Standards relating to flat roof and green roof construction distinguish between different types of green roof. These are not the same distinctions we discussed in our introductory post on green roofs, relating to the type of planting. Instead, they relate to the roof in terms of foot traffic and the load that will be applied – and therefore what type of thermal insulation should be specified.

Typically the standards distinguish between green roofs – which are bracketed with untrafficked and pedestrian roofs – and roof gardens, which are usable leisure spaces subjected to higher levels of foot traffic (and which may also be described as roof terraces, though this terminology is not used in standards).

Does the type of green roof affect the U-value of a roof?

A frequently cited benefit of green roofs is the thermal insulating effect of the growing medium, but no U-value calculation convention recognises it. Like any ballast layer installed over the thermal insulation and water control layer, neither a green roof or roof garden build-up is considered to contribute any thermal performance to an inverted warm roof.

No adjustment therefore needs to be made to a U-value calculation. A thermal benefit may be claimed if verified by third party certification, but we are unaware of any such certified systems.

Also, if the proposed green roof construction requires an anti-root grade of waterproofing, there is no difference in thermal performance compared to its standard waterproofing counterpart.

The only aspect of a green roof proposal that would influence overall thermal performance is if the proposed loads, mainly from predicted foot traffic, are enough to require a stronger grade of insulation with a slightly higher thermal conductivity.

What standard specifies insulation performance for green roofs?

ETAG 031 is the European Technical Approval Guideline covering the specification of inverted roof ‘kits’ – the name given to the combination of thermal insulation and water control layer.

It describes specifications for both extruded polystyrene (XPS) and expanded polystyrene (EPS) generally. However, it cannot be used to assess EPS insulation for green roofs “due to the current limited history of use in green roofs and roof gardens”.

According to ETAG 031, XPS insulation with a CS(10\Y)300 designation (i.e. a compressive strength of 300 kPa declared in accordance with EN 13164) is suitable for both green roofs and roof gardens. Polyfoam Roofboard Extra offers a declared compressive strength of 300 kPa.

What compressive creep performance should XPS insulation for green roofs achieve?

The standard also categorises insulation on the basis of compressive creep performance. ETAG 031 gives minimum compressive creep performance declarations that XPS insulation should achieve for different roof types:

    • CC(2/1.5/25)50 for green roofs.
    • CC(2/1.5/50)100 for roof gardens.

These designations mean that compression should not exceed 1.5% for compressive creep, and 2% for total thickness reduction, after 25 year extrapolation under a declared stress of 50 kPa for green roofs, and after 50 year extrapolation (but assumed to occur over 25 years) under a declared stress of 100 kPa for roof gardens.

It is important to consult fully with a structural engineer on any roof design where foot traffic is a consideration, but even more so where the exact definition of what a roof garden is, and how much load it might impose on the roof build-up, is unclear.

Architects and specifiers need to be clear with structural engineers, thermal insulation manufacturers and roof system suppliers about their proposed designs. If the phrase ‘green roof’ is used to describe a build-up that will actually be more heavily trafficked than that terminology suggests, there could be a resulting under-specification of the thermal insulation.

Being clear about the intended use of the roof will help ensure the receipt of comprehensive advice from other parties involved. To discuss your green roof or roof garden project, contact us.

Published in August 2019.