Skip to main content

CROSS Safety Report

Modular construction design concerns

Report ID: 1247 Published: 20 May 2024 Region: CROSS-UK


A reporter highlights concerns regarding fire travel in a modular floor cassette with inbuilt services.

Key Learning Outcomes

For designers and manufacturers:

  • Where designs use innovative techniques to build a premises, there should be available methodologies to enable the maintenance of incorporated systems for the lifespan of the building

  • If fire resisting compartmentation has to be removed at a later date to enable access, then there should be a methodology available to replace the fire resistance REI values (defined as capacity (R), integrity (E) and insulation (I)) as originally designed

  • Compartmentation lines should include the vertical plane of a floor cassette

For building control/ approved inspectors:

  • Understanding new methods of construction is critical to safe design. Many modern methods of construction cannot be regarded as a common building situation and therefore guidance such as Approved Document B or BS 9991 may not be adequate

Full Report

Find out more about the Full Report

Our secure and confidential safety reporting system gives professionals the opportunity to share their experiences to help others. If you would like to know more, please visit the reporting to CROSS-AUS page. 

A reporter describes a light gauge steel floor cassette, constructed offsite, with preinstalled services. The cassette consists of a floor, then structural elements below forming a void with various services, and then the fire-resisting ceiling of the unit underneath. It is intended to be used for student accommodation.

The reporter has identified that only the underside of the ceiling system is fire resisting and no cavity barriers are located within the cassette void. The reporter believes this system has a number of flaws that have not been addressed:

  1. In the event of a fire in the void space, combustion products would be able to spread extensively, laterally, via the void space. As the floor is not fire resisting, the fire could affect the floor structure and possibly get into the structural framing system.

  2. There would be a concern for the persons escaping above any fire. In addition, if there is a fire in the void space, the fire and rescue service would find tackling it at the source difficult due to the floor structure and smoke and flame travel through the void.

  3. There is a concern over the maintenance of the services above the fire rated ceiling, as there is no access to them. This may result in a contractor cutting the fire rated ceiling to access the services without an adequate system/methodology for resealing the ceiling's fire resistant integrity. Given that the lifespan of the building would be expected to be around forty to sixty years, this scenario could very well occur.

The reporter believes modular construction must be looked at further and in much greater detail. In their view, there is inadequate guidance as to how to build using modular constructions. They believe that the use of modular construction, such as the one in this report, has moved too fast with little supporting fire safety consideration.

Expert Panel Comments

Find out more about the Expert Panel

An Expert Panel 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-AUS Expert Panel page.

The Expert Panel acknowledge that information regarding the fire safety of modular structures is limited, and many ongoing projects are actively trying to address this matter.

Below are a range of design objectives that should be addressed and which would apply whether the framing of the modules is timber or light gauge steel.

Internal Fire Spread

The system must not facilitate the spread of fire or products of combustion through concealed areas. This is a fundamental design requirement of the building regulations. Although Approved Document B (ADB) is not specifically tailored for modular structures (and a modular building may not be regarded as a common building situation), there is guidance such as ADB Volume 1 Section 8 that should be considered. 

Additionally, there is a BRE study discussing the spread of flames through roofing. It should be said that the study is not specific to modular systems, yet it offers valuable insights into fire behaviour through voids and the necessary design considerations.

Suitable design detailing can mitigate this issue, including:  

  • Cavity barriers. These are a critical design solution to reduce the risk of concealed fire spread. Designers of modular construction products should provide robust details of cavity barrier construction. The Expert Panel share the reporter's concern that the industry is forging ahead with increased use of modular systems without having fully engineered the necessary details. 

  • Lightweight cassette's internal structural webbing. If a fire gets into the cassette cavity, the system may fail rapidly. This is due to its lightweight construction as compared to traditional construction. 

  • Fire compartment lines. These should not be bridged by voids under a floor.  

  • Protected escape routes. The reporter is referring to a panelised system covering a significant portion of the entire floor area. In such cases, if cavity barriers through the void are not provided, this type of detailing could elevate the risk if a fire reaches the cavity, potentially compromising escape routes and the structural stability of the floor. However, even under current regulations for these types of constructions, the floor should be designed as a compartment floor. Thus, it is critical that this issue is addressed through appropriate structural design and detailing.


Concerning the maintenance of the system, it is the duty of the Responsible Person to ensure that all relevant information regarding the installed system is retained. This ensures that any modifications made do not compromise the overall performance of the system.

If the concern is around the durability of the product and its expected lifespan (i.e. a separate concern), the Responsible Person should seek advice from the product manufacturer. For buildings in-scope for the Building Safety Act, this is part of the Golden Thread.

Such maintenance work should only be carried out by competent persons, and it should be recertified on completion

Part of the design criteria for such modular systems should include ‘approved maintenance and repair techniques’ to ensure works required to access the void are carried out and completed in accordance with the manufacturer’s instructions. Such maintenance work should only be carried out by competent persons, and it should be recertified on completion. The challenge to achieving this is the building owners/managers understanding a.) that they have a building with a modular system and b.) the importance of maintaining the structural and fire protection integrity.

Relevant information on the maintenance of critical safety systems should be contained in the building information pack i.e. Regulation 38 information, the fire strategy, Golden Thread information, safety case and other relevant information. Experience has shown in the past this data is often lost or rarely referred to even if provided. Responsible or Accountable Persons should ensure this information is retained and used appropriately throughout the lifetime of the building.  

For most buildings, this centres on having a suitable management structure and policies in place, to prevent unauthorised or unplanned works. Controls such as following The Construction, Design and Management Regulations (CDM 2015)  permits to work, and quality assurance should be used.

Submit a report

Your report will make a difference. It will help to create positive change and improve safety.

Our secure and confidential safety reporting system gives professionals the opportunity to share their experiences to help others.