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CROSS Safety Report

Fire hazards in historical modular timber framed buildings

Report ID: 1243 Published: 20 February 2024 Region: CROSS-UK


Overview

A reporter has raised fire safety concerns suggesting the potential existence of a systemic issue pertaining to the design and construction of modular timber frame systems.

Key Learning Outcomes

For responsible persons (RPs) and/or accountable persons (APs):

  • Persons responsible for the fire safety arrangements and/or the safety case for a premises must have a comprehensive understanding of the risks the building presents

For fire risk assessors:

  • It is essential to understand the construction type of the premises being assessed, in particular any common faults that may be found and the risks these present

  • Fire Risk Assessors should have the appropriate level of competence to identify and understand the risk, bringing in expertise where needed

For fire and rescue services:

  • Compartmentation and cavity barrier faults may allow unexpected fire spread. Firefighting tactics will need to consider this particularly if there is evidence of unusual fire or smoke spread

  • Fire safety audits should question the RPs/APs understanding of their premises and the construction method

For designers:

  • Many modern methods of construction cannot as yet be regarded as a Common Building situation and therefore guidance such as ADB or BS9991 may not be adequate

Full Report

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The reporter shares concerns they have about a modular system, that the reporter believes was used up to 2013.  They explain they have inspected multiple premises using a modular timber frame system, including structures such as care homes and student accommodations, with heights ranging from 3 to 7 storeys.

During the inspections, several issues were identified by the reporter:

  • Timber composite decks - some timber composite decks (I beams) were not underboarded
  • Expanded Polystyrene (EPS) - unenclosed EPS of 50 mm thickness on timber walkways 
  • Design details - an absence of design details for framing and lining services penetrations from composite decks
  • Inadequate fire stopping - an absence of test details to support fire stopping of walling systems and composite decks 
  • Voids and unprotected cavities - large voids and unprotected cavities within floors and external walls
  • Cavity barriers - a lack of cavity barriers to seal external wall service penetrations

The reporter believes the underlying causes of these issues can be attributed to several critical factors:

  • Poor design - inadequate design practices are considered to be a significant influence
  • Lack of testing of timber frame structures - the absence of rigorous testing protocols for timber frame structures introducing uncertainty regarding their fire resistance capabilities
  • Onsite buildability and quality control shortcomings - deficiencies in onsite construction practices, coupled with a lack of robust quality control measures, contributing to compromised fire safety elements
  • Lack of understanding and diligence from approved inspectors/Building Control - approved inspectors and Building Control authorities, involved during the construction process, not identifying poor workmanship, installation or design

    The reporter puts forth the following recommendations:

    • A comprehensive assessment of all buildings constructed using modular timber frame systems during the specified period
    • Steps to address the issues outlined above - the rectification of fire safety measures, proper insulation, installation of cavity barriers, and an overall improvement to construction quality

    Expert Panel Comments

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    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-US Expert Panel page.

    Modular construction is seen as the future for rapid, cost-effective building projects and has been embraced by significant publicly funded projects, including the proposals for the Hospital Improvement Project and the model Hospital 2.0.  However, modular buildings (timber framed or otherwise) have always had issues with the details around fire stopping, structural element protection, protection of connections and, in particular, cavity barriers to the voids created when modules are fitted together to form multi storey buildings.

    The issues the reporter raises are concerning. The Panel highlights the importance of robust and tested detail for the fire protection elements for larger multi storey modular buildings, and offers the following advice:

    • RIBA Stage 2 - Designers must understand that many modern methods of construction cannot as yet be regarded as a Common Building situation and therefore guidance such as ADB or BS9991 may not be adequate. Understanding new methods of construction is critical to safe design and, in particular, the detailing of key fire protection features is critical to ensuring construction is completed competently
    • RIBA Stages 3-5 - Contractors, Building Control and approved inspectors should ensure critical features of the design are appropriately detailed and installed. Looking forward, it is important to ensure the measures introduced by the Building Safety Act 2022 are followed with robust monitoring
    • All stages - RPs and/or APs must have a comprehensive understanding of the risks within, and construction type of, their buildings. Fire risk assessors can help identify issues but if doubts exist over the quality or competence of the construction or products used within, intrusive surveys may be required. This is particularly important where there exists a risk to the integrity of the compartmentation or there may be faulty/missing cavity barriers

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