Flat roofing

PVC roofing is another thermoplastic material capable of being welded in much the same way as TPOs. Performance of the material is generally good, with a design life of 30 years being expected. It was first developed in the early 1970s but suffered from age embrittlement and loss of plasticisers. Today, these performance difficulties have been largely overcome by the provision of polyester or glass reinforcement and plasticisers that are more stable.

Hot air and solvent welding is possible; early membranes suffered lap failures as a result of problems with solvent welding, and although this method is still used, heat welding using an electric hot air gun is more popular.

As with all sheet materials, the success of the roof depends on many factors, not just the durability of the membrane. PVC sheets can be fully adhered, partially attached, or loosely laid and ballasted. They are also available with a non-woven polyester fabric backing, which acts as a cushion to reduce the risk of perforation due to sharp ridges or material on the surface of the substrate.

PVC membranes are damaged by contact with bitumen or coal-tar pitch, as plasticiser from the PVC migrates into the bitumen, reducing the flexibility of the sheet and promoting premature failure. For this reason, an isolating membrane is usually specified.

Defects in PVC roofing are likely to follow similar patterns to some of the defects listed under EPDM, with seam failure and contamination issues being reported by ASTM Committee 8.

Attempts have been made to seal the joints in this PVC roof with an unsuitable sealant material