Asbestos fibre may be found in roofing felts, flashing tapes and damp proof courses. This is sometimes in the form of asbestos paper in bitumen matrix. These materials may become brittle with age but during normal use they do not present a hazard. Asbestos mixed with bitumen or bitumen reinforced with asbestos paper was sometimes used as a coating for corrugated steel. This was used as roofing and wall cladding in warehouses and factories. The asbestos is firmly bound but may be released if the bitumen is burned off.