15 January 2021
Changes to the test and requirements for fire and cladding
Although timber is historically one of the most commonly used materials for construction, the current climate emergency, and as a result the focus on sustainability and the reduction of carbon dioxide emissions, has prompted an increase in the specification of timber for construction projects.
Advances in timber technology have also enabled designers to use hybrid timber solutions not only for low-rise construction but also for medium-to-high-rise construction (such as the 58m-tall Brock Commons Tallwood House in Vancouver, Canada, the 49m-tall The Tree in Bergen, Norway, the 33m-tall Dalston Works in London, UK and several others).
Despite their advantages, timber structures are vulnerable when exposed to fire if not adequately protected. Fire safety of timber construction and the risks in terms of fire resistance and reaction to fire are specified in fire safety regulations.
Following the tragic fire at Grenfell Tower, several standards have been subject to reviews and redevelopment, which has had an impact on the timber industry. In England and Wales this includes an amendment to Building Regulation 7 – Materials and Workmanship, the review of the Approved Document B (AD B), BS 8414 and development of BS 9414.
The review of AD B led to amendments and clarifications. The main part subject to revision was part B4 of AD B dealing with external fire spread and the inclusion of the amendment text from Regulation 7. As a part of those changes, Regulation 7(4) defines so called ‘relevant buildings’, which means a building with a storey height of more than 18m* above ground level that contains one or more dwellings, an institution, or a room for residential purposes.** For this type of construction the external walls or specified attachments must be made from materials with a reaction-to-fire classification of A2-s1, d0 or better (when classified to EN 13501-1).*** For non-relevant buildings such as hostels, hotels or boarding houses with a storey height over 18m or other buildings with a height under 18m, the external wall can still be constructed with materials that do not achieve class A2-s1, d0 or better, provided that they comply with the recommendations given in paragraphs 12.3 to 12.9 of AD B (which offers guidance for external surfaces, materials and products, cavities and cavity barriers). The provision of the acceptable materials is given in paragraph 12.5 and table 12.1. The alternative route, as shown in paragraph 12.3 b, is to meet the performance criteria given in BRE report BR 135 for external walls using full-scale test data from BS 8414-1 or BS 8414-2.
How is timber cladding affected?
The new recommendations mean that timber cladding, which if treated to have an improved flammability performance, can achieve Class B at present but not A1 or A2, and must be tested to BS 8414-1 or BS 8414-2 (if used for construction above 18m for non-relevant buildings).
BS 8414-1 or BS 8414-2 are a series of well-known, large- scales tests developed in the UK to determine the performance of non-loadbearing cladding systems under fire conditions. These tests are among the most stringent fire tests, duplicating a post-flashover fire scenario in a room with a total heat output of 4500MJ over 30 minutes at a peak rate of 3±0.5 MW. These standards have recently been reviewed and the latest versions are currently being applied. Although not subject to fundamental changes, there were some additional points.
- fixing the height of the rig (where previously only a minimum was stipulated)
- an additional measurement level closer to the top of the rig (that can gather further information and can be used for more detailed classification of the cladding)
- specification of the block work required for BS 8414-1 test.
Aside from those points that directly affect technical aspects of the test, the latest version of the standards requires labs to provide more detailed information about the:
- observations during and after the fire test
- installation and installers of the cladding system.
Those points are particularly important because the performance of a cladding system is significantly affected by the detailing and installation of the key elements of the system.
The new standard
While the new regulations do limit the use of timber for construction above 18m, if the recommendations of the fire safety codes are followed (for example treatment of timber to achieve Euroclass B-s3,d0) and if the timber is appropriately and robustly detailed for construction, it could be tested to the relevant part of BS 8414 to demonstrate compliance. However, even within a single structure the detailing and cladding systems could vary. Therefore, as part of the review of the standards, the Government commissioned the development of a new standard BS 9414.
BS 9414 was developed with the remit of having harmonised rules for extending the scope of application of cladding systems tested to the BS 8414 test series. It would also ensure that a new system could not record temperatures higher than the tested system. The new test regime will allow for previously tested products to remain valid, while extending the scope of the test to allow testing of newer products with more applications. This approach not only accommodates the need for the practical aspect of the test, but also makes sure that the change in detailing would not have an adverse impact on the overall performance of the system.
Although the changing regulatory environment has had an impact on the use of timber cladding systems, the new regulations still allow for these to be used where the detailing is robust and fire test evidence is provided. The regulations will ensure the fire safety of the end-use product before they are used as a part of a construction.
* This height does not include roof-top plant areas or any storey consisting exclusively of plant rooms. The height is also under review and may be lowered further.
** This article discusses the changes applicable in England and Wales; similar regulations and guidance are published in Scotland with the limit set at 11m. Northern Ireland is likely to follow England and Wales in future.
*** Some exemptions exist for component parts that are documented in Regulation 7(3).
About the author
Dr Mostafa Jafarian, Principal Certification Engineer – Facades & Structures, Warringtonfire
|This is an extract from the new Timber 2020/2021 Industry Yearbook Online. Download the full article, including supporting images, references and further reading, here|
Learn with TRADA Live
Dr Mostafa Jafarian is speaking at two Learn with TRADA Live sessions: How to correctly demonstrate and specify the reaction to fire performance of timber cladding systems on 2 March 2021; and Proving compliance with UK Building Regulations when designing with mass timber on 11 May 2021. Find out more
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