A revised version of BS 6229, the code of practice for ‘flat roofs with continuously supported flexible waterproofing coverings’, came into effect on 30th November 2018. As part of the wide-ranging update, the standard’s guidance about inverted roof construction was amended.

For more information about what BS 6229 covers generally, we have written a separate blog post.

BS 6229 guidance about the thermal performance of inverted flat roofs

In addition to a general description of inverted roof construction, paragraph is the section of most interest in terms of inverted roof construction. It describes how the thermal performance of an inverted roof should be established to include for the cooling effect of rainwater that penetrates the insulation layer and reaches the waterproofing.

This procedure should be familiar to anybody who regularly works with inverted roof insulation manufacturers or system suppliers. Thermal transmittance (U-value) is calculated following the method in BS EN ISO 6946, using the design thermal conductivity of the insulation and rainwater cooling correction, as specified in the accompanying guidance of ETAG 031-1, BRE Report BR 443, and BBA Information Bulletin No. 4.

Furthermore, the drainage factor, f, can be obtained from third-party testing and certification if available – such as Polyfoam XPS’s BBA certificate for flat roofing.

What has changed in BS 6229’s guidance on inverted roofs?

The 2018 revision features a supplementary note to paragraph It says that imperfections occur in the water flow reducing layer installed over the inverted roof insulation, due to poor workmanship, poor detailing, or post-construction damage, increasing the volume of water likely to reach the waterproofing layer.

In turn, that increases the water cooling effect on the insulation and worsens the in-service thermal performance of the roof, beyond what was anticipated at design stage. To compensate, the supplementary note suggests increasing the thickness of the insulation layer by 10% until such a time as further evidence and performance testing is available.

What are the issues with this revised inverted roof guidance?

Polyfoam XPS has a number of issues with this supplementary note in BS 6229: 2018.

The Standard’s foreword is clear that “notes give references and additional information that are important but do not form part of the recommendations”, but readers may not be aware of that. The inclusion of this note could lead people to wrongly assume their designs and installations do not accord with current best practice, even though the correct calculation procedures have been followed.

Standards are typically written assuming a good level of workmanship. Workmanship is a requirement of national building regulations, and it is impossible to make provisions for every scenario of something being installed poorly. It is unusual to see a comment included in a standard which assumes poor workmanship, for which evidence has not been gathered.

As a manufacturer supplying inverted roof insulation and water control layers, Polyfoam XPS is not aware of widespread workmanship issues. If the committee responsible for BS 6229 has evidence to the contrary then industry – including manufacturers like Polyfoam XPS – should be given the opportunity to respond and address those issues.

Instead, the suggestion of a 10% increase in insulation thickness feels arbitrary. If changes to insulation thicknesses are necessary then they should have been included in the recommendations upon the completion of evidence gathering and further testing – not as a potentially scare-mongering supplementary note in advance of any such exercise.

Mechanisms exist within the calculation method to achieve a similar level of caution, such as using an increased value for the drainage factor, f. It is not clear why the supplementary note, if it had to be written at all, did not suggest this, rather than an arbitrary percentage increase in insulation thickness.


If clients and customers are concerned about the quality of workmanship then we advise them to speak to their contractor. However, if they wish to explore a compensatory factor in their U-value calculations then we are happy to work with them to discuss a measurable level of caution.

Otherwise, the recommendations of BS 6229: 2018 are clear. The current method for calculating the thermal performance of inverted roofs is valid and does not require any change in current practice. Polyfoam XPS will therefore continue to abide by all existing calculation standards and guidance.

The LRWA have published further guidance on the topic and it can be found on the LRWA website. To discuss inverted roof U-value calculations in more detail, contact our technical helpline on 01429 855120, or email

Published February 2019 / Updated March 2020