ASTM have issued Standard 6878, a specification for thermoplastic polyolefin ( TPO )-based sheet roofing membranes. TPOs are typically blends of polypropylene with ethylene-propylene rubber, and retain flexibility at low temperatures without the addition of plasticisers. Membranes for roofing applications are available in widths up to 3.66 m, usually comprising a layer of reinforcing fabric between layers of TPO, having a total thickness of 1-2 mm. The membrane is usually installed using mechanical fasteners and plates along the edge, with adjoining sheets being overlapped and hot air welded. Alternatively, adhesive fixing may be used. TPO membranes comfortably comply with the ASTM-specified severe accelerated ageing tests, and white membranes substantially exceed the US Environmental Protection Agency's Energy Star performance levels. Being thermoplastic, TPOs are recyclable.