How do changes in BS 6229:2025 change inverted roof installation?
The publication of BS 6229:2025 has introduced new recommendations for laying water flow reducing layers (WFRLs) as part of inverted roof build-ups. Among wide-ranging and significant revisions to the standard’s content, this highly specific area of guidance seeks to ensure the WFRL remains in place while installing the ballast layer that follows.
New recommendations aimed at inverted roofs are part of a broader change of approach in the updated standard. Overall, it can be said that BS 6229:2025 Flat roofs with continuously supported flexible waterproof coverings – Code of practice seeks to reduce the gap between how flat roofs are designed in theory and how they perform in reality.
This is similar to the approach adopted in BS 5250:2021 Management of moisture in buildings. Code of practice, and is to be welcomed.
Why were changes to inverted roof installation needed in BS 6229?
When the standard was last updated, BS 6229:2018 introduced a controversial guidance note for inverted roofs. While the note did not constitute a formal recommendation, this did not prevent confusion over whether it should be applied or not.
This is not the place for a detailed explanation of the note, but some understanding of it is helpful to put into context the revisions made for 2025.
An inverted roof is so called because the waterproofing layer sits below the thermal insulation layer and ballast finish. Some water is expected to penetrate the ballast and insulation layers, and drain from the waterproofing layer.
To reduce the amount of water reaching the insulation layer, a WFRL can be added above the 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 waterproofing 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.
That note has been removed for the publication of BS 6229:2025, bringing the debate about its application to an end once and for all.
What does BS 6229:2025 recommend for WFRL installations?
Above the waterproofing 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.
This concern is especially valid considering that different WFRL membranes have different properties and characteristics (which we discuss in a post about the qualities of a WFRL), and is what BS 6229:2025 has tried to address.
Manufacturers and suppliers of inverted roof kits (the name used in ETAG 031 to describe the combination of thermal insulation and WFRL) require the WFRL to be laid with a 300mm lap at the joints between separate runs of membrane.
Where a 300mm lap cannot be achieved during installation, BS 6229:2025 recommends that joints be taped and sealed. This is not a measure to make the layer more resistant to water penetration, but to make it less likely that the WFRL moves during installation. If the membrane is more likely to remain in place, then it is more likely to deliver its intended performance.
“All of the test results and correction factors used in U-value calculations for inverted roofs should therefore be more representative of the amount of water actually reaching the waterproofing layer,” said Rob Firman, Technical and Specification Manager at Polyfoam XPS. “That improves certainty for everyone involved in the design and specification of inverted roof build-ups, while eliminating the confusion caused by the note introduced in 2018.”
BS 6229:2025 has also published separate recommendations about WFRL installation for when inverted roofs are used as part of a blue roof arrangement. We will publish another blog post dealing with that subject in due course.
What to do next
Designers or specifiers reading this post might not notice much impact from these very specific new recommendations in BS 6229:2025. If you have direct involvement with specialist contractors installing inverted roof systems then you can check they are aware of the updated standard and applying its new recommendations.
Otherwise, the important thing is to recognise that inverted roofs should now be installed in a way that makes it more likely they perform their intended function across the entire roof.
“From the experience of running my family’s roofing contractor business, I know how important it is that specifiers make decisions about thermal insulation with installers in mind,” said Gary Ferguson, Polyfoam XPS’s Business Development Manager (North & Scotland). “They tend to have much less control over components like a WFRL – but that doesn’t stop this being a useful topic to be aware of, especially as it can be a good introduction to the benefits of inverted roofs more generally.”
If you would like to discuss whether an inverted roof build-up might be right for a current or future project, contact us.
For contractors reading this blog post, it’s important to emphasise that BS 6229:2025 represents a significant revision over its 2018 counterpart. The recommendation discussed in this post is just a single change among many, and a deeper knowledge of the whole standard is needed. Nevertheless, we hope this has served as a useful introduction to something that needs to be considered when planning and executing inverted roof installation work.
To make sure you stay up to date with our latest blog posts, subscribe to our monthly email newsletter, The Build-Up, or follow the Polyfoam XPS LinkedIn page. Extensive design guidance on inverted roofs is available in The Polyfoam Orange Book. Finally, if you found this blog post useful, please consider emailing it to a colleague or sharing it so that others might benefit from it.