The simplest forms of flat roof are those of short span laid on loadbearing wall or beam and column frame structures. They consist of boarding on joists, or an in situ or precast reinforced concrete slab. The surface of these roofs must receive a waterproof covering, and where their inherent thermal insulation is inadequate, an insulating slab or board must be incorporated in the roof system. Solid lightweight precast reinforced concrete slabs are produced by manufacturers of foamed concrete products (Fig. 5c). Low-density structural concrete has good thermal insulation, depending on the thickness employed, and the units may be used spanning directly on to precast concrete beams, portal frames or monitor frames, or on to purlins, so forming a deck of high fire resistance and durability. If there is likely to be interstitial condensation within the porous concrete, a continuous vapour barrier in the form of a sprayed pigmented plastic skin may be applied to the underside. Differential deflection between adjacent units is prevented by running a cement mortar grout into dovetail grooves formed by rebates cast in the edges of adjacent units. A splay on the lower arrises reduces the risk of damage during handling and masks slight differences of soffit level. If falls can be arranged by inclination of the structural deck itself, the waterproof covering is applied direct to the upper surface, without screeding.
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