A water flow reducing layer (WFRL) is not a waterproof layer, and technical guidance for flat roofing is consistent in reinforcing this. It is therefore not appropriate to design an inverted flat roof on the assumption that no rainwater will be able to reach the waterproofing layer.
ETAG 031 is a guidance document that describes the technical characteristics of an inverted roof system, comprising thermal insulation and WFRL. It says that a membrane used as a WFRL should be water resistant, rather than waterproof, and diffusion open to allow the passage of moisture vapour from inside to outside.
What is the advice of the flat roofing code of practice?
BS 6229:2018 carries weight as the relevant code of practice for inverted roof design and construction. It references BBA Bulletin No.4, which advises that a WFRL should not be assumed to be waterproof.
The publication of the revised BS 6229 in late 2018, and the guidance on inverted roof build-ups contained within it, have led to the inverted roofing sector working together to review and enhance the ETAG 031 test method, in conjunction with the BBA.
At the time of writing in March 2020, that work is ongoing. Stay up to date with its progress via the Polyfoam XPS blog, and communications from the Liquid Roofing and Waterproofing Association (LRWA). For accurate inverted roof U-value calculations based on up-to-date guidance, including using design lambdas and appropriate correction factors, contact Polyfoam XPS for technical support.