98
INSULATION
Inverting perceptions on roofi ng design decisions
Rob Firman at Polyfoam XPS looks at new recommendations for inverted roof buildups under the revised British Standard, and refl ects on how specifi ers’ decisions on roofi ng systems have ramifi cations through a building’s life.
Having the knowledge that installation techniques for WFRLs have become more robust improves certainty
T
he revised code of practice BS 6229:2025 (Flat roofs with continuously supported fl exible waterproof coverings) seeks to reduce the gap between how fl at roofs are designed in theory, and how they perform in reality. This is to be welcomed, especially as we continue to see inverted roof specifi cations increase from year to year.
Before we get into detail about what the 2025 edition of BS 6229 says about inverted roofs, it is worth revisiting what happened when the 2018 version of BS 6229 introduced a controversial and confusing guidance note for inverted roofs. An inverted roof is so called because the waterproofi ng layer sits below the thermal insulation layer and ballast fi nish. Some water is expected to penetrate the ballast and insulation layers and drain from the waterproofi ng layer. To reduce the amount of water reaching the insulation layer, a water fl ow reducing layer (WFRL) can be added above the
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insulation and below the ballast. It is not a waterproof layer, but inverted roof test results and corrections to inverted roof U-value calculations show WFRLs to be important and an effective addition. BS 6229:2018 posited that more water reaches the waterproofi ng layer than is predicted in theory. It featured a note saying that the thermal insulation layer could be increased in thickness by 10% to help address additional heat loss. The note was not a formal recommendation, and agreement on whether it should be applied or not was never reached within the fl at roofi ng sector. For the publication of BS 6229:2025, the note has been removed – bringing any debate about its application to an end once and for all. To try and reduce potential inverted roof performance gaps, BS 6229:2025 has dispensed with additional corrections to calculated insulation thicknesses. Instead, its focus is on practical considerations that should ensure WFRLs deliver their tested performance.
One new recommendation in particular seeks to address scenarios where a WFRL might move during the roof construction. Above the waterproofi ng layer, inverted roof systems are loose laid. They rely on the weight of the ballast layer to keep the thermal insulation and WFRL in place. However, installing the system on an advancing front, there is concern that the relatively lightweight WFRL membrane could move during installation. Manufacturers and suppliers of inverted roof kits (the name that describes the combination of thermal insulation and WFRL) require the WFRL to be laid with a 300 mm lap at the joints between separate runs of membrane.
ADF APRIL/MAY 2026
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