HOW DOES COMPARTMENTATION IN A BUILDING IMPACT INVERTED ROOF DESIGN?

A fire usually starts inside a building, so internal fire spread is as critical a part of fire safety regulations as external fire spread. It’s not enough to think only about the performance of a roof’s external covering, it’s also important to consider its resistance from the underside.

Internal fire spread is split into two areas: linings and structure. The internal ceiling or underside of the roof deck needs to meet the requirements for linings, so has little impact on the roof insulation specification above the deck.

How does compartmentation help the fire resistance of the structure?

We examine the role of roof coverings in protecting the roof structure from external fire spread in this blog post. Internal fire spread is tackled by designing the building layout to restrict a fire to discrete compartments of the building, making it more likely that fire and rescue services can tackle the blaze and bring it under control.

In addition, concealed cavities should not allow the uncontrolled and unseen spread of fire through a building.

A compartment wall divides two separate compartments and should provide a certain level of fire resistance to help contain a fire. Where a compartment wall meets the underside of the roof, it imposes certain requirements on the roof covering.

How should I design an inverted flat roof over a compartment wall?

Approved Document B in England says: “If a fire penetrates a roof near a compartment wall there is a risk that it will spread over the roof to the adjoining compartment.”

One solution is to carry the compartment wall up through the roof structure to create a definitive break, but this is often undesirable or impractical. The preferred option is for the roof construction to continue uninterrupted over the compartment wall, but this can be the subject of some confusion because of statements made in some BBA certificates for inverted roof systems.

The Polyfoam XPS BBA certificate, for example, says at paragraph 9.4: “The system should not be laid over compartment walls.”

That leads specifiers and contractors to believe a complete inverted roof construction should not span across two different compartments. Following discussion with the BBA, we know they are only referring to the insulation and water control layer in their use of the word ‘system’.

This is unusual, since these components are always laid on a structural deck, and would not be supported directly by a compartment wall. A compartment wall can therefore meet the underside of an inverted roof, and should be fire stopped accordingly (a detail that is outside the scope of an insulation manufacturer’s advice).

What is an appropriate inverted roof specification over a compartment wall?

Approved Document B currently requires a roof covering with a minimum rating of AA, AB or AC for a distance of at least 1500mm either side of a compartment wall. To align with European classifications, that should be read as a minimum rating of BROOF(t4).

Inverted roof systems completely covered by an inorganic covering automatically achieve this standard. Departing from one of those standard coverings may force alternative solutions, which is down to the design professional to address and make appropriate enquiries about.

This blog post is largely based on the guidance contained in Approved Document B for England. The requirements for other countries of the UK are similar, and even identical in some respects, but care should be taken to ensure that any design complies with the individual regulations that apply. 

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 in June 2019.

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