Fire performance of timber cladding

Approved Document B has previously and continues to permit the use of "timber cladding at least 9 mm thick" in the guidance related to the reaction to fire performance of external walls. The current format of this guidance from Approved Document B, Vol. 2, states in the footnote of Figure 12.1, "timber cladding at least 9mm thick is also acceptable".

The guidance makes no specific mention of the type or configuration of timber cladding, or indeed what may or may not constitute 'timber'. This led the reporter to investigate the origins of the recommendation to garner greater insight into how the guidance should be applied in practice, and its potential bounds of applicability.

The reporter is concerned that, given the current and necessary drive to reduce the embodied carbon in our buildings, greater use of this piece of guidance will be encountered in the future. It is, therefore, necessary to understand the context in which it can apply. 

It is the reporter's understanding, that this element of the guidance comes from the former prescriptive regulations and has been transferred from imperial to metric units, with the relevant extract from The 1965 Building Regulation, E7 below:

3/8 inch thickness converts to around 9.5mm. The reference to 3/8 inch can also be found in Fire Research Note 8 (FRN 8, see Reference [1] below for further details), which sets out the principles embodied in the regulations, with emphasis on the external wall. Reference is made to the experiments of Ashton and Malhotra in Fire Research Note 436 (FRN 436, see Reference [2] below for further details). Specifically, in reference to these experiments, Fire Research Note 8 says that:

Based on eleven large scale experiments with different wall constructions, in FRN 436 Ashton and Malhotra conclude that:

"… no undue hazard is introduced by use of a combustible cladding of solid timber. The use of other combustible materials needs separate consideration since they introduce undesirable hazards."

The statement in FRN 436 is broadly like that in FRN 8, apart from the mention of a dimension of 3/8 inch. The reporter has subsequently reviewed the experiments by Ashton and Malhotra to see what timber was tested. The cladding was said to have comprised of 1 inch cedar boarding on top of timber studs, backed by 3/8 inch plasterboard. 

A section is provided below, with the thickness figure differing from the text of the report, noting the cedarwood to be 7/8 inch in thickness and not 1 inch.

An elevation of the cladding is provided below to show the nature of the assembly that was tested:

The timber cladding was of a shiplap type construction, comprising 7/8 inch cedar boards, with planks spanning vertically, forming an essentially continuous and homogeneous flat surface. 

Upon reviewing the work of Ashton & Malhotra, the reporter concluded that the interpretation (in FRN 8) of FRN 436 was likely incorrect, with the timber thickness (7/8 inch as per the above figure) mistaken for that of the plasterboard (3/8 inch). 

This interpretation subsequently entered the regulations in 1965 and guidance, through Approved Document B, thereafter. 

This is potentially problematic as any evidence relating to the adequacy of timber cladding at largescale was premised upon a thickness of circa 22 mm, not 9.5 mm. The figure has subsequently been rounded down to 9 mm, meaning a difference of around 2.3 times that of the timber cladding originally tested. Given that the ignition of timber will be influenced by its in-depth heating, it is foreseeable that this potential error could result in 9 mm timber cladding more readily supporting vertical fire spread than was originally observed or intended in the 1960s. 

Further to this, the definition of ‘timber cladding’ relevant in this context is one of softwood (cedar) planks abutted in a manner to form an essentially flat and homogeneous timber surface. Any conclusions reached by Ashton and Malhotra would not be readily transferred to different configurations of timber cladding, for example, slatted systems with air gaps in between or behind planks, nor could it be readily extrapolated to timber based products such as plywood.

The underlying issue appears to be one of misinterpreting the research of Ashton and Malhotra when transitioning to National Building Regulations. Subsequently, there has not been a detailed review of the origins of the 9 mm value, with it reproduced through various iterations of the Regulations and, more recently, Approved Document B. The phrase ‘timber cladding’ has also been poorly defined, leaving it open to interpretation i.e., the types of materials and products that constitute timber cladding, and how they might be configured.

In the reporter's opinion, limited regard should be given to the 9 mm recommendation in Approved Document B or its reproduction in other codes and standards. 

Whilst there might be no evidence of a potential hazard owing to the longstanding nature of the recommendation and its potential widespread application, the premise upon which the value was recommended appears to be incorrect.

It would be the reporter's recommendation, that greater emphasis is placed on the guidance relating to reaction to fire classifications of external surfaces, i.e., Euroclass standards. In such instances, the Euroclass achieved by ‘timber’ should not be generalised, noting that the reaction to fire performance in apparatus such as the SBI (Single Burning Item) rig is not a function of the material, but a function of a system, i.e., with dependencies on the material, the dimensions of components, the configuration of components, substrates, etc. This would help to address both the dimensional error identified and any ambiguity regarding what constitutes timber cladding.

References

[1] G. J. Langdon-Thomas and M. Law, ‘Fire and the external wall’, Joint Fire Research Organisation, Boreham Wood, England, Fire Research Note 8, 1966.

[2] L. A. Ashton and H. L. Malhotra, ‘Fire Research Note 436. External Walls of Buildings - Part I. The Protection of Openings against Spread of Fire from Storey to Storey’, Fire Research Station, Boreham Wood, 1960.