Thermal insulation, to provide the necessary roof U-value, is bonded with hot bitumen. A wide variety of insulation boards or blocks including compressed cork, high-density mineral wool, fibreboard, perlite, cellular foamed glass, high-density extruded polystyrene and polyisocyanurate are suitable, although where insufficient rigidity is afforded by the insulation material, or if it would be affected by heat during application of the hot asphalt, it must be overlaid with firm heat-resistant boards to prevent damage during maintenance or construction. Where falls are not provided by the structure, the insulation may be set appropriately, providing that the thinnest section gives the required thermal properties. A separating layer of loose-laid geotextile material or Type 4A sheathing felt (BS 747: 2000) is then applied to allow differential thermal movement between the decking system and the mastic asphalt waterproof finish.

Fig. 6.5 Typical mastic asphalt roofing system
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