotting timber, mould and damage to brickwork and plaster.
Flat Roof Types
There are several choices of flat roof systems. They consist generally of a timber deck and a waterproof coating. In the past pour and roll bitumen felt roofs were common. They were easily damaged and created many problems, often needing regular replacement. Fortunately modern flat roofs have moved on considerably since then. The most usual constructions are:
Torch on felt roofing
Usually have three layers: vapour control layer; 2 mm reinforced felt layer; and a 4 mm cap felt layer with either a mineral flack finish or plain bitumen ready to accept solar reflective paint. The felt layers are welded together using a large blowtorch. Typically such a roof would be guaranteed for 10 to 20 years and it is cheapest option. Such roofs can often be repaired and patched.
EPDM rubber roof
The full name is ethylene propylene-diene-terpolymer, quite a mouthful, but it is an excellent innovation that came across from the US. A major advantage is that often it can be installed in a simple layer without the need for any joins. Usually it is attached to the deck using adhesive. EPDM is light weight and tough, and if treated well can last from 25 to 50 years. Although more expensive than torch on felt roofing, it is still a highly affordable solution. It can look good, but only when professionally installed.
Fibreglass or GRP (glass reinforced plastic)
With this solution sheets of fibreglass are laminated to the roof deck using special resins with overlapping sheets. When partially cured a top coat of resin is applied and allowed to cure. With modern resins, the structure is strong and long lasting; you can expect one to last 30 years or longer. This tends to be the most expensive option, but it is both attractive and strong and the most damage resistant.