Flat roofing

Single-ply membranes have been popular for several years for both warm and inverted roof construction. Membranes usually comprise a reinforcement layer of polyester or fibreglass

matting or fibres sandwiched between 2 layers of flexible polymer.

Broadly there are 2 generic types (aside from the bitumen-based materials): thermosets (TS) and thermosplastics (TP). Thermosets are not modified by heating and therefore use bonded joints. A typical example of a thermoset single-ply membrane would be EPDM (ethylene propylene diene monomer). Thermoplastic materials are modified by heat and can be welded. The family of thermoplastic sheets includes thermoplastic polyolefins (TPOs), now a particularly popular material. The terminology is confusing because 'olefins' is not specific, it simply describes any molecule containing carbon-carbon double bonds or 'alkenes'.

TPOs do not contain plasticisers, they exhibit similar UV resistance and heat resistance as EPDM but are weldable. Some forms are fairly difficult to install as they tend not to hold shape and do not relax quickly.

The durability and life expectancy of TPO roofing materials is very good, but in common with many roof systems the success of the system relies heavily on good workmanship and the selection of materials that are appropriate for the particular circumstances in which they will be used.

According to a study conducted by NRC and others, membranes reinforced with continuous polyester fibre scrim had significantly higher tensile breaking strength than those reinforced with random short glass fibre mat. (Liu, K.K.Y., Paroli, R.M. and Smith, T.L., Blistering in SBS Polymer Modified Bituminous Roofs, Construction Technology Update No. 38, June 2000, National Research Council Canada.) In North America, experience with TPOs appears to have been mixed; the product was marketed as a cheaper alternative to PVC roofing, but its use has been marked by seam failures and age-related embrittlement. However, the products used in North America tend to be thinner than their European counterparts, where there is greater experience in use. (Russo, M., TPO problems require a closer look, Roof Consultants Institute, Raleigh, USA, 1 July 2001.)

Some TPOs are reported to fade over time, but the most likely failure mode would probably be in detailing or welded joints. Precise temperature control is needed to ensure proper fusion. Too cold and an improper joint will be formed: too hot and the material may burn through. Since large scale welds are made by machine, the most likely problem areas will be at the start and end of a run.

In very cold weather conditions, the material may become brittle and suffer tensile failure. However, for the most part a review of manufacturer's data should enable selection to be made based on the likely glass transition temperature (Tg).