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Technical Information Article

​Timber treatments and finishes: An introduction

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Timber can be physically altered and chemically engineered through the application of various treatments and finishes to meet demanding performance requirements. Covers preservation, treatments to improve performance in fire and various finishing treatments.

Timber is one of the world’s most versatile materials. It has a remarkably diverse range of uses from fuel to a structural and/or decorative material in buildings bridges and boats. It is now recognised as environmentally friendly and provides a natural warmth and beauty unmatched by other products.

Although we think of timber as one material, in reality it is a whole range of materials and products. Timber species vary in density, strength and durability from balsa (density around 100kg/m3) to greenheart, used for building bridges, wharves and jetties (ten times denser at about 1050 kg/m3). In addition there is an increasing number of wood products from board materials, such as plywood, particleboards and fibreboards, to engineered structural components, such as glued laminated timber, timber frame panels and I- beams.

Just as timber can be physically altered and ‘engineered’ to meet our needs so it can also be chemically ‘engineered’ through the application of various treatments and finishes to meet demanding performance requirements.Finishing and preservation treatments can transform the performance and appearance of timber used externally.

These broadly fall into three categories:

  • preservative treatments to prevent biological attack – by fungi, insects and marine borers
  • treatments to improve behaviour in fire
  • finishing treatments to protect from weather or wear and to provide decoration
Preservation treatments

Timbers vary in their susceptibility to attack by biological agents with some being naturally extremely durable. Others are less durable but it is important to note that all timber which is kept dry (ie below a moisture content of 25%) will not sustain fungal attack.

There are three main types of wood preservative: tar oils, waterborne and organic solvent-borne. Waterborne and organic solvent types are the most widely used for the protection of building timbers. Their primary function is to penetrate and be retained usually in a ‘shell’ of the outer few millimetres of timber to guard against stain, decay, mould growth and insect attack.

Most water-borne preservatives are applied by vacuum/pressure impregnation. After impregnation, the copper-chromium-arsenic (CCA) types most frequently encountered become fixed and insoluble in the wood, giving high durability to the timber. Other types of water-borne preservatives may require the protection of a finish to prevent long-term leaching in exposed situations. Timber treated with water-borne preservatives must be dried to an appropriate moisture content before use.

Organic solvent preservatives can be applied by dip, double-vacuum or vacuum/pressure impregnation treatments. Some form of finish is essential for satisfactory exterior performance. They have the advantage that the timber is not wetted during treatment so the moisture content is not altered. The treatment should always be allowed to dry before a finish is applied.

Contact: BM TRADA

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