Authors: Mostafa Jafarian, Mark O'Connor, Octavian Lalu, Tom Lennon, Jose Cullen Torero
1. Introduction
Modern Methods of Construction (MMC) has been a recognised concept since the Second World War, primarily developed to meet the pressing demand for new housing and a shortage of skilled labour. Nonetheless, the methodology faced significant setbacks following the collapse of Ronan Point, which led to a decline in its adoption (Hayes, 2024). In the years following 1998, however, there has been a resurgence of interest in MMC, driven by the increasing recognition of its commercial advantages. Key benefits such as accelerated construction timelines, enhanced quality, and improved reliability have garnered attention, alongside considerations of sustainability inherent in these construction techniques (Lawson et al., 2014).
In 2005, the Office of the Deputy Prime Minister and the Housing Corporation tasked the National Audit Office (NAO) with conducting an independent review aimed at discovering effective strategies for achieving optimal value through modern construction methods. The purpose was to offer practical guidance to Registered Social Landlords and private developers (RNAO, 2005). Furthermore, this approach has been embraced by the government, which has shown strong support for MMC, recognising them as integral to achieving ambitious housing targets and delivering the Affordable Homes Programme (UK Government, 2019; House of Commons Housing, Communities and Local Government Committee, 2019b; NFCC, 2022).
In 2017, the government established a working group aimed at supporting the adoption of MMC. The primary goal of this group was to standardise and clarify the terminology associated with MMC by defining the diverse range of innovative construction techniques currently utilised in the residential market, as well as those anticipated for future application (UK Government, 2019). This committee categorised MMC into four typologies, seve general materials and seven categories (see Figures 1 and 2).
| Building typology |
Material genre |
| Houses |
Mass engineered timber (MET) |
| Low-rise apartments (<5 storeys) |
Timber framed (TF) |
| Mid-rise apartments (6-9 storeys) |
Light gauge steel framed (LGS) |
| High-rise apartments (10 stroreys and above) |
Hot rolled fabricated steel (HRS) |
| |
Hot rolled / light gauge steel combination (SC) |
| |
Concrete & cement derived (C) |
| |
Timber framed / concrete combination (TFC) |
Figure 1: Range of buildings and materials covered by MMC
Figure 2: MMC catergory definitions
Despite its many benefits, volumetric modular construction poses unique challenges, particularly in the realm of fire safety. Concerns include the potential for fire spread between modules, the integrity of connections under fire conditions, and the adequacy of existing regulations to address these novel construction techniques. Some of these issues have been raised through different means, such as CROSS report: Volumetric modular buildings and fire (Report 1065) (CROSS-UK, 2021), NFCC (2022) report, etc which identifies those issues likely to be critical and recommends a way forward to ensure the safe adoption of MMC in the growing market. The government has already funded different projects to study those risks including the work by Brennan et al. (2024) and the development of PAS 8700: 2025 by the British Standards Institution (BSI).
In addition to the actions taken by the government, the Institution of Structural Engineers has commissioned the authorship of detailed guidance. This situation reflects the fact that addressing fire safety issues will require detailed consideration based on the needs of individual projects.
It is important to note that while some fire safety aspects discussed will focus on MMC, the key principles of fire safety are equally relevant to traditional construction methods. Therefore, the factors highlighted should not be viewed solely as concerns specific to MMC, but rather as critical considerations for all construction methods.
This article aims to provide a high-level overview of key fire safety aspects that can affect both the safety and structural integrity of the building design. For more comprehensive information on this topic, readers are encouraged to refer to upcoming publications from IStructE and other relevant documents providing more detailed information on the mentioned topics.
2. Regulatory framework in the United Kingdom
Regardless of the construction type, the initial step for any project is to address the regulatory requirements that apply within the jurisdiction where the construction will take place. These regulations can range from being highly prescriptive to performance-based. In the United Kingdom, the fire safety regulatory framework is fundamentally grounded in the functional requirements set forth in the Building Regulations. These regulations prioritise safety, health, and welfare, establishing legal obligations to protect both building occupants, firefighters and the public. Guidance documents, such as Approved Document B (ADB, 2022), play a crucial role in interpreting these requirements. It should be noted that additional requirements may need to be considered, depending on the insurance provider, such as those outlined in Chapter 11 of NHBC (2025).
Key elements of Part B of Schedule 1 of the Building Regulations include:
- B1: Means of warning and escape
- B2: Internal fire spread (linings)
- B3: Internal fire spread (structure)
- B4: External fire spread
- B5: Access and facilities for the Fire Service
However, achieving compliance often requires a nuanced understanding that extends beyond mere prescriptive measures. This is particularly true for MMC, where the manual on building regulations (DLUHC, 2020) indicates that fire safety considerations for these structures are not directly addressed within the approved documents. Consequently, more detailed evaluation and consideration are essential. Among the outlined fire safety requirements, Regulation B3 significantly influences structural and fire safety design. Regulation B3 is especially critical for volumetric modular construction, as it mandates the maintenance of structural stability during a fire for a "reasonable" or "appropriate" duration. Compliance with Regulation B3 would also require structural engineers to have a thorough understanding of the requirements stated in both Approved Document A (ADA, 2013) and Regulation B3 itself. It highlights the importance of three subjects including the technical considerations, considering the structure from a construction point of view and the necessity of employing competent professionals who can interpret the guidance, assess risks, and implement suitable solutions. These three aspects are briefly discussed in this article.
3. Fire safety concerns in modular construction
As mentioned above, MMC encompass a diverse array of forms and material choices. It is imperative to design these systems with careful consideration of the selected materials, ensuring compliance with relevant standards and regulations. This approach not only enhances the structural integrity and performance of the construction but also aligns with contemporary practices in the field. Furthermore, it is important to note that material selection can significantly affect the fire safety of MMC constructions. This includes factors such as smoke development, interactions with occupants, and the spread of flames both within the structure and on the external envelope; however, these important topics are beyond the scope of this article.
3.1. Material performance and connections
Independent of the materials and technologies used for the MMC system, it is evident that modular systems response depends significantly on the integrity of the connections between prefabricated units. During a fire, these connections may experience thermal expansion and contraction, which could jeopardise structural stability. These forces are generally induced due to differential heating of the connected elements that induces forces within the connection itself. Consequently, it is essential to ensure that these interfaces perform reliably under fire conditions through rigorous analysis and robust design strategies, making this a fundamental aspect of the overall design process. Depending on the system, detailing, and boundary conditions, additional analysis beyond standard fire tests may be necessary. However, this critical requirement can sometimes be overlooked by the design team.
3.2. Risk of fire spread between modules
The other aspect which can have a significant influence on both fire safety and structural design is the potential for fire and smoke to spread between modules. This issue presents one of the most critical challenges in modular construction. The risk is heightened by the voids that exist at connection points.
To address this, effective cavity barriers are essential for the following reasons:
- Preventing unseen fire spread: Strategically placed barriers can obstruct the conduits formed by voids
- Maintaining integrity under thermal stress: These barriers must be able to endure the expansion and contraction caused by fire
- Ensuring long-term performance: Regular inspections and maintenance are vital to preserving their effectiveness over time
4. Compliance challenges
Ensuring regulatory compliance can be particularly challenging due to the off-site fabrication of modules and the variations site-specific conditions. Modular construction requires comprehensive documentation and validation of compliance, which is achieved through a combination of factory-based quality controls and on-site inspections.
4.1. Prescriptive vs. performance-based design
Depending on the system chosen the requirements for fire safety, design can shift between prescriptive and performance-based approaches. While prescriptive methods offer straightforward solutions, they may lack the flexibility needed to address the unique challenges inherent in modular construction. In contrast, performance-based design emphasises meeting objectives tailored to specific scenarios, facilitating optimisation and innovation, although it demands more advanced assessment techniques which requires the design team to go back to basic principles to address the needs of the project.
4.2. Prescriptive design
Prescriptive methods depend on tabulated data, such as fire resistance ratings established by standard fire tests. For example, in a concrete based modular system when designing a four-story residential building, one might reference Table B4 of ADB to ascertain a 60-minute fire resistance requirement for structural elements. Though this approach is cost-effective and straightforward, it necessitates a degree of confidence in the conservative nature of the test outcomes.
4.3. Performance-based design
Performance-based design offers customised solutions by going back to basic principles and taking into account specific building characteristics such as applied loads, spans, and alternative load paths. This approach allows for greater flexibility and optimisation, although it involves a more complex design process. In the context of modular construction, where unique geometries and materials are often utilised, performance-based solutions are essential for meeting functional requirements effectively. For instance, in case of volumetric systems or when engineered timbers are employed.
4.4. Advanced calculation methods
For complex or non-standard modular buildings, the use of advanced calculation methods is crucial. These techniques enable precise modelling of fire scenarios, going beyond the limitations of prescriptive approaches. By simulating real-world conditions, professionals can evaluate the effects of fire on structural stability, ensuring both compliance and safety.
However, relying solely on finite element analysis (FEA) is inadequate. While FEA provides valuable insights into structural responses during fire exposure, these simulations must be calibrated and validated using actual fire test data. Without such validation, the results may not accurately represent real-world conditions, potentially compromising safety objectives.
4.5. The role of certification and inspection in modular construction
Certification and inspection are essential components of modular construction, ensuring both compliance and performance. Key elements include:
- Factory-based quality control: This involves verifying material certifications, assembly processes, and compliance with design specifications throughout the manufacturing phase
- On-site inspections: These inspections concentrate on the connections between modules, the continuity of fire separations, and the installation of site-specific elements
- Certification schemes: These schemes establish standardised assessment methods that enhance confidence among stakeholders while streamlining regulatory approval processes
5. Competency and early engagement of design teams
Ensuring fire safety in modular construction necessitates a high level of expertise among the professionals involved in the project. Early collaboration among architects, structural engineers and fire engineers is crucial to developing a cohesive design that prioritises safety. According to PAS 8700: 2025, key considerations include:
- Competency requirements: Design team members must have the appropriate training, experience and qualifications to navigate the complexities of modular construction. This encompasses an understanding of fire safety implications, structural interactions and innovative construction methods
- Collaborative design: Engaging all stakeholders at RIBA Stage 1 facilitates the integration of fire safety considerations into the early design phases. This proactive approach minimises the need for costly retrofits and ensures compliance with performance-based objectives
- Defined roles and responsibilities: Clearly delineating roles and responsibilities within the design team promotes accountability and streamlines decision-making processes
In addition to the points mentioned above, it is important to consider the recommendations outlined in Phase 2 of the Grenfell Tower Inquiry report (Moore-Bick et al., 2024). These include:
- Introducing mandatory accreditation for fire engineers and assessors
- Ensuring rigorous training and continuous professional development
- Requiring a comprehensive understanding of fire safety principles, material behaviour, and building regulations
- Establishing clear accountability and oversight mechanisms for professionals in fire safety roles
5.1. Recommendations from industry reports
From a practical point of view the National Fire Chiefs Council (NFCC) and PAS 8700: 2025 highlight the importance of the following in projects (which are also needed for traditional form of constructions):
- Developing project-specific fire safety strategies instead of relying on historical modular design precedents
- Implementing large-scale fire testing protocols to validate the fire performance of MMC
- Including fire and structural engineers with expertise in MMC within project teams
- Enhancing the focus on addressing firefighting challenges presented by modular layouts, void spaces, and compartmentation
6. Conclusion
Fire safety in MMC requires a thorough understanding of regulatory frameworks, design methodologies and inspection protocols. By combining prescriptive guidance with performance-based solutions and integrating insights from industry research, professionals can effectively address the challenges of modular construction while ensuring compliance, safety, and innovation. Early collaboration between architects, structural and fire engineers and a high level of competency among design teams significantly enhances the delivery of safe and efficient modular projects. As both structural and fire engineering continues to evolve, adopting a proactive and balanced approach remains crucial for the protection of lives and property.
References
ADA 2013. Approved Document A National Building Specification. London: HM Goverment-DLUHC.
ADB 2022. Fire safety: Approved Document B. London: HM Goverment-DLUHC.
Brennan, J., Vokes, C. & Massey, T. 2024. Research and analysis: Volumetric Modular Construction research. London: Ministry of Housing,Communities & Local Government.
CROSS-UK 2021. Volumetric modular buildings and fire-Report ID: 1065 London: CROSS.
DLUHC 2020. Manual to building regulations (England). London: DLUHC.
Ministry of Housing, Communities & Local Government 2019a. Modern Methods of Construction working group: developing a definition framework. MHCLG
House of Commons Housing, Communities and Local Government Committee, 2019b. Modern methods of construction: Fifteenth Report of Session 2017–19. House of Commons.
Hayes, P. 2024. Editorial: MMC in the spotlight. The Structural Engineer, 102, 5.
Lawson, M., Ogden, R. & Goodier, C. I. 2014. Design in modular construction, CRC Press Boca Raton, FL.
Moore-Bick, M., Akbor, A. & Istephan, T. 2024. Grenfell Tower Inquiry: Phase 2 Report Overview.
NFCC 2022. Modern Methods of Construction Policy Position Statement. London: National Fire Chiefs Council.
NHBC 2025. NHBC Standards, Chapter 11 MMC Systems. London.
PAS 8700:2025. Modern methods of construction for new build residential properties. BSI.
RNAO 2005. Report by the National Audit Office - Using Modern Methods of Construction to Build Homes more Quickly and Efficiently. RNAO London, UK.
UK Government, 2019. Government Response: Modern Methods of Construction inquiry.
Note: detailed guidance on how to address the concerns raised in this article has been commissioned, and is expected to be published in late 2025/early 2026
Related Resources & Events