Torchflex TP-180-SF-Base (3.5) Heat Welded Base Sheet consists of a tough non-woven reinforced polyester mat strengthened with select glass fiber strands that is coated with SBS polymers and premium quality asphalt both top and bottom to a thickness of approximately 3.5 mm (138 mils). The product’s top surface is sanded to prevent sticking in the roll during application and the underside is covered with a thin micro-perforated film surface that conveniently disappears upon heat welding to the substrate. Torchflex TP-180-SF-Base (3.5) can also be used as a vapour retarder.