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All You Need to Know About: WATER FLOW REDUCING LAYERS... In the first of a new series of articles from Polyfoam XPS, Rob Firman (pictured) looks at water flow reducing layers (WFRLs) – an essential part of inverted flat roof construction. Designers and installers of flat roofing solutions are often expected to be experts in the products they’re using, as well as how to use them. Such expectations can be unrealistic when the marketplace is full of conflicting advice concerning what you ‘should’ know about every available system or component. In this new series of articles, I will look at the different parts of flat roofing design and construction and explain what contractors need to know. This month, I will focus on water flow reducing layers (WFRLs), which are an essential part of inverted flat roof construction... What is the role of a WFRL on an inverted roof? An inverted flat roof features the waterproofing layer installed directly on the structural deck. Above the waterproofed deck, an inverted roof system, or ‘kit’, is loose laid and secured by ballast. The system is a combination of thermal insulation and a membrane, called a water flow reducing layer. The ballast is typically gravel or paving slabs. This arrangement allows a roof to be waterproofed sooner and protects the waterproofing layer from UV exposure and freeze thaw actions. Why is it important to install a WFRL correctly? Because of the location of the waterproofing below the thermal insulation, WFRLs were developed as a way to limit the ingress of moisture into inverted roof systems. The membrane is loose laid over the thermal insulation layer and acts as a barrier to significantly reduce the volume of rainwater reaching the waterproofing. The WFRL is not a waterproof layer and no standards or guidance relating to inverted roofs suggest otherwise. Some moisture is still expected to reach the waterproofing layer and relevant corrections are applied to U-value calculations accordingly. If you have to obtain U-value calculations for an inverted roof construction, there are two adjustments that should be applied: • The thermal conductivity of the insulation should be adjusted by a moisture correction factor. • A rainwater cooling correction should be applied to the whole calculation to give the final U-value. Both adjustments are important, but the accuracy of the rainwater cooling correction is particularly dependent on a high quality WFRL installation being achieved on site. A poorly installed WFRL, or no WFRL at all, would lead to a much greater level of rainwater cooling occurring on the completed roof. How should a WFRL be installed? 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 extruded polystyrene (XPS) – should then be loose laid in a brick bond pattern, again in accordance with instructions and details provided. A summary of WFRL installation: • Lay the WFRL 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 “WFRLs were developed as a way to limit the ingress of moisture into inverted roof systems” laid, it is recommended to install only as much of the system as can be ballasted at the time. Further guidance is available in LRWA Guidance Note 14. What are common issues with WFRL installation? BS 6229:2018 refers to post-construction damage as being 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. Correct drainage and roof falls are a topic for discussion all on their own, but they are critical to the success of a WFRL 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 barrier to rainwater, but not waterproof, the inverted roof should be designed with dual-level drainage. Rainwater outlets are detailed to drain at both WFRL level and waterproofing level, and should be installed accordingly. Part of that correct installation is ensuring that the WFRL is turned down at outlets. Contact Polyfoam XPS www.polyfoamxps.co.uk @PolyfoamXPS 60 TC MAY <strong>2021</strong>
NON-COMBUSTIBL E EXTERNAL UPSTA AND BOARD ROCKFACE A2 www.q uantuminsulation.com A2 MAY <strong>2021</strong> TC 61
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May 2021

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