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Timber Preservation

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The durability of many commercial softwood timber species can be improved with the application of a wood preservative by an industrial pressure process thereby allowing effective use in outdoor situations. The process involves a high pressure treatment plant / autoclave like the one shown here.

Much of the decking and landscaping timber available through trade suppliers has already been treated in this way, but to varying levels of protection.

Treatment parameters vary according to the end use of the timber and therefore it is crucial to be aware of where such purchased timber can be on.


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The Use Class system

Softwood species decking components classified as semi or non-durable must be pressure treated, before installation, with an approved preservative to provide the service life required. The amount of preservative protection required is governed by its intended end use situation and these are categorised according to a Use Class system which is detailed in a British/European Standard BS EN 335-1.

  • Use Class 3 : includes timber deck components such as balustrades and deck boards that are not in contact with the ground.

  • Use Class 4 : includes timber deck components such as posts, beams or joists (occasionally deck boards) that will be in contact with the ground or fresh water*.


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*The TDCA recommends that any decking timbers providing exterior structural support should be preservative pressure treated to Use Class 4 - regardless of whether they are in ground contact or not.

This is to ensure the enhanced durability and safety of deck substructures is in line with British Standards – BS 8417 Preservation of Wood: Code of Practice (which is to adopt this change shortly) and the Wood Protection Association's new Code of Practice for Industrial Wood Preservation

The Wood Protection Association is the UK authority on wood protection. Its website is packed with information about extending the life of wood and protecting it against rot, insects and fire.

the 'Make sure it's 4' campaign

Learn more at the WPA website

The Buyer's Guide to Preservative Treated Timber

Download pdf

We strongly advise you to ask your timber supplier to confirm in writing the Use Class your timber is treated to and the desired service life specification that applies either 15, 30 or more years, the industry default being 15 years.

Brush-applied wood preservatives or dip treatments are not suitable for the long-term protection of timber used for decking and other outdoor landscaping applications and should not be used.


Timber Treatments

The preservative product for outdoor use is generically described as copper organic. An example is TANALITH wood preservative. The resulting TANALISED treated timber has a soft green/brown hue which mellows and matures as the wood ages eventually becoming silver grey in colour.  Additives can also be incorporated such as colour for added decoration or a water repellent that helps further protect the wood from weathering.

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When buying any timber component for decking look for the DeckMark® quality assurance symbol. This is your proof that the product has been independently assessed by the TDCA. DeckMark® products are widely available throughout the UK through timber and builders merchants.

Quality assessed timber decking manufactures, distributors, suppliers and installers can be found on this site.

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WPA Code of Practice: Industrial Wood Preservation. A valuable and current reference of standards and good practice for treaters, specifiers and users of treated wood. This new publication supports the ongoing education campaign to raise awareness within the wood supply chain for the accurate specification of preservative treated wood products.

This document replaces the former WPA Manual: Industrial Wood Preservation - Specification and Practice.

BS 8417: Preservation of wood. Code of practice: Provides information for the specifier concerning the treatment of timber for use in the UK, drawing upon appropriate portions of the relevant BS EN documents.