The dangers of cross-laminated timber in multi-storey buildings

The CROSS-UK Fire Safety Expert Panel discusses the interpretation and application of the Building Act 1984 with regards to the use of cross-laminated timber in multi-storey buildings.

In report 966, the reporter presented concerns about the fire safety of multi-storey buildings employing cross-laminated timber (CLT).

CROSS has subsequently received additional comments on this report that have highlighted the associated need for improved understanding by architects and engineers of both the law and related technical matters.

One commenter noted that many architects and engineers currently believe that compliance with the Approved Documents guarantees compliance with Building Regulations. This observation aligns with the findings of Dame Judith Hackitt’s Independent Review of Building Regulations and Fire Safety – that “the cumulative impact of the Approved Documents changes an outcome-based system of regulation to one that is often inferred by users to be prescriptive” (Paragraph 1.28).

Approved Document B (2019) includes explanatory text to explicitly remind readers about their responsibilities. It states that “those with responsibility for meeting the requirements of the regulations will need to consider for themselves whether following the guidance in the Approved Documents is likely to meet those requirements in the particular circumstances of their case” (Page i).

The importance of the Building Act 1984

A key question is: if I follow Approved Document B, will the building be safe, and will I have complied with the law? The Building Act 1984 (Section 7), which underpins the issuing of the Approved Documents by the Secretary of State, states that: “A failure on the part of a person to comply with an approved document does not of itself render him liable to any civil or criminal proceedings; but if, in any proceedings whether civil or criminal, it is alleged that a person has at any time contravened a provision of Building Regulations– (a) a failure to comply with a document that at that time was approved for the purposes of that provision may be relied upon as tending to establish liability, and (b) proof of compliance with such a document may be relied on as tending to negative liability.”

Compliance with the Approved Documents may tend to show compliance with the law, but this is not absolute. Regarding the use of CLT in multi-storey buildings, designers and engineers should be aware of the caveat set out in the introductory paragraphs of each of the Approved Documents. For Approved Document B, which was last updated in 2020, these introductory paragraphs include:

“The Secretary of State has approved a series of documents that give practical guidance about how to meet the requirements of the Building Regulations 2010 for England. These Approved Documents give guidance on each of the technical parts of the regulations and on regulation 7 … The Approved Documents provide guidance for common building situations.

“It is the responsibility of those carrying out building works to meet the requirements of the Buildings Regulations 2010. Although it is ultimately for the courts to determine whether those requirements have been met, the Approved Documents provide practical guidance on potential ways to achieve compliance with the requirements of the regulations in England.

“Although Approved Documents cover common building situations, compliance with the guidance set out in the Approved Documents does not provide a guarantee of compliance with the requirements of the regulations because the Approved Documents cannot cater for all circumstances, variations and innovations. Those with responsibility for meeting the requirements of the regulations will need to consider for themselves whether following the guidance in the Approved Documents is likely to meet those requirements in the particular circumstances of their case.”

Chapter 7 of the Manual to the Building Regulations states that: “The Approved Documents provide guidance for common building situations. They may not provide appropriate guidance if the case is unusual in terms of its design, setting, use, scale or technology. Non-standard conditions may include any of the following:

  • difficult ground conditions;

  • buildings with unusual occupancies or high levels of complexity;

  • very large or very tall buildings;

  • large timber buildings; and

  • some buildings that incorporate modern construction methods.

“The full list of the Approved Documents and what they cover is given in Table 1.1 in Chapter 1. The Approved Documents will be relevant in many common building situations. Anyone using the Approved Documents should have sufficient knowledge and skills to understand the guidance and correctly apply it to the building work. This is important because simply following the guidance does not guarantee that your building work will comply with the legal requirements of the Building Regulations.”

The CROSS-UK Fire Safety Expert Panel believes the use of CLT in multi-storey buildings is not a “common building situation” as defined in the Approved Documents as they currently stand. CLT is a relatively new technology, particularly in relation to the standard recommendations in Approved Document B for structural fire resistance, which were developed decades ago. It therefore cannot be automatically assumed that for these types of buildings, the guidance of Approved Document B is sufficient to ensure compliance with Building Regulations.

Lack of understanding of fire resistance periods

One commenter on report 966 highlighted a problematic but common misconception – that the periods of fire resistance cited, for example, in Approved Document B represent the period of time that a construction element would remain stable in a real fire. For example, if a construction element is rated to achieve a fire resistance of 60 minutes, one might assume it will perform adequately in a real fire for 60 minutes (but not necessarily longer). However, this assumption is false – real fires subject building elements to different thermal and mechanical conditions from those that may occur during a standardised fire resistance test within an accredited testing laboratory.

Similarly, the duration of a real fire is limited primarily (but not entirely) by the fuel load, whereas a fire resistance testing furnace follows standardised temperature versus time curves recommended within the relevant testing standards. For buildings with non-combustible primary structural materials and where the height and use require high fire resistance ratings, it is therefore possible, or even likely, that the fuel within the building may burn out before structural failure occurs.

As noted in report 966, this idea is the original basis for the longer fire resistance standards specified in Approved Document B, for example for multi-storey residential buildings, and is commonly referred to within the fire engineering community as ‘design for burnout’. However, this concept breaks down in situations where the structural frame is itself combustible, and in such cases it is considered inappropriate to rely blindly on the historical fire resistance design framework – or indeed on the required periods of fire resistance quoted within, for example, Approved Document B. 

Further reading

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