Covered up - external timber cladding regulations

Wood Focus magazine
13 Aug 2013

Peter Wilson examines the issues surrounding the use of external timber cladding in the UK, and looks forward to the introduction of regulations to improve design and installation.

Over the past decade or so, external timber cladding has become increasingly evident on new buildings around the UK. Unfortunately, aside from the many excellent examples to be seen, there is plenty of poor species selection, design and installation, leading to what might charitably be described as less than satisfactory performance in practice. Certainly, conflicting building standards (especially those relating to control of fire and moisture) have not helped, but neither has the lack of fundamental knowledge among architects and other construction professionals about timber performance, nor indeed has a lack of understanding within the carpentry and joinery fraternity of relatively basic cladding detailing and installation requirements.

Until relatively recently, timber façades in the UK have been restricted mainly to low-rise and often low-status construction, such as social housing, rural dwellings, and agricultural and forestry buildings. This has now changed and in the past 15 years, timber façades have become increasingly common on mainstream housing, apartment buildings, and non-domestic applications such as schools and offices. Several implications arise from this expansion of interest in the use of external timber cladding. Notably, associated risks such as fire spread have increased, cladding manufacturers and installers often lack sound technical knowledge, and cladding design and installation standards are highly variable and sometimes really quite poor.

It should be said – because it is not at all self-evident – that façades clad with timber can be more complex to design and construct than those of equivalent size made of other materials such as metal or masonry. Yet, when compared with these other products, the technical information on timber façades is still very limited – internal and external timber wall coverings, for example, are often guided by the same published standards, even though the performance issues affecting the latter are much more complex. In addition, façade engineers have unfortunately tended to ignore timber in favour of more conventional cladding materials such as concrete, metals and glass, resulting in (until very recently) timber exteriors mainly being designed by architects and timber specialists working outside the discipline. Consequently, the primary performance guidance for façades in the UK barely mentions timber, and the scope of the best-known publication on external timber cladding allows for only limited discussion of performance. Similarly, BSI has published British Standards for all of the main external cladding materials except timber. This may have been acceptable when timber façades were uncommon in the UK, but this is no longer the case and a BSI code of practice for external timber cladding is now urgently needed.

To this end, in 2012 the Wood Products Innovation Gateway (a £1.468m EDRF-supported project managed by the Wood Studio within Edinburgh Napier University’s Forest Products Research Institute) committed funding to develop the necessary standards. Led by Dr Ivor Davies and with a steering group that includes representatives from all relevant timber research organisations, trade associations, manufacturers and distributors, work is now well advanced and the final documents should be available in early 2014. Structured in two parts, the new British Standard will for the first time set out codes of practice for product manufacture (BS8605-1) and design (BS8605-2), giving recommendations for use by manufacturers and distributors, and by designers of external timber cladding.

The first of these – BS8605-1 – will list relevant specifications or manufacturing recommendations where available, recommend manufacturing standards, give characteristics of key timber species, recommend suitable timber grades and cladding profiles, and give relevant measurement and test procedures. This part of the standard will be applicable to timber cladding products intended for use as a rainscreen or solar shade on the exterior wall or roof of a building or other structure, and therefore covers cladding of new construction, overcladding of existing construction during their refurbishment, and systemised cladding such as prefabricated panels. The types of timber cladding covered by this standard are boards, shingles, shakes and wood-based sheet materials along with associated components such as support battens, fasteners and cavity barriers. The standard will not apply to siding, pressure-equalised cladding, double-skin façades, curtain walls or cladding made from wood-plastic composites.

Complementing this, BS8605-2 will give recommendations to enable building designers who are not necessarily structural engineers to design and oversee the installation of external timber cladding on most buildings while also highlighting the circumstances in which specialist advice should be obtained. The standard will provide performance criteria for external timber cladding, describe key materials and products, provide guidance on types of cladding and their construction detailing, give simplified calculation methods for wind loadings and recommend accepted good practice for installation and maintenance. BS8605-2 will be applicable to the same range of products and construction systems as BS8605-1, with the additional caveat that some clauses will be subject to specific limitations, such as maximum permitted height.

Annexes to these standards will provide further information on measurement and test methods, grading of cladding and cladding battens, characteristics of timber species, geometrical characteristics, examples of cladding profiles, nomenclature (all BS8605-1), simplified calculation of wind pressure, fastener sizing and calculation, protection against wood destroying organisms, construction details and weathering (all BS8605-2).

The new standards will for the first time set out what is accepted good practice for the design and workmanship of external timber cladding in the UK, as well as offering a series of good practice options for the range of circumstances typically affecting timber rainscreen cladding in the UK and the accepted ways of achieving them. Additionally, BS8605-1 and BS8605-2 will enable readers to make immediate use of the results of recent scientific investigations at Edinburgh Napier University, BRE and Trada Technology. Scheduled for publication in early 2014, this may well be the point at which a new age of creativity and technical excellence in the design, installation and performance of external timber cladding in the UK begins.

Further information
Peter Wilson is an architect and director of the Wood Studio within Edinburgh Napier University’s Forest Products Research Institute.