Tapered insulation

The designed slope built into a nominally flat roof so that rainwater drains towards gutters or outlets. Falls may be formed by the structure, screeds, firring pieces or tapered insulation, and design gradients are normally set steeper than the minimum finished fall to allow for construction tolerances and deflection.

A roof construction in which the thermal insulation is placed above the structural deck, keeping the deck and supporting structure at close to internal temperature. A vapour control layer is normally installed on the warm side of the insulation to limit the risk of interstitial condensation.

A measure of thermal transmittance, the rate of heat flow through unit area of a building element per unit temperature difference between inside and outside, expressed in watts per square metre kelvin. Lower U-values indicate better insulation, and limiting values for roofs and other elements are set by national building regulations.

A rigid closed-cell foam chemically related to polyurethane but with a higher proportion of isocyanurate linkages, which improves fire performance and thermal stability. It is widely used as the insulation core of flat roof boards and metal-faced cladding and roofing panels.