What is wood preservation?

Wood preservation is treating timber so that it resists attack by wood-decay fungi (rot) and insects for longer. It lets you use a fast-growing, naturally non-durable species such as spruce or pine outdoors. In Dutch it is called verduurzamen van hout. There are two main routes: bringing a protective product into the wood, or changing the structure of the wood itself.

Chemical: impregnating

Impregnation brings a wood preservative into the timber, usually in a vacuum-pressure cylinder (pressure treatment). How deep it penetrates depends strongly on the species: in spruce only a few millimetres, whereas pine sapwood takes it up well. See the article on impregnating for how it works and how it differs from a coating.

Modification: changing the wood itself

Wood modification alters the cell wall so that fungi can no longer break the wood down. No biocide is left behind.

• Thermal modification: the timber is heated with steam to around 200 °C in a low-oxygen atmosphere. It becomes more durable and more stable, swelling and shrinking less, but also more brittle. Examples are Thermowood and Platowood.
• Acetylation: the wood reacts with acetic anhydride, so the cell wall takes up less water. The best-known brand is Accoya.
• Furfurylation: the wood is impregnated with furfuryl alcohol, which cures inside it (for example Kebony).
• Silicification: silicon compounds are bonded to the wood fibres under pressure and form a barrier against fungi (for example OrganoWood).

There are also surface treatments such as shou sugi ban, in which the surface is charred.

Measuring durability

How well timber resists wood-decay fungi is expressed in a durability class under EN 350, from DC 1 (very durable) to DC 5 (not durable). EN 335 describes five use classes, from dry indoors to permanently in seawater. The aim of preservation is to make the timber suitable for the use class in which you apply it.

Application

Preserved timber is used for fences, posts, decking, cladding, window frames and garden structures. Modified wood is chosen mainly where dimensional stability matters, such as cladding and joinery.

Tips

• Choose by use class: timber in ground contact needs a heavier treatment than timber above ground.
• Treat cut ends: in pressure-treated wood the preservative often does not reach the core. Brush a suitable end-grain preservative onto every fresh cut.
• Design still matters: let water drain, ventilate and keep timber off the ground. Even treated wood rots if it stays wet.
• Do not burn it: offcuts of chemically treated timber do not belong in a stove or fire pit.

Related terms

• Impregnating
• Durability class
• Wood modification
• Accoya wood
• Thermowood
• Wood rot

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