WFRL and the importance of correct installation

The installation of the Water Flow Reducing Layer in inverted roof insulation systems has been highlighted as a concern. It has been suggested that poor installation could significantly affect U-Values achieved. It is therefore important that the WFRL is installed correctly.

 A WFRL is a loose laid, rot resistant, UV stable, water impermeable (vapour permeable) membrane installed over inverted insulation to reduce the effect of rainwater cooling on the waterproofed deck and enabling thinner insulation to achieve the target U-value.

It is imperative that the inverted roof system itself is also designed and installed correctly. Back falls or negative falls should be avoided to ensure that water does not flow below the WFRL, reducing its effect on the overall thermal value. The roof should also be designed with drainage at both the WFRL level and the waterproofing level, with the WFRL turned down at drainage locations to allow water to flow to the outlets.

The Insulation and WFRL are loose laid. Only as much as can be ballasted should be installed at any one time, thus protecting the insulation and WFRL from wind uplift and foot traffic. 

The WFRL is laid over the insulation at right angles to the designed fall of the roof, starting at the bottom ensuring that rainwater does not track under the 300mm overlaps. Side and end overlaps should also be 300mm. Any cutting required should be done with a sharp knife or scissors. The WFRL should be turned up at penetrations and upstands, finishing above the roof surface. The WFRL should be “star cut” and turned up around soil pipes, with additional WFRL wrapped around the pipe to ensure continuity. 

Inverted Green Roofs and Blue Roofs carry additional design requirements which could affect the installation of the WFRL and information should be sought from the system provider.

On completion, the contractor and the waterproofing system holder should ensure that the WFRL has been installed correctly and care should be taken during maintenance that it is also reinstated correctly.

Installed correctly the WFRL will achieve the thermal benefits for the lifetime of the system.