The application of roofing asphalt should be in accordance with BS 8218 and BS 8000: Part 4.
FLAT ROOFS UP TO 10°
Roofing asphalt is normally laid in two coats, breaking joint, to a nominal thickness of 20mm on an underlay of black sheathing felt laid loose with 50mm lapped joints.
Where thermal insulation is laid beneath the weather proofing it is recommended that the roof pitch does not exceed 5°.
SLOPES OVER 10° AND VERTICAL SURFACES (EXCLUDING SKIRTINGS)
Roofing Asphalt laid on concrete or screeded sub-structures of 10° – 30° slope is applied in two coats to a nominal thickness of 20mm direct to the concrete.
For vertical work over 300mm and slopes over 30° the asphalt is applied in three coats, the first coat being applied very thinly with a steel trowel or small wooden float. A further two coats are then applied, breaking joint, to give a nominal thickness of 20mm. For these applications the concrete must be left with a roughened surface to form a key for the asphalt.
In the case of sloping timber surfaces over 10° a layer of black sheathing felt is first nailed to the timber. Bitumen primed expanded metal lathing is then fixed at 150mm centres with galvanized clout nails or staples to form a key for the asphalt which is applied in three coats to a nominal thickness of 20mm.
With the exception of vertical and steeply sloping work, clean sharp sand should be rubbed evenly into the finished surface of the asphalt with a wooden float while it is still hot. This rubbing breaks up the skin of bitumen brought to the surface at the time of application. Gradual crazing of the surface due to the action of the sun is minimized by sand rubbing in this way.