WHAT IS THE FIRE PERFORMANCE OF XPS INSULATION IN FLAT ROOFS?

As one of the two main areas of building design and construction where extruded polystyrene (XPS) insulation is commonly specified, it’s useful to understand how flat roof constructions are viewed by national building regulations when it comes to fire safety.

A separate blog post deals with the difference between reaction to fire and fire resistance generally, while we have also looked in some depth at the fire performance of XPS insulation in ground floor and basement build-ups. For information about the impact of the December 2018 revisions to the Building Regulations in England, check out our post on AMENDMENTS TO BUILDING REGULATIONS FOR FIRE SAFETY.

Is a flat roof a loadbearing element of structure?

Fire safety regulations do not generally class roofs as ‘loadbearing elements of structure’. Unlike the walls, a roof does not usually take the weight of other parts of the building – unless it performs ‘the function of a floor’, such as forming part of an escape route or being used for parking vehicles.

In those specific situations, where an inverted roof with XPS is commonly specified because of the extra load imposed, the roof should have a minimum fire resistance as specified by the regulations. For a roof forming part of a means of escape, the typical standard is 30 minutes when measured from the underside (putting the onus on ceiling and deck specification).

For a roof acting as, say, a parking deck, the performance requirement is much more dependent on the variables that affect how a fire might develop in the specific building – things like building height and occupancy, and whether the building is sprinklered or not.

How is resistance to fire spread declared for roofs?

National building regulations are concerned with the potential for fire to spread from one building to another, and across the external walls and roofs of buildings.

A roof’s resistance to external fire exposure, in terms of fire spread across the surface and penetration through the construction, is classified in accordance with EN 13501-5. A roof can be rated one of the following, from best performance to worst:

    • BROOF(t4)
    • CROOF(t4)
    • DROOF(t4)
    • EROOF(t4)
    • FROOF(t4)

EN 13501-1 refers to four separate roof tests, detailed in ENV 1187. The (t4) in the above classifications refers to the use of test 4, which is the only one sufficiently rigorous to demonstrate compliance with UK fire safety requirements. The performance rating of a roof typically dictates how far from a boundary or another building the particular construction may be used.

Like reaction to fire classifications, the external fire exposure of roofs used to be measured under national tests. While the European classifications should be the norm, approximate national class equivalents, when tested to BS 476-3, are as follows:

    • BROOF(t4) – AA, AB or AC
    • CROOF(t4) – BA, BB or BC
    • DROOF(t4) – CA, CB or CC
    • EROOF(t4) – AD, BD or CD
    • FROOF(t4) – DA, DB, DC or DD

In Scotland, the terms low, medium and high vulnerability are also used.

Restricting fire spread across an inverted flat roof

ETAG 031 is a European technical assessment guidance document which defines the criteria against which inverted roof ‘kits’ (thermal insulation plus water control layer) are assessed.

It points readers toward a list of roof coverings that meet the necessary standard for fire spread without needing further testing. Among the options are kits fully covered by one of the following inorganic coverings:

    • Loose laid gravel at least 50mm thick, or with a mass greater than 80 kg/m2 (subject to maximum and minimum aggregate sizes, to resist wind scour).
    • A sand cement screed at least 30mm thick.
    • Cast stone or mineral slabs at least 40mm thick.

These inorganic coverings feature in the majority of specifications, and drive the design of most inverted roof systems. Roof coverings not listed in the annex should be tested to ENV 1187 and classified to EN 13501-5.

Fire performance of (inverted) green roofs

Green roof designs should follow guidance issued by the Green Roof Organisation (GRO), featuring details of fire testing. The advice includes the correct use of fire breaks and non-combustible growing medium, and employing sufficient irrigation and maintenance to guard against a build-up of dry vegetation.

Fire performance of conventional warm roof constructions

An inverted roof imposes performance requirements on the roof covering (ballast, paving etc.). By contrast, a conventional warm roof imposes requirements on the waterproofing. Occasionally, extruded polystyrene insulation boards are specified in warm roof constructions. The external fire exposure of a warm roof is tested to the standards described above, but the onus is usually on the waterproofing manufacturer to comment on fire performance.

Whatever the type of roof construction, we continue to encounter a lack of confidence, awareness and understanding about how individual materials work together in complete build-ups. While this post is not intended to be a comprehensive guide to flat roof fire performance, we hope it is useful in summarising the key criteria and terms, and allowing specifiers to identify the best solution for the proposed building design. 

For more on the use of Polyfoam XPS products in different types of flat roof construction, visit our flat roof application pages. Section 4.9 of BS6229:2018 also provides useful guidance on the topic of fire safety in flat roofs. If those don’t answer your question, then contact us with any questions and we will get back to you.

Published June 2019.

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