Built-up roofing consists of two or more layers of bitumen sheets bonded together with self-adhesive or hot bitumen. Bitumen is the residual material produced after the removal by distillation of all volatile products from crude oil. The properties of bitumen are modified by controlled oxidation, which produces a more rubbery material suitable for roofing work. In the manufacture of bitumen sheets, the continuous base layer of organic fibres, glass fibre or bitumen-saturated polyester is passed through molten oxidised bitumen containing inert filler; the material is then rolled to the required thickness. The bitumen sheet is coated with sand to prevent adhesion within the roll or with mineral chippings to produce the required finish. A range of thicknesses is available and for ease
of recognition the classes relating to the base fibre layer are colour coded along one edge of the rolls.
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