Fire compliance best practice for flat roofs – ADC

One of the principal responses to the Grenfell tragedy in legislative terms have been the amendments to Part B of the Building Regulations. According to the approved document, compliance for flat roofs requires the installed system to achieve BROOF(t4) in accordance with BS EN 13501-5. But this is only part of the story – increasingly stringent demands from key stakeholders, including fire safety officers and building insurers, is making fire safety compliance not only more onerous and complex, but also more time consuming to achieve.

So what are the main issues? How should architects adapt their approach to flat roof design and specification? And what do they need to do to ensure compliance across the board? SIG Design and Technology’s Ross Finnie (Sales Director), Andy Wells (National Supply Chain Manager), and Daniel Bosworth (Design & Technical Manager), together with Graham Ford, Principal of Graham Ford Architects, discuss these questions and more with Architecture Today’s Technical Editor John Ramshaw.

How should architects approach flat roof design and specification with regards to fire safety?

RF: Compliance has changed dramatically since Grenfell for several reasons. To start with, the amendments to Part B of the Building Regulations are somewhat ambiguous and open to interpretation, producing some confusion in the marketplace. Alongside this, we have independent fire officers who have their own approach to fire strategy, which is frequently project-based and aimed at bettering the legislation.

This in turn seems to be driven by the insurance companies. Certain insurers will only permit non-combustible materials on the projects they insure, even though a BROOF(t4) build-up incorporating combustible products would still comply with Part B. The specification of completely non-combustible products, particularly insulation, has become more prevalent because of this. In short, insurers are further mitigating their risk, probably by going over and above what the legislation demands.

This can be problematic for specifiers as they have to consider not only the legislative issues, but also what roof systems are appropriate and if these will meet the expectations of the fire officer – even if they already comply with the Building Regulations. We’ve had instances where specifiers have undertaken a lot of design work to produce regulatory compliant designs that then do not meet the requirements of the fire officer and/or insurer. This can result in significant re-design costs.

DB: The current situation has also resulted in a (rightly) cautious design approach, particularly where membranes and insulation are concerned. The Building Regulations state a requirement for BROOF(t4) systems to be used for flat roofs and terraces, but there have been cases where fire officers have insisted on covering waterproofing membranes with non-combustible materials and switching BROOF(t4) tested insulation, such as XPS or PIR, to ‘non-combustible’.

So what would you advise to avoid these types of situation arising?

RF: It’s critical that architects know who the project-specific influencers are with regards to fire compliance at the outset – most notably the name of the building insurer and what the fire strategy is. This will help both the specifier and our technical team to design a roof system that will meet the requirements of all the key stakeholders.

Testing is another important area. Fire officers are increasingly asking for project-specific fire tests relating to roof build ups, rather than relying on data from indicative tests. The latter, for example, might cover the fire performance of a flat roof system with a plywood deck, so common sense would indicate improved capabilities with a concrete or steel deck. This is creating a backlog as independent test centres struggle to keep pace with demand. We used to be able to get test results turned around in two months, now it’s more like six months, which is affecting lead times. Early engagement with roofing solution suppliers, such as SIG Design & Technology, is therefore crucial.

AW: A good example of the current fire testing situation is the Sands Leisure Centre in Carlisle where we had three types of metal roof deck, each of which had to be fire tested, resulting in added time and financial costs.

DB: Significantly, the Sands Leisure Centre client had similar projects planned for other areas with the same roof build-ups, so they could justify the cost, even though the system would have been BROOF(t4) compliant with an OSB 3 deck, which is the most flammable substrate commonly used.

AW: Early engagement with regards to fire compliance is also critical because it can impact on the detail design of the roof. For example, the requirement to use non-flammable insulation can make it harder to achieve best practice design for door thresholds and upstands, due to the added depth of material needed over flammable equivalents. Of course, roof build-ups with flammable or non-flammable insulation can both achieve BROOF(t4).

GF: Complying with the Building Regulations is already challenging due to their complexity and onerousness. The additional demands of insurers and fire officers means that the design and specification process is becoming a minefield.

Project-specific fire tests have the potential to torpedo building schemes. The issue of fire in relation to flat roof specification should be discussed very early on with the building inspector or building control. If, for example, a combustible membrane is planned as part of BROOF(t4)-compliant roof, the architect should ask the building inspector to run this by his or her fire officer to check that there are no issues.

The last thing that the architect wants is a notification of a project-specific fire test just before the scheme goes to site. This situation is pushing flat roof design higher up the project risk register – much more so than in the past, and architects need to mitigate it from the word go.

Is it easier to achieve key fire performance criteria with certain types of flat roof construction, such as warm, cold or inverted?

RF: It’s the lightweight structures that inevitably invite more testing rather than inverted roofs.

DB: If the roof system is ballasted with 80kg/m² of inert finishes, such as cast stone or mineral slabs of at least 40mm thickness, 30mm screed, or 50mm of 4-32mm ballast, it is noted as satisfying fire performance within Approved Document B as defined in Commission Decision 2000/553/EC.

How important is insulation type?

DB: Both combustible and non-combustible insulation products can achieve BROOF(t4), however a building insurer may request a more ‘robust’ solution, ie, non-combustible. Legislatively speaking, you are not gaining anything by using one type of insulation over another, providing they comply with BROOF(t4). It’s a simple pass or fail. But architects need to understand and manage the specification process with regards to insurer and fire officer requirements.

What detailing issues should specifiers be particularly aware of?

RF: There’s a lot of confusion over the definition of what constitutes a balcony or terrace in Part B of the Building Regulations and how to fireproof them.

DB: BS 8579:2020, which is the standard for balcony and terrace design, clearly says that a terrace should be treated like a ‘roof’ and must therefore achieve BROOF(t4). A balcony requires materials of at least A1 or A2-s1, d0. Waterproofing membranes are exempt from this requirement; however, an assessment of the risk should be carried out to prove adequate resistance to the spread of fire is maintained.

What about fire warranties and insurance cover?

RF: Some insurance companies have been removing fire cover from architects’ PI insurance. In these instances, architects have struggled to get cover for their designs, which has had a knock-on effect, whereby they’ve been going to their materials suppliers to get the fire cover they need. They have also had to tell clients that the design aspects related to fire are no longer protected. However, if there is problem in the future related to fire design, the architect could be personally liable in the absence of appropriate PI cover. It’s an issue that needs addressing urgently.

GF: Architects who are unable to get PI cover relating to the fire design of flat roofs might have to inform their clients, so that other parties involved in their construction projects can assume responsibility for this liability in their absence. The latter might be a fire engineer or a subcontractor possibly. This is new territory, particularly as architects tend to use standard/traditional membranes that comply with the Building Regulations, but not necessarily with the latest demands of insurers and fire officers.

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