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CROSS Safety Report

Volumetric modular buildings and fire

Report ID: 1065 Published: 16 December 2021 Region: CROSS-UK

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A report has been received concerning volumetric modular construction, in the form of permanent stacked modular buildings. The reporter is concerned that the existence of extensive cavities within the compartments, combined with the lack of appropriate care when connecting the modules could lead to the concealed spread of fire and smoke, noting some recent fire events.

Key Learning Outcomes

For the construction team:

  • Be aware that passive fire protection components should be installed in accordance with the manufacturer’s specifications while considering that some proprietary products may not be suitable unless specifically tested with the modular technology being used
  • Consider introducing a quality assurance process that covers the correct use and installation of fire protection products and components

Fire and Rescue Service:

  • When FRS are attending a fire, they are, in many cases, no longer aware of the nature of the building and the key structural elements; this can lead to mistaken assumptions regarding committing firefighters into the building
  • Care should be taken to be aware of the nature of the structure and its possible performance in fire
  • Consider the possibility of early structural collapse and therefore, very early evacuation of the buildings

Architects and Lead Consultants on projects:

  • Establish a matrix of design responsibilities to avoid confusion

  • Account for differences in quality assurance between off-site and on-site processes

  • Consider requesting a complete photographic record of the installation of as-tested fire stopping as identified in RISCAuthority’s Form IQ8 to be submitted to the satisfaction of the “Responsible Person” under the 2005 Regulatory Reform (Fire Safety) Order within the Regulation 38 Fire Safety Information at Handover

Structural Engineers, Fire Engineers, and Site Engineers:

  • Ensure communication between teams so that all aspects of the design are fully addressed
  • When contracted to do so, attend site at key stages to inspect the works to ensure they are being built in accordance with the design
  • Consult with Insurers and the Fire and Rescue Service early in the design process
  • Interrogate any modular system warranty for the fire situation


  • Consider requesting RISCAuthority’s Form IQ8 - Building system questionnaire: Permanent Stacked Modular Buildings to be completed by the client supported by their consultants and contractors
  • Insist upon full disclosure of system fire tests for the cavity construction

Authorities having jurisdiction:

  • Check that system fire tests for the cavity construction correspond with the installation of fire stopping at every instance in the volumetric modular stack

Full Report

Find out more about the Full Report

The Full Report below has been submitted to CROSS and describes the reporter’s experience. The text has been edited for clarity and to ensure anonymity and confidentiality by removing any identifiable details. If you would like to know more about our secure reporting process or submit a report yourself, please visit the reporting to CROSS-UK page.


A report has been received referring to permanent stacked modular buildings, which are considered a form of Modern Method of Construction (MMC). To avoid confusion on the terminology of MMC, in 2019 a Joint Industry Working Group on MMC by the then Ministry of Housing, Communities & Local Government (MHCLG), now Department for Levelling Up, Housing & Communities (DLUHC) produced a new categorised definition framework. This aimed to standardise the terms used for the different forms of innovative construction methodologies.

This CROSS report touches upon concerns in permanent stacked modular buildings, that fall within Category 1 of the above definition framework. Their use is common in temporary buildings, but over the recent decades this practice has been used for long-term construction, hence the term ‘permanent’. The term ‘modular’ refers to pre-manufactured systems that form modules, whereas the more commonly used term ‘volumetric’ indicates cases where the modules are of a three-dimensional nature. This is why the practice is widely known as volumetric modular construction. The latter will be the term employed in the rest of this report. Similarly, reference will be made to the Building Regulations in England, but the comments apply for devolved administrations in the UK as well.

These modules are manufactured off-site, and the structural chassis can be non, partially, or fully fitted out. This approach can support the stacking of modules for the construction of a whole systemised building in which the modules are the primary structural system. There can also be some other form of hybrid solution, which employs traditional cores, podiums, or any other secondary structure to enhance system performance. A variety of configurations and architectural arrangements is possible, depending on the placement and stacking of the modules. The modules are transported to the site and using a crane can be fixed on to stair and lift core(s), allowing them then to be interconnected with fixings. A general MMC material categorisation was also provided in this framework, outlined below:

  • Mass engineered timber (MET)
  • Timber framed (TF)
  • Light gauge steel framed (LGS)
  • Hot rolled fabricated steel (HRS)
  • Hot rolled / light gauge steel combination (SC)
  • Concrete and cement derived (C)
  • Timber framed / concrete combination (TFC)

An additional combination in the form of construction materials can be a hot rolled steel frame with timber frame infill. Similarly, the use of structurally insulated panels (SIP) in volumetric modules has been observed as well.

Volumetric modular construction can be a favourable option for a construction project, in terms of costs, speed of erection, or lack of local materials and workforce. Given though, that with every innovation arise new risks, care should be taken that every safety aspect has been appropriately addressed – in this case, structural and fire safety.

This method of construction can involve cavities within the compartment floors and walls. With the proliferation and development of volumetric modular, there now can be cases where multiple modules (open from one side by employing corner load-bearing elements) are used to form larger spaces that comprise one compartment. In such cases, in order to inhibit the concealed spread of fire, smoke, and heat through the joints of connected modules that together form a larger compartment, appropriate fire stopping in all dimensions is required. This performance characteristic is outlined in Requirement B3.(4) in Schedule 1 of the 2010 Building Regulations.  An example is shown in Figure 1 below.

Figure 1: firestops in volumetric modular construction. Image Courtesy of SCI (The Steel Construction Institute)

To ensure best practice, volumetric modular cavity fire stopping requires consistent installation in accordance with the system’s fire resistance testing requirements, using only the materials and products that are tested for fire stopping, and proper installation during stacking and connecting the modules according to the manufacturers’ instructions.

The reporter also highlighted the need to have in place technical solutions with a compatible service life to that of the building. Products used as fire stopping in inaccessible areas will have to match the building's service life because of the inability to access them and replace them. Additional consideration should also be given to installing products within their tested remit - if a product is used on a substrate that it has not been tested on then there are no guarantees that it will perform as intended. 

The reporter cited two fire events that are of interest in the Shetlands; a location where environmental constraints and ferry timetables favoured the choice of a volumetric modular solution. The Fair Isle Bird Observatory was destroyed by fire in Spring 2019, and the Moorfield Hotel in Brae was also lost in 2020, its construction process is well presented in this video along with details of the construction methods used by the developer. These two events have raised some concerns in the press.

Note: CROSS has no information on the way in which these buildings were designed or constructed, or on the causes of the fires.

The reporter was alarmed because these buildings were of volumetric modular construction. In neither case, this did not give rise to a life safety issue, but the properties were completely lost and the Observatory’s mission was interrupted. 

The reporter considers these recent fires to be “near misses” in terms of life loss, but not in terms of property protection.

If a proprietary system uses combustible materials within the cavities, it should be highlighted by the designer or manufacturer. Care should also be taken that the Functional Requirements are satisfied for the duration of the structure’s service life.

Alongside the fire events mentioned, there were press reports indicating that a residential building of modular construction in another location had to be evacuated because of fire and structural safety issues.

A property and business protection issue and involvement of insurers

Even when the Functional Requirements are satisfied, other involved parties might have their own requirements. Homeowners, funders, lenders, and insurers can request assurances that innovatively constructed properties will be sufficiently durable for their service life, which is usually 60 years. That is why third-party assurance schemes have been developed, in order to demonstrate durability and quality of construction. However, these schemes may treat fire testing as voluntary, and the scope of the scheme should be reviewed closely.

With volumetric construction, as with other forms, a technical solution should be in place to address issues of durability and repairability in case of fire. Specifically, possible water damage to the fire stopping, compartmentation and structure, and the difficulty of replacement of damaged components should be considered.

Possibility for a life safety concern

Concern was also raised by the reporter about the employment of this approach for the construction of high-rise buildings. The reporter notes that the use of modular construction is promoted for other residential developments with different sleeping risks and high-rise escape strategies. It is indicative so far that off-site volumetric modular construction is not cost-competitive for every project type, but it can be for specific occupancies where there are sleeping risks, which indicates an expanded role in the industry.

It can be argued that volumetric modular is not a common building situation, as it incorporates modern construction methods. It should be noted that the technical guidance found in the Approved Documents might not be exhaustive for such projects. Chapter 7 of the MHCLG Manual to the Building Regulations provides clarifications that can assist designers on how to approach the issue of technical guidance.

One of the cases recognised as non-standard conditions, which may indicate that the building is not a ‘common situation’, is for “some buildings that incorporate modern construction methods”. In such cases, where the guidance in the Approved Documents does not fully cover the application, it is up to the designer to ensure that the Functional Requirements, as they are set out in Schedule 1. of the Building Regulations 2010, are satisfied. Specifically:

  • In case the extended cavities present between modules are not fire stopped appropriately, then the unseen spread of smoke and fire will not be inhibited, failing to comply with Requirement B3.(4). 
  • If this kind of fire spread cannot be limited or controlled, then it is possible that the stability of the building will not be maintained for a sufficient period, as outlined in Requirement B3.(1), posing a threatening situation for firefighters or people around the building. 
  • If a modular building is placed in a short distance from a neighbouring building, then in case of its full involvement in a fire, the spread of fire from one building to another cannot be eliminated, going against Requirement B4.(1).

Care should be taken by those responsible for the procurement of a high rise building that a competent fire engineer is consulted at the early stages of the project, to ensure that all aspects of fire safety are addressed and that a site engineer or inspector is present to ensure that all the necessary cavities are adequately constructed and protected. The reporter is of the view that records of fire stopping installation are kept and submitted in the Regulation 38 Fire Safety Information handover. Another consideration should be how the structure is fire protected. If structural fire protection is provided through fittings in the module construction, then issues of durability and maintenance should be addressed in the chosen technical solution.

Another area of importance in such construction methods is the necessary close cooperation between the fire and structural engineers. The structural engineering assessment should be able to provide ground displacements, differential vertical movements due to different loading scenarios, site tolerances, and analysis for accidental disproportionate collapse. Of interest to the fire engineers, are the differential vertical movements and possible deflections (even though foundation movement is usually minimal in low rise situations), and the effect these can have on the performance of cavity barriers and fire stopping.

The fire engineer should be able to provide information on the heat transfer processes (for both the heating and cooling phases) and key element temperatures that could affect the loadbearing capacity of any module, or even further, the performance of the whole system’s stability. In any event, consultation amongst the involved parties is key. 

Expert Panel Comments

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Expert Panels comment on the reports we receive. They use their experience to help you understand what can be learned from the reports. If you would like to know more, please visit the CROSS-UK Expert Panels page.

This is another interesting case that overarches many vital concepts. Fire stopping is an essential part of the building fire safety design, and it should be borne in mind that the issues involved in volumetric modular construction can be three-dimensional, which are more difficult to identify, detail, and highlight on two-dimensional drawings.

The significance of the scenario of fire entering (potentially combustible) voids should not be downplayed, as it can complicate fire service intervention for suppression. That is why it should be explicit that a rigorous and robust system of supervision and inspection (fully or of an appropriate number of sample installations) be part of the contractor’s method statement for installation, by a suitably qualified and experienced person at all times. This can ensure that any difficulties in physically accessing the space between the modules are overcome at the construction stage, given how these spaces become practically inaccessible, hence not inspectable without destructive means, in the occupation stage.

All of the above presumes that it is the building designer's responsibility to ensure appropriate fire stopping is specified in the first place. So once again, the need for competent people in the design stage, along with thorough inspections during construction, and extensive record-keeping in all phases cannot be overstated.

As it was presented, volumetric modular buildings are not necessarily a ‘common building situation’, so it is essential that the design teams consider the issues holistically, without ‘blindly’ relying on prescribed guidance. This necessitates the employment of a fire engineered approach, while government research projects may be ongoing concerning the review of the Approved Documents, their scope, and future policy decisions for possible improvements or changes to the technical guidance which could cater for these cases. Even if the guidance was relevant in the domain, the Manual to the Building Regulations clearly states that anyone “using the approved documents should have sufficient knowledge and skills to understand the guidance and correctly apply it to the building work”.

The same argument of ‘uncommon building situation’ can be made for the provisions available in Approved Document A. Requirement A3 of Schedule 1 clearly states that “The building shall be constructed so that in the event of an accident the building will not suffer collapse to an extent disproportionate to the cause”. It should be highlighted that for the case of volumetric construction, providing adequate vertical ties to prevent disproportionate collapse on-site can be more challenging than it is for other methods of construction. One reason is that access to connect the modules in every point may be restricted and will be dependent on the construction sequence. The double structure of beams and columns in hot rolled fabricated steel could also behave in a different manner than in a regular steel structure. Appropriate thought should be given to the issue from the design stage. 

Finally, constructional tolerances should be kept in mind, to avoid any lack of fit or misalignment. This can affect essential connection bolts between units, the fixing of rain screens on substrates, and the fit of fire breaks. The designers should accommodate for such cases in their approach, and site supervision should reference any issues back to the design team. An example of such a case is shown in the picture below where a gap still existed between modules until it was spotted and rectified.

Figure 2: an example from a gap in the firestopping between modules

Past CROSS reports that appear to be related

Incidents related to sub-groups of modular construction or cavity protection have been raised with CROSS in the past. 

The risks related to off-site manufacture and the possible confusion in design responsibility have been brought up in report 529 Risks from off-site manufacture and hybrid construction. The issue of cavity barriers and fire stopping has been reported, yet for conventional buildings, in report 1039 on detailing issues with fire compartmentation. The presence of cavity barriers on-site does not necessarily mean that they have been certified or installed correctly. In terms of water damage, report 852 on the rotting of cross-laminated timber roof panels can be of interest to draw corollaries on water damage. Finally, a CROSS feature article on why CLT (a sub-category of possible modular systems) does not constitute a “common building situation” can be informative on the responsibilities of volumetric modular construction designers. 

Further reading

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