CROSS Feature Article
Cross-laminated timber (CLT) in multi-storey buildings
The CROSS-UK Fire Safety Expert Panel share their views about the interpretation and application of the Building Act 1984 with regards to the use of cross-laminated timber (CLT) in multi-storey buildings.
In report 966, the reporter presented concerns about the fire safety of multi-storey buildings comprised of CLT. CROSS has subsequently received additional comments on this report which have highlighted the associated need for improved understanding of both the law and related technical matters by architects and engineers.
One commentor noted that many architects and engineers currently believe that compliance with the Approved Documents can be assumed to guarantee 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].
'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’
The importance of The Building Act 1984
Key questions are “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. In the instance of the use of cross-laminated timber 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:
What is an approved document?
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 (see the back of this document). 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.
Manual to the Building Regulations
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:
a. difficult ground conditions
b. buildings with unusual occupancies or high levels of complexity
c. very large or very tall buildings
d. large timber buildings
e. 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.
Is the use of CLT in multi-storey buildings a “common building situation”?
It is the opinion of the CROSS-UK Fire Safety Expert Panel that the use of CLT in multi-storey buildings is not a “common building situation” as defined in the Approved Documents as they are currently published. CLT is a relatively new technology, particularly in relation to the standard recommendations in Approved Document B for structural fire resistance that were developed decades ago. It therefore cannot be automatically assumed that for these types of buildings, the guidance of ADB is sufficient to ensure compliance with Building Regulations.
It is the opinion of the CROSS-UK Fire Safety Expert Panel that the use of CLT in multi-storey buildings is not a “common building situation” as defined in the Approved Documents as they are currently published
Lack of understanding of fire resistance periods
One commentor 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 an element of construction would remain stable in a real fire. There is a common misconception that, for example, if an element of construction is rated to achieve a fire resistance of 60 minutes – then it will perform adequately in a real fire situation for 60 minutes (but not necessarily any longer). However, this assumption is false – since real fires subject the building elements to different thermal and mechanical conditions from those that may occur during a standardised ‘fire resistance’ test within an accredited fire testing laboratory.
since real fires subject the building elements to different thermal and mechanical conditions from those that may occur during a standardised ‘fire resistance’ test within an accredited fire 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 has an effectively unlimited supply of fuel which is deployed so as to ensure that the furnace follows the standardised temperature versus time curve recommended within the relevant testing standards. For buildings where the primary structural materials are non-combustible and where the height and use result in the need for high fire resistance ratings, it is therefore possible, or even likely, that the fuel within the building may ‘burnout’ 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
There is a wealth of literature on this topic, some of which the Expert Panel have assembled below for the information of CROSS readers. Of particular note for designers of mass timber buildings is the STA ‘compliance road map’ guidance.
Approved Document B, Volumes 1 and 2, 2019
Fire Resistance and Burnout Resistance of Timber Columns. Fire Safety Journal, 2021
The Rise and Rise of Fire Resistance, Fire Safety Journal, 2020
We Need to Talk about Timber: Fire Safety Design in Tall Buildings, The Structural Engineer, 2020
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