Rob Firman at Polyfoam XPS looks at new recommendations for inverted roof buildups under the revised British Standard, and reflects on how specifiers’ decisions on roofing systems have ramifications through a building’s life.

The revised code of practice BS 6229:2025 (Flat roofs with continuously supported flexible waterproof coverings) seeks to reduce the gap between how flat roofs are designed in theory, and how they perform in reality. This is to be welcomed, especially as we continue to see inverted roof specifications 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 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 water flow reducing layer (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.

The note was not a formal recommendation, and agreement on whether it should be applied or not was never reached within the flat roofing 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 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.

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.

Where a 300 mm 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.

Design professionals’ influence

This may all sound more pertinent to contractors than design professionals. However, it is worth noting from the point of view of understanding how design and specification decisions carry through the construction and use phases of a building.

If the aim of BS 6229:2025 is to address performance gap issues, then we should not forget the role that design professionals play in influencing what happens on site.

Knowing installation techniques for WFRLs have become more robust improves certainty for everyone involved in design and specification, while also eliminating BS 6229:2018 confusion.

It is important to understand that different WFRL membranes have different properties and characteristics. Not all membranes are the same, so engaging with inverted roof system suppliers during design and specification phases offers the opportunity to understand what qualities can best support successful installation.

A typical design life for many buildings is 60 years, with 100 or even 120 years sometimes the aim. If a better standard of WFRL installation is evidenced during construction, will that be maintained if maintenance work disrupts the membrane? If invasive work is required to inspect, maintain or repair some aspect of the roof, it is relatively easy to take up the inverted roof system. But will it be reinstated to the condition in which it was found?

Making and recording specification decisions with construction and maintenance in mind can allow the building owner/occupier to make decisions that align with the original intent.

Rob Firman is technical and specification manager at Polyfoam XPS