A water flow reducing layer (WFRL) is a loose laid membrane used as part of inverted flat roof constructions. It is installed over the roof’s thermal insulation layer, and below the roof covering. A WFRL is a barrier to rainwater, reducing the volume of water that can reach the waterproofed roof deck. It also stops fines from entering the roof system.

The properties of a WFRL are defined in the technical guidance document ETAG 031. It should be a synthetic non-woven membrane that is water resistant, diffusion open, UV stable and rot resistant.

Crucially, a WFRL is not a waterproof layer. The membrane may perform well in independent testing, but some rainfall is always expected to reach the roof deck. That means design lambda values should be used for the insulation layer, and an appropriate rainwater cooling correction applied to inverted roof U-value calculations.

Why is a WFRL important?

Without a WFRL, more water reaches the waterproofing layer and increases the effect of rainwater cooling on the roof U-value calculation. A significantly thicker layer of insulation is required to compensate, increasing the depth of the roof build up and the cost of the inverted roof system.

Specifying WFRLs, and accounting for them accurately in U-value calculations for inverted roofs, is an essential part of ensuring that buildings achieve their intended performance once occupied and in service. Polyfoam XPS have been at the forefront of industry efforts to improve guidance around WFRL installation, leading to the creation of LRWA Guidance Notes 14 and 15.

Roofboard Extra and Roofboard Super are extruded polystyrene insulation products suitable for use as part of inverted roof build-ups, together with Polyfoam Slimline Zero membrane performing the function of WFRL. For any technical queries or U-value calculations relating to inverted roofs, please contact us.

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