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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. 

Please note that the treatment parameters vary according to the end use of the timber and therefore it is crucial to be aware of where the timber you have purchased can be used.

Timber is made highly durable in a high pressure treatment plant / autoclave like the one shown here.


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

Use Class 4 = deck components such as posts, beams or joists (occasionally deck boards) that will be in ground or fresh water contact

Use Class 3 = deck components such as beams, joists and deck boards that are in not ground contact.

BS8417 is the principle standard governing timber treatment in the UK.


The correct preservative product for outdoor use is generically described as copper organic. An example is TANALISED TIMBER. The resulting treated timber has a soft green/brown hue which mellows and matures as the wood ages eventually becoming silver grey in colour.  Sometimes colour additives are used to give a stronger or different colour to the timber.

As well as preservative protection some timber treatments incorporate a built-in water repellent that helps further protect the wood from weathering.


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We strongly advise you to ask your timber supplier to confirm in writing the Use Class your timber is suitable for 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.

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.

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.

Find a preservative supplier


The Wood Protection Association

Visit the WPA website


The WPA Wood Preservation Manual. Industrial wood preservation specification and practice. This Manual is an extensive guide to the preservation of wood and panel products using controlled industrial processes. It covers wood preservation selection, specification and practice and includes the WPA Commodity Code System of Specification called up by organisations such at NBS (National Building Standards), NHBC and Zurich.

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.