What Is Timber Treatment? The Complete Guide to Treated and Tanalised Wood

Why Timber Needs Treating in the First Place

Wood is a natural, organic material, which means it is food for fungi and insects, and vulnerable to rot when it stays damp. Untreated softwood left in contact with soil, or exposed to the British weather without protection, can start to decay within just a few years. Treatment does not make timber indestructible, but it dramatically slows that process by making the wood inhospitable to the fungi and insects that cause decay, extending a fence post’s working life from a handful of years to potentially decades.

Not every piece of timber needs the same level of protection. A roof joist sitting dry inside a house faces a completely different risk to a fence post buried in wet clay, which is why treatment is not a single, one-size-fits-all process. It is matched to how and where the timber will actually be used.

What ​‘Tanalised’ Actually Means

‘Tanalised’ is one of those brand names that has become the everyday word for the whole category, in the same way people say ​‘Hoover’ for any vacuum cleaner. Tanalith is a well-known preservative brand used across the UK timber industry, and ​‘tanalised timber’ has come to mean pressure-treated timber generally, regardless of which specific preservative was actually used. You will sometimes see it spelled ​‘tannalised’, but the meaning is the same: timber that has had preservative forced into its structure under pressure, rather than simply brushed or dipped onto the surface.

How Pressure Treatment Actually Works

Pressure treatment, sometimes called vacuum-pressure impregnation, is an industrial process, not something that can be replicated by brushing a product onto the surface at home. It typically runs through four stages:

Kilning: the timber is dried down to a controlled moisture content, since preservative will not penetrate wet wood effectively.

Vacuum: the timber is loaded into a sealed cylinder, known as an autoclave, and air is drawn out of the wood’s cell structure.

Pressure: preservative is then forced into the timber under high pressure, pushing it deep into the cells rather than leaving it sitting on the surface.

Fixation: the preservative chemically bonds to the wood fibres over a period of time, so it cannot simply wash or rub off.

This is why treated timber protects all the way through, not just on the outer surface, and why cutting a treated post in half still shows discolouration well below the surface, not just on the outside face.

Use Classes: The Bit Most People Skip, and Should Not

This is the single most useful thing to understand when buying treated timber, and the part that is most commonly ignored. Not all treated timber is treated to the same level, and the level that is right for your project depends entirely on how exposed the timber will be. The UK and Europe define this using a Use Class system, set out in British Standard BS EN 335, typically summarised as follows.

Use Class 1: timber kept permanently dry indoors, such as roof timbers. Protected against insects but not decay, since it should never get wet.

Use Class 2: indoor timber with occasional risk of dampness, such as roofing battens.

Use Class 3: timber used outdoors, above ground, either coated (3.1) or left uncoated and exposed (3.2). This covers most fencing, cladding and decking boards.

Use Class 4: timber in direct or near-permanent contact with the ground or fresh water, such as fence posts and gravel boards and sleepers. This needs the deepest, most thorough treatment of the four.

The practical takeaway is this: a fence post in Use Class 4, going straight into the ground, needs a significantly higher level of protection than the horizontal rail sitting above it, or the cladding on the side of a shed in Use Class 3. Buying timber that is only treated to Use Class 3 and burying it in the ground is one of the most common reasons a ​‘treated’ fence post still rots within a few years; it simply was not treated for that level of exposure in the first place. If you are not sure what level a product is treated to, ask before you buy. A reputable merchant should be able to tell you.

Green Versus Brown Treated Timber: What the Colour Actually Means

This trips up a lot of people, so it is worth being direct about it: colour is not a reliable guide to how well timber has been treated. Both green and brown treated timber can be treated to exactly the same Use Class and offer the same level of protection. The colour is typically down to a dye or pigment added during treatment for appearance, not the strength of the preservative itself. Green has traditionally been the more common shade for outdoor structural and agricultural timber, while brown has become popular for a more natural-looking finish on fencing and garden timber, but neither colour is inherently better or stronger than the other. If you want to know how well a piece of timber is actually protected, ask about its Use Class rating, not its colour. You can read more in our separate guide to green and brown treated timber.

Is Treated Timber Safe to Handle?

This is a fair question, and one with a genuinely reassuring answer for anyone buying timber in the UK today. Older wood preservatives containing chromium and arsenic, known as CCA, were banned from sale in the UK in September 2006. Modern preservatives used across the UK timber industry are copper-based systems combined with organic co-biocides, which do not carry the arsenic risk associated with the older chemistry, and are approved for use in exactly the kind of everyday domestic settings, such as fencing, decking, raised beds and garden structures, that most people are buying timber for.

That said, sensible precautions are still worth following with any treated wood, in the same way you would take care with any building material.

Wear a dust mask and eye protection when cutting or sanding treated timber, and work outdoors or in a well-ventilated space where possible.

Wash your hands after handling treated timber and before eating.

Never burn offcuts of treated timber on a bonfire or in a wood burner. Burning concentrates the preservative in the ash and smoke.

Treated timber is not suitable for surfaces in direct, prolonged contact with food, such as chopping boards.

How Long Does Treated Timber Actually Last?

This depends heavily on the Use Class, the timber species, and how exposed the finished structure is. As a general guide, timber treated to the correct Use Class for its application, and reasonably well maintained, can be expected to last well over a decade, with ground-contact fence posts treated to Use Class 4 often carrying manufacturer guarantees of 15 years or more against fungal decay and insect attack, such as the guarantee on our own PermaTimber® range.

The single biggest thing that shortens treated timber’s life is not the treatment itself; it is what happens after installation. Cutting, notching or drilling treated timber exposes untreated wood beneath the treated outer layer, and that cut face is far more vulnerable to moisture than the rest of the piece. Always treat freshly cut ends with an end-grain preservative before installing, particularly on fence posts, and avoid unnecessary cutting where a pre-sized product will do the job.

Can You Paint or Stain Tanalised Timber?

Yes, but timing matters. Freshly pressure-treated timber typically still holds a fair amount of moisture from the treatment process, and paint or stain applied too soon will not adhere properly and may fail early. As a general rule, let treated timber weather for a few weeks to a few months, depending on the season and how it is stored, before painting, and check that the surface looks and feels dry first. Water-based, breathable finishes tend to perform better on treated softwood than solvent-based ones, since they let any remaining moisture continue to escape gradually rather than trapping it in.

How to Tell If Timber Is Already Treated

If you are buying from a proper timber merchant, treated stock should be clearly labelled, and staff should be able to confirm the Use Class on request. As a rough visual guide, treated softwood usually shows a consistent green or brown tint running through the cut end, not just on the surface. If a cut end looks pale and untinted right through, the timber likely has not been pressure treated. When in doubt, always ask; it is a much more reliable method than guessing from appearance alone.

Why This Matters More Than It Might Seem

Treatment is, in a very real sense, the difference between a fence, deck or raised bed that lasts one Herefordshire winter and one that lasts fifteen. Buying the right Use Class for the job, understanding that colour is not a quality indicator, and looking after cut ends during installation are three small pieces of knowledge that make a disproportionate difference to how long a timber project actually lasts.

Buying treated timber for your project? Browse our range of sawn and pressure-treated timber, treated fence posts and gravel boards, and timber care and treatment products, all treated at our own FSC-certified plant in Ross-on-Wye, or speak to your nearest yard if you are not sure which Use Class your project needs.