Underlay Usage

In lined buildings and dwellings, an absorbent permeable underlay is required under metal roofs. The same applies to direct-fixed steel wall cladding, but underlays used behind a drained cavity are not required to be absorbent.

For aesthetic reasons, a foil-faced (or white-faced) vapour check layer may be used in unlined commercial or industrial applications to reduce heat radiation from the roof cladding and provide enhancement of light. For insulated applications, foil is normally used as a vapour check under the insulation and an absorbent permeable underlay used above with a 20 mm gap from the roof cladding.

On non-residential dwellings, profiled roof-light sheeting running in continuous lengths from the ridge can have multiple skins to avoid condensation dripping from the sheeting or plastic sheet with a spacer can be used to lessen condensation. Roof underlays should not be laid continuously under translucent roof or wall cladding.