Different class

Dr Chris Coggins, director of the Wood Protection Association, discusses choosing timber for Use Classes 2 and 3

TIMBER in construction is typically used in BS EN 335-1 Use Classes 2 or 3. These Use Classes cover:

  • interior or covered timbers (Use Class 2) with a risk of occasional wetting; typical UC 2 timbers include the external frame of a timber frame construction, valley boards and tiling battens
  • exterior timbers, exposed to the weather but protected, normally with a coating (Use Class 3.1); typical UC 3.1 timbers include painted external joinery, soffits and barge boards
  • exterior timbers, exposed to the weather unprotected (Use Class 3.2); typical UC 3.2 timbers include unpainted cladding and deck boards

The common feature for all these timbers is that wetting in service is either probable or inevitable but in either case carries with it the risk of fungal decay. For interior timbers in the UK there is a risk of wood boring insect attack but this is generally disregarded as an influence on the decision process regarding the durability of such timbers unless used in roofs in the Hylotrupes area as defined in Building Regulations where treatment is mandatory.

So timber elements to be used in these situations must have either adequate natural durability (rated on heartwood only) or be processed to confer the necessary degree of durability.

Durability has for many years been conferred on timbers of inadequate natural durability, or timbers from which sapwood cannot economically be excluded, by preservative treatment. This option is well standardised from the tests that must be performed to demonstrate efficacy (BS EN 599) to the penetration and loading of individual preservatives required to achieve a service life of 15, 30 or 60 years (BS 8417). In addition, since it is appropriate to consider the health, safety and environmental impact of preservatives and treated timber, it is important to understand that these aspects have been regulated in the UK for over 20 years; furthermore a new EU-wide scheme has been launched, bringing such regulation into countries like France and Germany for the first time, and which raises the bar for safe use of preservatives and treated timber throughout Europe.

Now there is another route to conferring durability (and indeed other improved characteristics): wood modification. Modified wood (defined as wood or wood-based products whose properties have been altered by a wood modification process) is available in commercial quantities now the UK. Wood modification in current commercial practice involves the action of a chemical or physical agent upon the material, resulting in a desired property enhancement during the service life of the modified wood. If the modification is intended for or confers claimed improved resistance to biological attack, then the mode of action should, as far as can be determined, non-biocidal.

Wood modified by heating (thermal modification) and wood modified by exposure to, and reaction with, a chemical (for example acetylated wood) are examples of commercially available modified wood on the UK market today.

Although as mentioned, durability may not be the only enhanced property of modified wood (colour, hardness, stability, strength and performance of coatings may also be improved depending on the wood and the process used), for the purposes of this review we are looking only at durability.

The Wood Protection Association’s scope includes all forms of wood protection, not only the traditional preservative treatment option, but also modified wood products. We have a long tradition of publishing specifications and guidance on preservative treatment and this is now being extended to cover modified wood principally through a manual describing the properties, performance and specification of modified wood products.

The WPA’s manual Industrial Wood Preservation – Specification and Practice includes a range of commodity specifications that set out what level of natural durability is required and, if timbers of otherwise inadequate durability are preferred, which preservative treatments are suitable, for different timber commodities in the various Use Classes for typical or recommended service lives.

The way in which similar recommendations might be achieved for modified wood so that designers and specifiers have a common basis and a benchmark for comparing naturally durable, preservative treated and modified wood products requires a new approach and this is the aim of the new WPA modified wood manual now in development.

BRE Digest DG 504 Modified Wood – an introduction to products in UK construction - sets out the position clearly: The European Standards related to specifying preservative-treated timber, BS EN 351: 1996 Parts 1 and 2, have been published for 10 years. They require the specification to be written in terms of the results of the treatment process. A required penetration of the preservative into the wood and the retention (concentration) of preservative within a defined zone (the analytical zone) of the treated timber is specified.

This immediately highlights an issue with wood modification technologies not fitting within this framework. In the UK, BS 8417 sets out a framework for specifiers to interpret the European standards and to base specifications on penetration and retention requirements thought to reflect what the old British process specifications actually achieved. Provision is made in the document for the range of traditional preservatives with recommended penetration/ retention combinations for different timber types, end uses and service-life requirements. These are based on best estimates of what has been achieved in practice. Guidance is also given for new preservatives whose performance is demonstrated by testing according to BS EN 599-1 but which may have little or no evidence from longer term tests or service data. For example, in BS 8417 for exterior window joinery (Use Class 3), a 15-, 30- or 60-year service life is satisfied by choosing, as a minimum, a timber of BS EN 350-1 natural durability Class 4, 3 or 2, respectively. In addition, the service factor based on safety and economic factors is Class C where remedial action or replacement would be difficult and expensive; thus natural durability or preservative treatment is desirable.

For modified wood products, the mechanism by which the durability is enhanced may be different and in some cases is not completely understood. This is the opposite of the case with wood preservatives where the loadings of product required to control organisms that can damage wood is precisely known or has been established by appropriate testing. The durability therefore of modified wood may best
be considered alongside natural durability, as if the modified wood was a new timber species. Specification for durability should therefore match the end-use environment of the timber (Use Class) of the modified wood expressed in the same terms as natural durability of unmodified timber.

Natural durability ratings are taken from BS EN 350-2. As this standard states, the ratings are “based upon information drawn from various sources, including historical records, practical experience, laboratory tests and other data”. For all species the durability rating for fungal attack includes data from ground contact field trials with timber stakes. Thus it should be understood that the link between durability rating and service life is, for naturally durable solid timber, a rather robust one reflecting long experience with each species.

For modified wood, durability data may have been derived from a number of laboratory tests, perhaps supplemented with field trial or even service experience. It is the aim of the WPA in this area to establish a benchmark for durability assessment of modified wood in the context of Use Classes and service life expectation that will establish a level playing field for fair comparison with naturally durable and preservative-treated timber.

The Wood Protection Association manuals are supplied in electronic format (PDF) and are available to download from www.woodprotection.org . For construction professionals registering on the website the download is free or manuals can be bought on CD. Current manuals include Industrial Wood Preservation and Industrial Flame Retardant Treatment.