The volume of rainwater that reaches the waterproofed deck of an inverted roof depends on two things. First is the building’s location, and the average rate of rainfall that location experiences during the heating season. The second is the result achieved from testing the thermal insulation and water flow reducing layer (WFRL) in combination to see how effective a barrier to rainfall the system is.

These two values are used as part of the rainwater cooling correction applied to inverted roof U-value calculations. The average rate of rainfall is denoted by the value p and based on location-specific data from the Met Office.

A building in the south-east of England, for example, experiences significantly less average rainfall than one in the Outer Hebrides. Inputting accurate rainfall data for the location improves the accuracy of an inverted roof U-value calculation.

How is an inverted roof tested for rainwater penetration?

The technical guidance document ETAG 031 describes how thermal insulation and a WFRL should be tested in combination to establish what fraction of the rainwater volume denoted by p will reach the waterproofed roof deck. This fraction is denoted by the value f.

Test results are usually extremely good, but a WFRL should never be assumed to be waterproof. The guidance of BBA Bulletin No.4 – which is referenced in the code of practice BS 6229:2018 – says that a minimum fraction (and therefore f value) of 0.025 should be used to represent 2.5% of the rainwater reaching the waterproofing layer.

As of the first quarter of 2020, an insulation committee set up by the Liquid Roofing and Waterproofing Association (LRWA) is working with the BBA on new test procedures that will enhance the accuracy of the value of f used in rainwater cooling correction calculations.

Polyfoam XPS is heavily involved with this work and we continue to update our blog and our calculation procedures in accordance with industry findings. For any queries regarding accurate U-value calculations for inverted roofs on your project, contact us and we will be happy to discuss current best practice.