A water flow reducing layer (WFRL), is a loose laid membrane installed over inverted warm roof insulation. The performance of a WFRL is independently tested, and the result incorporated into the Agrément certificates of inverted roof solution providers.

It is not a waterproof layer, but acts as a barrier to severely reduce the volume of rainwater entering the roof’s insulation layer and reaching the waterproofed roof deck. The primary benefit of a WFRL is to reduce the rainwater cooling effect in thermal transmittance calculations, allowing target U-values to be met using thinner insulation solutions.

What does BS 6229 say about water flow reducing layers?

At the end of 2018, a heavily-revised version of BS 6229, the code of practice for ‘flat roofs with continuously supported flexible waterproof coverings’, was published. The standard’s updated guidance referred to “imperfections” occurring in the WFRL when installed on site.

A separate post goes into more detail about our concerns with the way this guidance has been written.

Putting to one side issues over the stance adopted by the standard towards inverted roof construction and water control layers, one thing the construction industry can unite on is the general need to address issues of construction quality. Otherwise, the performance gap – the discrepancy between building performance as-designed and building performance as-built – will never be closed.

Polyfoam XPS is not aware of widespread issues with the way WFRLs are installed. Our colleagues in the inverted roof sector have expressed similar surprise at the revised standard. Nevertheless, in an effort to improve the availability and accessibility of information, a blog post on WFRL installation seemed like a worthwhile idea.

How to install a water flow reducing layer

The structural roof deck should be fully waterproofed in accordance with the manufacturer’s design guidance and details. Insulation suitable for inverted roof applications – such as the Polyfoam XPS Roofboard range – should then be loose laid in a brick bond pattern, again in accordance with instructions and details provided.

Water control layer installation is as follows:

    • Lay the WFRL – e.g. Polyfoam Slimline Zero membrane – over the roof insulation, at right angles to the slope of the roof.
    • Make sure all side and end overlaps are a minimum of 300mm, and that end overlaps are in the direction of the downward slope.
    • Turn up the membrane at upstands and penetrations so it finishes above the surface of the ballast.

As both the insulation and the WFRL are loose laid, it is recommended to install only as much of the system as can be ballasted at the time. BS 6229:2018 contends that post-construction damage is one cause of poor WFRL performance, so another reason for laying ballast on an advancing front is to protect the membrane from site traffic as materials are moved across the roof.

Getting roof falls and drainage right with a water flow reducing layer

Correct drainage and roof falls are critical to the success of a water flow reducing layer installation. Back falls should never be allowed to occur on a roof, and are a potential reason for worse-than-expected performance of a WFRL. If a back fall causes rainwater flow across the roof in the opposite direction to the intended fall, it may flow under an end lap.

Because a WFRL is an effective (albeit not waterproof) barrier to rainwater, the roof should be designed with dual-level drainage. Detail rainwater outlets so that drainage takes place at both the WRFL level and the waterproofed deck level. As part of that detailing, ensure the WFRL is turned down at outlets.

The Polyfoam XPS BBA certificate for roofing contains further guidance on our solution for inverted warm flat roofs. Up-to-date guidance on the best practice for the installation of water flow reducing layers is also available, in Guidance Note No 14, via the LRWA website or you can contact us with any questions.

Published in June 2019 / Updated March 2020