For architects and specifiers looking to include an inverted flat roof as part of a construction project, ETAG 031 is a document that is very likely to be referenced in technical literature for inverted roof solutions. But what is ETAG 031, and how much do you need to know about it to help you successfully specify an inverted roof?

An ETAG, or European technical approval guidelines, provides a basis on which certain construction products can be assessed. A formal assessment body can use an ETAG to issue a European technical assessment (ETA). ETAs are a route by which products can be conformity assessed (i.e. CE marked) when no harmonised standard exists for that product.

What parts of an inverted roof does ETAG 031 cover?

The guidelines of ETAG 031 cover what is known as the inverted roof ‘kit’, a name given to the combination of thermal insulation and water flow reducing layer (WFRL).

The kit is installed, loose laid, on the waterproofed roof deck. The performance of the waterproofing is not impacted by the inverted roof system above it, or vice versa. Inverted roof waterproofing is therefore not part of ETAG 031’s guidelines.

The insulation and WFRL are secured by means of ballast such as gravel, paving slabs or a green roof system. ETAG 031 recognises these components as a “set generic specification” in terms of how they affect, or may be affected by, the performance of the kit. Together with the ‘kit’ these components make up what ETAG 031 calls the inverted roof ‘system’.

Are inverted roofs assessed to ETAG 031 in the UK?

Thermal insulation materials used in inverted roof systems are already the subject of harmonised standards. Extruded polystyrene, for example, is covered by EN 13164. This means they can be conformity assessed without the need for an ETA – and, indeed, must carry conformity assessment in order to comply with the Construction Products Regulations.

ETAG 031 refers to two insulation materials as being suitable for inverted roofs: extruded polystyrene (XPS) manufactured to EN 13164, or expanded polystyrene (EPS) manufactured to EN 13163. Both harmonised standards are referenced in ETAG 031, so an insulation material conforming to either standard is therefore considered suitable by ETAG 031.

Inverted roof kits are assessed at the national level for an Agrément certificate, such as a BBA certificate. Certification is made more straightforward thanks to the insulation being manufactured to a harmonised standard.

Certificates may not directly reference ETAG 031 – for example, the BBA certificate for Polyfoam XPS’s inverted roof kit does not mention it – but the guidelines are still important.

ETAG 031 and inverted roof performance

Arguably the most significant aspect of ETAG 031 is its description of how to test and assess the performance of inverted roof kits in terms of the WFRL restricting the passage of rainwater through the insulation layer to the waterproofed deck.

Insulation manufacturers looking to obtain an Agrément certificate for their kit must have the WFRL tested in accordance with the method set out in ETAG 031. The results from this testing have a direct impact on every U-value calculation undertaken for an inverted roof featuring that kit.

This, then, is the area in which architects may take most interest in ETAG 031. An in-depth knowledge of ETAG 031’s contents is not essential. Being confident that the chosen insulation manufacturer is using the declared results of WFRL testing, however, and understanding that a project’s inverted U-value calculations have been properly adjusted, is important from a compliance and performance perspective.

About Polyfoam XPS

This blog post is simply an overview of what aspects of ETAG 031 are likely to be of most interest to design professionals and specifiers. For a much deeper exploration of inverted roof performance, standards, and Agrément certification, download Polyfoam XPS’s white paper.

Polyfoam XPS supplies an inverted roof kit comprising Polyfoam Roofboard Extra insulation, manufactured to EN 13164, and Polyfoam Slimline Zero membrane WFRL. Our solution is certified by the BBA.

Find out more about Polyfoam XPS and our complete range of products,  or learn more about current industry issues and product performance in our  blog.

Alternatively, to find out how we can help with flooring or roofing solutions for your project, contact us.