Wood modification is the permanent alteration of the cell structure of timber — chemically or with heat — so that it absorbs less moisture, moves less and resists wood rot better. Unlike impregnation, no biocide is introduced: the durability comes from the altered wood substance itself.
The principle behind both routes is the same. Fungi need moisture; by blocking or breaking down the water-attracting (hydroxyl) groups in the wood, the timber stays drier and the fungus has nothing to work with.
The two main methods
- Acetylation (chemical). The wood reacts with acetic anhydride. For the best-known product, Accoya, the manufacturer states that swelling and shrinkage are reduced by at least 75 percent and that the timber reaches durability class 1 to EN 350 — the highest class attainable. The process does not weaken the wood
- Thermal modification. The timber is heated in a low-oxygen environment to above roughly 180 °C. It darkens, becomes considerably more stable and more durable, but also more brittle: bending and splitting strength are reduced
| Acetylation (chemical) | Wood reacts with acetic anhydride; for Accoya the manufacturer states swelling and shrinkage are reduced by at least 75 percent and durability class 1 to EN 350 is reached |
|---|---|
| Thermal modification | Heated in a low-oxygen environment to above roughly 180 °C; darkens, considerably more stable and more durable, but more brittle with reduced bending and splitting strength |
There are also processes in which timber is impregnated with a resin that then cures inside the wood.
Application
- External joinery. Windows, doors and frames in modified timber stay dimensionally stable, so sealant joints and paint films last longer
- Cladding and decking. The high stability limits cupping, twisting and checking
- Fixings. Use stainless steel (preferably A4 / 316). Acetylated wood is mildly acidic and attacks ordinary galvanised steel more quickly
- Working it. Pre-drill thermally modified timber, as it is more brittle. Cut ends do not need end-grain preservative treatment — the whole section is modified
- Cost. Expect a purchase price well above spruce or pine, roughly in the range of good hardwood. The gain lies in less maintenance and a longer service life
- Colour. Left untreated, modified timber weathers to grey like any external timber. To keep the colour you need a pigmented finish
Related terms
- Impregnating
- Durability class
- Hardwood
- Wood rot
- Cladding
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