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Timber preservation

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The key sources of guidance on enhancing the durability of wood are:

  • BS 8417: Preservation of wood – recommendations
  • Specification Manuals of the Wood Protection Association (WPA)
    • Industrial Wood Preservation Code of Practive
    • Modified Wood Specification Manual

BS 8417 provides broad guidance on the pre-treatment of solid timber and panels with the preservatives approved for use in the UK.

WPA is the UK authority of wood protection technology and is principal advisor on the development of wood preservation, flame retardants and modified wood technology. Its specification manual on wood preservation includes Commodity Specification codes for the most popular applications.

Care should be taken to source materials that are treated in strict compliance with BS8417. Inappropriate levels of treatment or poor site practice may result in in a treated component having lower durability than that required in Use Class 3 cladding applications.

The WPA Commodity specification code for the preservative treatment of External Timber Cladding is C6. WPA wood preservation commodity codes are also referred to and form the basis of NBS specifications (Code Z12).

TDCA recommends that preservatives be manufactured in accordance with ISO9001 quality certification and applied by treaters that are WPA Benchmark approved or approved by the preservative manufacturer. The main treatments for cladding are shown in the table below.


Suitable wood preservatives to give a 30 year desired service life based on service factor C.

High pressure Low pressure / double vacuum Low pressure / double vacuum
Coated 3      coated Yes Yes Yes
Uncoated 3 uncoated Yes No No


The preservative types are based on generic groupings from BS8417: Wood preservation - Code of practice.

Copper organics are water-based preservatives which use copper as the principal active ingredient.  Other key ingredients are used but these vary from supplier to supplier. They give a green colour to freshly treated timber (unless a colour additive is used – typically brown). They are widely available across the UK where they are applied to timber in an industrial high pressure, industrial treatment plant. Copper organics have the widest range of uses and due to the nature of the ingredients, can be used successfully outdoors without the need for a coating.

Micro-emulsions are water-based preservatives which use active ingredients that are not directly water soluble so they are formed into an emulsion to make them homogenous in water. They are applied in a low pressure / double vacuum, industrial treatment plant. They are restricted for use inside buildings unless they are weather protected e.g. finished with an appropriated and maintained surface coating. They are widely available in the UK and can be clear or more commonly have a colour marker to denote that treatment has taken place.

Solvent based and boron compound products are rarely used in the UK.

Please note that treatment varies according where the timber will be used. Make sure that you obtain documented evidence to support that your timber has been treated to an appropriate level for it's intended use.