Feature

The Future of Fire Safety in Tall Buildings

On 29 November 2018, the UK government published details of previously announced changes to the Building Regulations banning the use of combustible materials in the external walls of certain types of building. The intention of these changes is shown in table 1. In addition, a revision to Approved Document B has been issued to reflect the ban.

Scope

The ban applies to any building with a storey 18m above the ground that either contains a dwelling, a room for residential purposes or an institutional use such as student accommodation, care homes, dormitories in boarding schools, and hospitals. Hotel type uses are not included. The ban applies to the whole height of the “relevant buildings”. The new regulations came into force on 21 December 2018 although there is some leeway for buildings already in the planning process, providingconstruction started within two months of the ban.

Clarification of items covered

Regulation 2 deals with definitions and this has been changed to include definitions of “external walls” and “specified attachments”, the items covered by the ban. Anything within the build-up of an external wall is covered – therefore structural elements within the walls of relevant buildings must not contain combustible materials. In addition, “specified attachments” include items such as balconies, sunshades and solar panels and they are also covered by the ban. Clearly, for relevant buildings, timber balconies will not be permitted.

Thermal upgrades

Regulation 4 sets out the requirements for building work. This has been changed to confirm that the ban applies to work carried out to upgrade the thermal performance of relevant buildings by stating that such work must comply with regulation 7.

Change of use

The changes to regulations 5 and 6 describe how the ban will work where there is a material change of use. Any building that, through modification, becomes one of the relevant building types must comply with the updated regulations. Therefore, if the external walls contain combustible material, that material will need to be replaced. This could have significant implications for office-toresidential conversions or vertical extensions.

Banned materials

Regulation 7 covers materials and workmanship, and here the changes provide details of the ban. The revised regulation states that materials in the external walls of relevant buildings must comply with European fire classification class A2-s1, d0 or A1. Specifiers should take note that this A2 limited combustibility class differs to that required for buildings outside of the ban, as given in the existing Appendix A of Approved Document B. Regulation 7 acknowledges that a number of items used in external walls may not be readily available in a non-combustible form, and a relatively short list of items are exempted from the ban.

This includes specific items such as cavity trays between two leaves of masonry, seals, gaskets, fixings, sealants and backer rods. However, there are more generic items such as window and door frames listed. It is accepted that the list will need to be regularly reviewed. Approved Document B Approved Document B was also revised in November 2018 and implements the change predominantly by modifying section 12. In addition to regulation B4 (surface spread of fire) the relevant part of regulation 7 (materials and workmanship) is included.

The introduction has been significantly shortened, stating that the purpose of the section is to reduce the risk of vertical fire in tall buildings and the risk of ignition from adjacent buildings. Additional requirements for fire resistance are highlighted, but rather than simply referring to Appendix A (which provides a table of resistances for different members), the text now refers to the relevant sections in the approved document. For buildings other than those defined in regulation 7(4), requirements for the combustibility of external walls are largely unchanged.

This means that insulation materials for buildings that have a storey above 18m but which fall outside of regulation 7(4) in terms of use must still either meet an acceptable standard of performance in the BS 8414:2015 system fire test or be of limited combustibility. The definition of limited combustibility remains as per the previous version.

Therefore, testing to BS 476-11 is acceptable. The equivalent BS EN 13501 criteria is A1 or A2-s3, d2 or better. The difference in classification for these buildings and those in regulation 7(2) is not helpful, particularly the fact that different A2 classes apply to buildings above 18m depending on whether they are required to comply with regulation 7(2) or not. It is not clear at present why these requirements have not been harmonised.

Masonry cavity wall construction in buildings that do not fall under regulation 7(4) remain exempt from the insulation requirement using the existing diagram 34 of the approved document.The requirements for external surfaces remain defined via reference to diagram 40 of the approved document, though the requirements of regulation 7(2) take precedence for relevant buildings. Under the new Approved Document B, it is now stated that the internal faces of cavities should also comply with diagram 40. The revised Approved Document B then contains an additional section dealing with regulation 7.

In addition to restating much of the regulation as presented above, it confirms that the ban includes student accommodation, care homes, sheltered housing, hospitals and dormitories in boarding schools. It is clarified that regulation 7 applies in addition to the requirements of B4. On this basis, Approved Document B notes the requirement to consider the impact of any products incorporated into the walls (including exempt items), and provides performance criteria for some exempt items. There is a requirement to consider the impact of other attachments to the wall which could affect risk – so even if the item is not a “specified attachment”, both the regulations and Approved Document B require that its potential to spread fire across the wall is considered.

It should be noted that Approved Document B was revised again on 18 December 2018 to restrict the use of assessment in lieu of tests.

Summary

Overall, the regulatory changes have delivered a ban with significant implications. The practical impact on change of use of existing structures will need to be considered. In addition, the completeness of the list of excluded items will be challenged. As these areas are written into the regulations there is no “legal” way around the requirements. The changes result in no change to the materials used in buildings that fall outside of the ban. This leads to subtle differences in requirements particularly with regard to the definition of limited combustibility.

Care should be taken to ensure that the appropriate rules are chosen – although given the rules on material change of use, it may be pragmatic to use the more onerous requirements for all new buildings over 18m.

Photo Ian Stewart / Shutterstock

Opinion: Too Much and Not Enough

Tony Jones argues that while the ban lets combustible structures off the hook, it may inadvertently outlaw safe forms of construction

The government’s decision to detail the ban on combustible materials in external walls within the Building Regulations may have unintended consequences for demonstrably safe forms of construction. In addition, restricting the ban to cladding does not address concerns about combustible structure. The government’s decision to change Building Regulations probably reflects the need to re-establish confidence in the industry, but it does have several downsides in comparison to implementing the ban through approved documents alone.

Approved documents provide an accepted means to satisfy the Building Regulations. While it is possible to demonstrate compliance without following the approved document, the onus is then on the design team to demonstrate how they meet the regulations. This means that alternative approaches that can be shown to be safe can be compliant. The relevant previous regulation was B4(1) and this remains unchanged: “The external walls of the building shall adequately resist the spread of fire over the walls and from one building to another, having regard to the height, use and position of the building.”

This regulation avoids prescription and is therefore aligned with the Hackitt report “Building a Safer Future”, which advocated outcome-based regulation. It would be completely compatible for the approved document to ban combustible material in cladding as the means to satisfy this regulation while accepting that other first principles approaches could be used.

By implementing the ban primarily through the regulation, the use of combustible materials in external walls will become “illegal”. This means that, for example, concrete sandwich panels that require rigid insulation for their construction will need to use less efficient insulation products, despite over 50 years of satisfactory performance from products where insulation that would not comply with the new criteria is sandwiched between two inert, fire-resistant concrete panels.

Similarly, the application of new regulations for material change of use will mean that office-to-residential conversions where insulated concrete cladding panels have been used may become less viable, as will the upward extension of such buildings to include a residential top floor. Again, given the good historic performance of these systems in fire, this seems an unnecessary restriction and requires review.

As not all components that make up an external wall are available in non-combustible forms, there is an exclusion list written into regulation 7. The list will require updating both to address omissions and presumably to remove items as non-combustible components become available. As a minimum, there will need to be further guidance on interpretation – for example, does a non-metallic wall tie count as “thermal break materials where the inclusion of the materials is necessary to meet the thermal bridging requirements of Part L of Schedule 1”?

If not, its inclusion is banned in relevant buildings. Writing the exclusion list into law leaves no room for professionals to apply their judgement. It is not desirable for further clarification to be established by case law. Overall such an approach is likely to limit innovation and may hamper the ability of cladding to improve performance in other areas such as internal comfort.

The changes to the regulations and Approved Document B do not address non-cladding combustible elements in high-rise structures. This reflects the scope of the consultation but ignores concerns about the performance of combustible structures in residential towers. In January 2018, Arup and the University of Edinburgh published a paper highlighting the contribution to the fuel load of exposed cross-laminated timber (CLT) in a fire test simulating a compartment fire.

In addition, they demonstrated the inadequacies of current fire encapsulation systems and the failure of timber fire design methods to capture real behaviour. Predicting charring rates was shown to be unreliable due to the delamination of the material. The points raised in this paper have not been addressed.

In a similar vein, the International Association of Fire Chiefs has raised concerns about extending the scope of the International Building Code to cover buildings constructed from CLT above five storeys tall.[2] In fact, there are restrictions on combustible structures in some parts of the UK. The Scottish Handbook 2017 effectively prohibits the use of CLT in residential buildings above 18m unless a fire safety engineering approach is used. This is because both separating elements and elements of structure supporting them are required to be non-combustible in high-rise residential buildings.

The regulations in England appear to be out of step in not considering any limitations on the use of combustible structures in high-rise buildings. These omissions also need to be addressed.

REFERENCES

1 Deeny et al, “Fire safety design in modern timber buildings”, The Structural Engineer, January 2018
2 International Association of Fire Chiefs “Th e Use of Cross-Laminated Timber in the Construction of Tall Wood Buildings”, 12 April 2018

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