The fire safety of cladding on multi-storey buildings is key to reducing risks to life and property. Maintaining effective fire safety standards can appear to discourage the innovative use of new and natural materials, but this need not be the case, as Antonia Crawford reports.
The level of susceptibility of external walls to fire is a vitally important building safety issue that is addressed by provisions in Approved Document B of the Building Regulations in England and Wales, and in similar documents in Scotland and Northern Ireland.
Examples of safety concerns in this area include the possibility of a fire breaking out on a lower floor of a multi-storey building, spreading up the cladding system and penetrating back into the building through a window. This form of fire spread could undermine a building’s other fire provisions, such as fire compartmentation and phased evacuation strategies.
Other risks include those faced by people on the ground, such as firefighters and evacuating occupants, from the collapse of a building’s cladding system as a result of fire.
These concerns demonstrate the importance of properly tested, certified, installed and maintained cladding systems, and the contribution this makes to reducing the risks to life, property and business continuity. The potential problem of external flame spread is particularly important for buildings with a storey 18m or more above the ground.
Complying with Approved Document B
Approved Document B (AD B) contains recommendations that are intended to limit the spread of fire via external cladding.
In a building with a storey 18m or more above the ground, AD B guidance states that any product used in the external wall construction (except for gaskets, sealants and similar components) must be of limited combustibility. Or the system as a whole must satisfy the performance criteria set out in the publication, Fire performance of external thermal insulation for walls of multi-storey buildings (BR 135), a new edition of which will shortly be produced by BRE Global.
Small-scale reaction-to-fire tests of cladding materials
There are a limited number of products that can satisfy the requirements of the small-scale, reaction-to-fire tests referred to in AD B. Approving cladding systems only using these small-scale tests therefore restricts the materials that can be used in the external envelope of buildings, and may prevent the inclusion of materials that could in fact be safely used.
One likely outcome of this is that the use of innovative insulation materials – such as those giving greater sustainability discussed below – will be hindered.
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| A demonstration of a cladding fire test with plasterboard standing in for the cladding sample |
Testing the cladding system as a whole
If the products in an external building envelope cannot meet limited-combustibility requirements, the alternative is for manufacturers and specifiers to demonstrate compliance with AD B by testing the cladding system as a whole (to BS 8414), and satisfying the performance criteria in BR 135.
This method requires a complete wall system to be installed and successfully tested, thus ensuring that the performance in testing relates directly to real fire performance. The tests were developed to demonstrate that, under a simulated whole-room fire in a compartment being emitted from an opening in the external wall, the cladding system will not permit excessive fire spread up the outside of the building.
The installation and testing of a complete cladding system allows materials that may themselves be combustible to be used, provided that the cladding system meets the requirements of BR 135. This performance-based approach allows building professionals to use new and innovative materials whilst maintaining appropriate safety standards.
Reducing property and business risk
The provisions of AD B are focused on reducing risks to life. Insurers and building users, however, may want an additional performance specification in order to further reduce the risks to property and business continuity.
In these cases it is possible to further assess a cladding system to Loss Prevention Standards, LPS 1581 and LPS 1582, new versions of which have recently been published by LPCB, the third-party certification body incorporated into BRE Global. These standards, which were developed in conjunction with the insurance industry, require a higher level of system performance under fire conditions, but are still based on the same test methodology for BR 135 classification.
Sustainability and innovation
The sustainability drive across the UK and beyond has had a significant impact on the types of cladding systems used on buildings.
An important issue for sustainability is the ability of a building to retain heat, which is often achieved by reducing the U-values of the building’s envelope. This has led to new and innovative external insulation being included in cladding systems. In some cases – such as phase-change materials – the insulation itself is combustible.
There is also a growing trend to use building products with lower environmental impacts – but that again may be combustible. The full cladding tests described above have the advantage of testing how a cladding system works as a whole when exposed to fire on a practical scale. As a result, the combustibility of individual elements need not prevent them from being used, provided that the whole system meets the performance requirements of BR 135.
In conclusion, it is important to re-emphasise that, whatever their composition and other characteristics – and however desirable these are – cladding systems must be properly tested, installed and maintained. Systems that do not meet the requirements of the appropriate standards pose a real threat to life and property. As the drive for greater sustainability and innovation gathers pace, it is vital that key principles in the fire safety of tall buildings are recognised, whilst not stifling innovation.
Antonia Crawford is a member of BRE Global’s Fire & Security Group. For more information – Tony Baker, 01923 665141, [email protected]
