What does tanalised timber actually mean?

Two things about treated timber are widely misunderstood and both cost people money. The first is that the treatment is a shell rather than a soaking, so cutting the timber undoes it locally. The second is that the colour is dye — added so you can see that a piece has been treated at all — and carries no information about how heavily it was treated or what exposure it is suitable for.

Safety

Step by step

1. Understand the process.The timber goes into a sealed cylinder, a vacuum removes air from the cells, preservative is flooded in and then driven into the wood under pressure, and a final vacuum recovers the excess. It is a factory process — there is no brush-on equivalent that achieves the same thing.
2. Understand where the preservative goes.Into the sapwood, which is permeable, to a depth of a few millimetres on ordinary carcassing. Heartwood is largely impermeable and does not take treatment however long the cycle runs. The middle of nearly every treated section is bare timber.
3. Treat every cut, notch and hole.The moment you saw a treated piece you expose an untreated end. End grain absorbs water many times faster than a face, so an untreated cut end sitting in a joint is the fastest route to rot there is. Brush end-grain preservative on generously and let it soak in before assembly.
4. Ignore the colour as a specification.Green comes from copper-based preservatives, brown from added pigment, and some treatments are almost colourless. The colour is there so treated timber can be told apart from untreated at a glance. It says nothing about the loading or the use class.
5. Read the use class, not the colour.Treatment is specified by use class: above ground and sheltered, above ground and exposed, in ground contact, in fresh water. Ground contact needs a much heavier loading than a fence rail does. Ask which class the stock is treated to.
6. Expect it to be wet, and let it dry.Treated timber often arrives soaking from the vessel. It is heavier than it will be, it will shrink as it dries, and no finish will take properly until it has dried. Stack it with air round every face for several weeks before painting or staining.
7. Use fixings that survive the preservative.Modern copper-based preservatives are corrosive to ordinary steel and to plain zinc plating. Use fixings rated for treated timber — hot-dip galvanised or stainless — or the fixings will fail long before the wood does.
8. Dispose of offcuts properly.Treated timber must not be burnt. Take offcuts to a household waste site and put them in the stream for treated wood, which is handled differently from clean timber.

Tips

• Spruce and whitewood are hard to treat and are often incised — pierced with rows of small slits — so the preservative can get in. Incising is not damage; it is a sign the treatment has been taken seriously.
• If you can, order treated timber cut to length so the mill's cuts are the only ones and the yard treats them. Failing that, cut everything first, treat all the ends, then assemble.
• A piece stamped with a treatment mark and a use class is worth more than a piece that is merely green. On anything going in the ground, look for the mark.

Common mistakes

• Assuming treated means waterproof — It is not a water barrier at all. Treated timber still absorbs water, still swells and shrinks, still weathers grey. The preservative stops fungi digesting it; it does nothing about the water itself.
• Burning treated offcuts on a bonfire or in a log burner — The preservative goes into the smoke and concentrates in the ash. It is a real health hazard, it is prohibited, and the ash must not go on a garden.
• Using treated timber for a raised vegetable bed and worrying about it afterwards — Modern UK preservatives are not the old CCA arsenic formulations and are widely used for raised beds, but if it concerns you, line the inside faces with a membrane or use a durable untreated species instead — decide before you build, not after.

Questions people ask