Single-ply roofing systems consist of a continuous membrane usually between 1 and 3 mm thick, covering any form of flat or pitched roof As water – proofing is reliant on the single membrane, a high quality of workmanship is required, which is normally provided by the specialist installer. In refurbishment work where the substrate may be rough, a polyester fleece may be used to prevent mechanical damage to the membrane from below. Life expectancies are typically quoted as 25 years. The wide range of mem – brane materials used may generally be categorised into thermoplastic and elastomeric products. In many cases the single-layer membrane is itself a laminate, incorporating either glassfibre or polyester to improve strength and fatigue resistance or dimensional stability, respectively. Both thermoplastic and elastomeric products are resistant to ageing under the severe con ditions of exposure on roofs. Fixings offered by the proprietary systems include fully bonded, partially bonded, mechanically fixed and loose laid with either ballast or concrete slabs. Joints are lapped and either heat or solvent welded, usually with tetrahydro – furan (THF). A final seal of the plastic in solvent may be applied to the joint edge after the lap joint has been checked for leaks. Most manufacturers provide a range of purpose-made accessories such as preformed corners, rainwater outlet sleeves and fixings for lightning protection. The standard BS EN 13956: 2012 Annex E lists an extensive range of flexible sheet materials (plastics, rubbers and thermoplastic rubbers) used for roofing systems across Europe, but the market in the UK for non-bitumen-based products is dominated by plasticised PVC and EPDM.