Modular construction and fire risks: mind the gap

In the autumn of last year, the London Fire Brigade (LFB) issued what it titled “a thematic report on modular construction”, highlighting potential safety shortcomings and the considerations needed to avoid them.

LFB has produced the report based on its experience of reviewing modular building proposals, which showed that building design teams have not sufficiently evidenced compliance with the building regulations in modular designs and that the fire performance of poorly designed or constructed modular buildings isn’t fully understood. “It is often found that crucial details are missing, especially regarding the specific structural elements of projects incorporating modern methods of construction (MMC),” says the report.

The report then goes on to set out areas that would benefit from additional consideration (design; structural; testing; firefighting; general fire precautions) and suggests questions for design teams to address as part of their design review process.

The LFB’s report follows a position statement in autumn 2022 by the National Fire Chiefs Council (NFCC) outlining various concerns with modular buildings, particularly those employing volumetric construction and engineered mass timber products. The NFCC report also warned that buildings are being designed, approved and built “despite a lack of understanding about their performance” and called on the government to tighten rules for testing MMC and to embed competence standards for practitioners.

While the NFCC called for strategic changes – better guidance, testing regimes, competency frameworks and regulatory clarity for MMC broadly – LFB takes those concerns and applies them to modular or volumetric building practice. As well as urging better evidence of compliance, it emphasises the need for early fire performance considerations, consistent benchmarks and understanding of whole-system behaviour.

As a spokesperson for LFB points out: “This thematic report was produced following joint-working that took place between LFB and colleagues from the NFCC in relation to a review of the NFCC’s position statement on MMC.”

In November, CROSS (Collaborative Reporting for Safer Structures) issued a safety report after a fire in modular residential accommodation spread rapidly across an external wall system. This was due to a combination of inadequate fire resistance and a lack of cavity barriers, breaching compartments and compromising facade fixings. This created significant risks to both occupants and responders.

CROSS said that where a premises uses “modern” lighter construction methods, these elements can be more vulnerable to fire than traditional concrete and heavy structural steel frames. Any fire travel within the structure is a critical failure, and these elements should be protected appropriately. CROSS recommended that fire engineering expertise should be integrated from the outset of modular projects, with a focus on joint detailing, cavity barriers, and compartmentation.

It went on to say that: “Many modern methods of construction cannot be regarded as a common building situation, and therefore guidance such as Approved Document B or BS 9991:2015 may not be adequate,” echoing government guidance and underlined by the LFB report.

LFB describes modular or volumetric buildings as prefabricated units, constructed off site, before being stacked in a bespoke arrangement on site, usually to form columns of modules connected to one another through a series of rigid connections. Module stacks are then often linked to one or more traditionally built steel/concrete core(s) and/or podium decks. 

The LFB report says: “Modular buildings therefore lack key features which are present in buildings of traditional construction such as a uniform floor plate, whilst introducing new features such as perimeter floor channels within the floor deck/slab, an increased number of voids and additional, (side by side) structural elements.

Lack of communication and responsibility falling between the different teams is a common problem, the report points out: “In design meetings attended by LFB on modular buildings, a common assumption has been that structural aspects fall solely under the purview of structural engineers, while fire-related matters are handled separately by fire engineers. When it is questioned why the construction methodology was not included, or even mentioned within the design documentation, the response is often that it did not impact the fire strategy.

“While this division of responsibility may not impact schemes using more traditional methods of construction, it may not hold true for MMC, especially when the fire performance of MMC structures is not yet well understood. There is a need for more considered ‘structural fire engineering’, where a connection is established between the two disciplines to better understand the structural fire performance resulting from non-traditional construction techniques,” it urges. 

Offsite construction is generally associated in the sector with better management of health and safety risk since transferring more work to factories speeds up construction onsite and should reduce accidents associated with the large number of workers and intersecting trades on a traditional project.

The LFB is positive about modular construction in principle, as the report acknowledges. It says: “The LFB supports the government’s and mayoral ambition to build homes quickly and sustainably, reducing the environmental impact wherever possible, and recognises the role that MMC can play in achieving this.”

It goes on to say, however, that “assurance is needed that fire performance of materials, elements, and complete systems have been fully considered, have been tested appropriately, and provide the level of safety that residents and firefighters should expect”. 

The report has been largely welcomed. Patrick Hayes, technical director at the Institution of Structural Engineers, says: “This report highlights issues which we ourselves are aware of and we are publishing a detailed guide this year (The Performance in Fire of Volumetric and Panellised Modular Steel Framed Construction), which will describe how designers can demonstrate the fire safety of MMC steel structures.”

“Whilst the report mainly concerns MMC, many points around the adoption of appropriate verification methods to demonstrate functional compliance with building regulations were highlighted in the Grenfell Inquiry Phase 2 report and equally apply to traditional construction,” he adds.

Dirk Vennix, executive director of Buildoffsite, comments: “We are pleased with the LFB’s report insofar that it acknowledges the importance of using MMC in the push to deliver the government’s target of 1.5 million new homes. It is also mainly constructive, with some interesting suggestions such as better design team collaboration at critical stages and taking a more holistic approach to structural design and fire engineering. 

“The LFB rightly emphasises the limitations of Approved Document B and identifies modular construction as complex in comparison to ‘bricks and mortar’ that cannot be assumed to be a common building situation as defined by the (English) building regulations.”

He goes on to add: “The testing proposals it sets out in Part 8 would prove difficult to implement and unaffordable for small to medium modular developments, which is exacerbated by the lack of empirical data available to assist designers, manufacturers, building users and the fire service.”

“Testing would add major cost and longer timeframes to offsite construction specifications if every modular building had to be individually tested. It would be a challenge to implement because there are not enough testing facilities in the UK to meet demand. As a result, significant delays in pipeline and construction would be expected which is the last thing our members want given they have the capacity to help deliver commercial projects and build 1.5 million homes.”