Flat roofing

In the late 1960s, aside from metal coverings, the choices facing a designer of a flat roof were generally fairly limited: either asphalt or built-up felt systems. Most roofs were of cold construction with minimal insulation while solar control was generally provided by chippings or promenade tiles.

Since then, much has changed. While warm roof systems are now almost universal, there are a bewildering number of different membrane options, all of which, if properly detailed, can deliver a good life expectancy. Defects in roof coverings are often due to inappropriate selection and use rather than a failure of the material itself (save for the expiration of normal life expectancy) with detailing accounting for a high proportion of failures. This subsection deals with the various types of covering in common use together with some of the defects to look out for. Common themes of workmanship and sensible design run throughout the various roof types.

Broadly there are 2 variations of flat roof:

• cold roofs (in which the insulation is located at ceiling level); and
• warm roofs (in which the insulation is located either immediately below the roof membrane or above it (inverted roof).

Earlier flat roofs tended to be of cold construction and were prone to condensation problems, although roof membranes such as asphalt performed well because the roof deck could act as a heat sink. With the gradual introduction of greater amounts of roof insulation, warm roofs became more popular. However, when insulation was located immediately below the membrane, the greater surface temperatures that developed would lead to quicker deterioration of the membrane. To avoid these problems inverted roofs became more popular. See Different types of roof construction.

Asphalt roofing performs well when it is protected against extremes of temperature. The material is thermoplastic and when heated tends to flow under gravity if unconstrained. At cold temperatures it becomes brittle and can be damaged under impact loads. Ponding on asphalt has not been shown to cause major problems but proper solar control measures are usually essential. See Mastic asphalt.

Built-up felt roofs (BUR) performed badly at one time, but the advent of polyester reinforcement brought a sea change in the industry with much greater flexibility and fatigue resistance than before. Attention to good detailing is needed to prevent damage to the covering from deck or substrate movement. Blistering due to entrapped air can be a difficulty but unless water-filled or greater than about 600mm, blisters are best left untouched.

Single ply membranes have become more popular and EPDM, TPO and PVC systems are common. EPDM needs adhesive bonded seams while TPO and PVC can be heat welded. Lap joints in these systems are all vulnerable if poorly executed. Loss of plasticiser can be a problem with some types, leading to embrittlement and cracking over time. See Thermoplastic polyolefin roofing.

Liquid membranes are also common. Hot applied systems are often used for new work but need solar protection. Cold applied systems often incorporate a polyester fleece for reinforcement and can be very effective, particularly where there is difficult detailing. However, adhesion failure arising from contamination of the substrate is a problem and the systems should never be used as a substitute for poor design and detailing. When used carefully and with compatible substrates, the use of liquid membranes for remedial work is often a good alternative to traditional roofing systems. See Liquid waterproofing systems.

In the 1900s the concrete flat roof and the system-built flat roof heralded a new outlook for the design and construction of housing. Unfortunately they left a poor impression on the buying public, as they failed repeatedly due to a lack of tolerance to thermal movement and the poor performance of early felts. See Defective flat roofs in the 1900s.