28 July 2020

External timber cladding – updated guidance

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Much has happened since the publication of the 3rd edition of External timber cladding in 2013. The market for external timber cladding has continued to expand, driven by the material’s unique combination of practicality, visual appeal and performance benefits. At the same time, however, questions have been raised about how appropriate timber is as a cladding material on some types of building.

 

Why is updated guidance needed?

When external timber cladding is used on small buildings and sheltered sites, it is straightforward to design and install. But the growing use of external timber cladding on larger and more complex building projects has increased the associated risks. This means that external timber cladding now needs to meet similar performance standards to those cladding materials with a longer history in demanding applications. Accurate and up-to-date design guidance is needed.

 

These issues were brought to the fore by the Grenfell Tower disaster in June 2017. Although no timber was involved in the fire, questions have since been raised about the safety of all types of combustible cladding material. The four sets of UK Building Regulations are also being updated.

 

Some timber industry bodies have responded to these challenges by updating their guidance on the fire safety of timber-clad facades. While these initiatives are an essential first response, they are not a substitute for the more wide-ranging treatment of the subject that only a new and fully revised edition of External timber cladding can provide.

 

A further impetus for updating the guidance is the BSI publication of revised BS 8605 – the British Standard for external timber cladding. The revised Standard is in two parts:

  • BS 8605-1 gives methods of specifying the characteristics of external timber cladding; it does not include other components in the cladding assembly, such as support battens.
  • BS 8605-2 (not yet published) gives best practice recommendations for designing and installing all parts of the external timber cladding assembly, and it provides specification guidance on those components in the assembly that BS 8605-1 does not cover.

 

External timber cladding 4th edition

The 4th edition of External timber cladding will mainly focus on the key recommendations in BS 8605-2. It explains the background to and practical application of those recommendations, while giving additional guidance – on timber-clad roofs, for example – that is not available in the BS 8605 series. For ease of use, the guidance will be structured in a similar way to BS 8605-2.

 

The new edition of External timber cladding will include:

  • an outline of the benefits of timber compared to other cladding materials
  • discussion of each stage in the design and construction process
  • expanded guidance on fire safety and structural performance
  • new construction details for the main connections and junctions on a timber-clad wall
  • information on the latest material options such as modified woods and surface coatings.

 

The guidance on fire safety and structural performance is particularly important.

 

Fire safety

In recent years considerable effort has been devoted to developing fire-safe solutions for timber-clad facades. This work has been carried out in several countries, most notably in Finland, Germany, Switzerland and Austria. Building regulations in these countries have been updated to take account of the research findings and this has created new opportunities for timber. In Finland, for example, market research has found that people feel safer in a timber-framed and timber-clad building, built to the new generation of fire regulations, than in an all-concrete building constructed in accordance with previous legislation.

 

The UK is in the process of adopting the latest fire safety measures for facades, although their implementation varies between the four sets of UK Building Regulations. This is unavoidable because these regulations need to take account of local conditions such as building practices and climate. Statutory documents supporting these regulations restrict the use of combustible cladding materials (such as timber) in some circumstances. In most cases, however, timber-clad facades on low- and medium-rise buildings are capable of meeting provisions in the statutory documents providing that the cladding design and construction follows best practice as described in BS 8605-2.

 

Therefore, although external timber cladding might be restricted in some applications, such as on tall buildings, this is not necessarily a problem. Most countries with widespread use of timber construction already restrict timber cladding on tall buildings and some high-risk uses. The revised guidance will highlight where the current legislative changes could be an opportunity for external timber cladding to expand its market share in specific applications.

 

Structural performance

The 4th edition also offers guidance on the structural performance of external timber cladding. UK Building Regulations require consideration of the structural performance of external cladding. This is because external cladding can present a life safety hazard if it becomes detached from the building due to wind damage, or if there is inadequate provision for movement or self-weight. The risk of wind damage is greatest on high-rise buildings and those in exposed locations. Movement and self-weight risks can occur on any size of building.

 

External timber cladding assemblies are usually fixed to and supported by a structural backing wall. Although such assemblies are non-loadbearing, they need to be capable of carrying their self-weight and any imposed loads and transmitting them to the building’s structure. This capacity needs to be maintained for the design life of the building, assuming anticipated maintenance.

 

Traditionally, structural engineers have not been involved in the design of external timber cladding assemblies, except on some tall buildings or when checking the work of others during post-completion assessments. This can lead to problems if the cladding assembly is not designed to ensure that it will be robust in its service conditions. There are four issues to consider:

  • loads acting from the cladding onto the structure
  • self weight
  • attachment of the cladding assembly
  • movement and tolerances.

 

Loads acting on the structure are the responsibility of the building’s project engineer, whereas the other three issues are normally the concern of the cladding designer. Consequently, BS 8605-2 gives simplified engineering design guidance that can be used by non-engineers in most circumstances and highlights those conditions where a full structural analysis is advisable.

 

Guidance for external timber cladding connections has developed over time and is often based on ‘rule of thumb’ instead of engineering calculation. Consequently, BS 8605-2 has updated the guidance on cladding connections so that it is evidence based. This means that some long-established practices are no longer recommended.

 

The structural design of an external timber cladding assembly needs to be in accordance with the relevant structural Eurocodes. The guidance in BS 8605-2 is consistent with these documents although – being simplified – it is necessarily conservative. External timber cladding will give clearer and more concise structural information than can be provided in the Standard.

 

About the author

Dr Ivor Davies is a wood scientist and technologist. He is a Lecturer and Research Fellow in the School of Engineering and the Built Environment at Edinburgh Napier University, member of the British Standards Institution technical committee responsible for the BS 8605 series, Chair of the relevant drafting panels, and technical consultant for the Timber Decking and Cladding Association. He also undertakes expert witness work on timber-related disputes.

 

This is an extract from the Timber 2020 Industry Yearbook. Download the full article, including supporting images, references and further reading, here

 

External timber cladding 4th edition will be published in 2020 and will be available to buy from the TRADA Bookshop.