Treated Timber (Tanalised): When You Need It, What to Buy, and What Not to Do With It

Your builder orders untreated timber for the wall plates because it's cheaper and "it'll be fine, it's in the roof void." Three years later, moisture wicking up through the blockwork has rotted the plates where they sit on the masonry. The roof structure above is now bearing on soft, crumbling timber. Fixing it means stripping the roof covering, lifting trusses, replacing the plates, and rebuilding. A £30 saving on treatment has become a £5,000+ repair.

Treated timber is one of those materials where the rules are simple and the consequences of ignoring them are severe. If timber touches masonry, sits near ground level, or goes outside, it must be treated. That's it. The rest of this page covers how to specify it correctly, what the treatment actually does, and the handling mistakes that catch people out.

What it is and what it's for

Treated timber (the trade calls it "tanalised" after the Tanalith brand of preservative) is softwood that's been pressure-impregnated with chemical preservative in a factory. The process is called VPI (Vacuum Pressure Impregnation): timber goes into a sealed steel cylinder, a vacuum pulls air from the wood cells, liquid preservative floods in under hydraulic pressure, and a second vacuum removes excess. The preservative bonds chemically with the wood fibres and stays there permanently.

The active ingredients in modern treatment (Tanalith E, the standard in the UK) are copper compounds and triazole biocides. The copper gives treated timber its distinctive green tint when new. That colour fades over time to honey-brown, then eventually silver-grey if left unfinished outdoors. Brown-tinted treated timber is also available; it's identical protection with a dye added to the preservative. Costs marginally more, looks better from day one.

Treatment does two things: it prevents fungal decay (rot) and it deters insect attack (primarily woodworm and, in parts of Surrey, House Longhorn Beetle). Untreated softwood in damp conditions can begin to decay within two to five years. Properly treated timber in the same conditions lasts 15 to 60 years depending on the exposure level.

The cost premium over untreated timber is small. Expect to pay roughly £1 – £3 per metre more for treatment on standard carcassing sizes. On a typical extension using 25-30 metres of treated wall plate and battens, that's £30 – £75 total. The insurance policy it buys you is worth vastly more than the premium.

Where treatment is mandatory

Building regulations don't leave this to personal preference. Approved Document C (resistance to moisture) requires that structural timber at or near ground level must be either naturally durable or preservative-treated. Specifically:

• Any timber within 150mm of a DPC (damp-proof course), which in practice means wall plates sitting on blockwork.
• Timber fillets embedded in concrete ground floors, unless they're above the damp-proof membrane.
• Any timber in contact with masonry, where moisture can wick from the block into the wood.
• External timber: cladding, fascias, barge boards, anything exposed to weather.

Approved Document A adds a separate requirement for roof timbers in designated House Longhorn Beetle areas (primarily Surrey and adjacent districts). In those zones, all softwood roof timbers must be treated with preservative that includes insecticide. Outside Surrey, treatment of general roof timbers isn't mandatory, but it's cheap insurance.

Use classes explained

BS 8417 (the British Standard governing timber preservation) defines four use classes that matter for domestic construction. Each one describes an exposure level and dictates what treatment the timber needs.

The rule is simple: match the use class to the exposure, not the other way around. UC3 timber used in ground contact (a common mistake with fence posts) will rot at ground level within a few years. The treatment hasn't penetrated deeply enough for that level of exposure.

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One change worth knowing about: BS 8417 was updated in 2024. The revision upgraded deck substructure beams and outdoor wall plates from UC3 to UC4 protection. If you're building a raised deck, the joists and bearers now need UC4 treatment, not UC3. This doesn't affect internal building wall plates (still UC2D), but it's a trap if you're ordering timber for both a deck and an extension on the same project.

Types, sizes, and specifications

Treated timber comes in the same grades and sizes as untreated. The two you'll encounter on a domestic extension are:

C16 is the standard structural grade for general carcassing. Most wall plates, battens, and non-load-bearing structural timber will be C16. It's what builders' merchants stock by default when you ask for "treated carcassing."

C24 is the higher structural grade, with better bending strength and stiffness. Your structural engineer specifies C24 where loads are higher, typically for floor joists and roof members in wider spans. C24 treated is widely available but costs 10-15% more than C16.

Common treated sizes for domestic extension work:

Wall plates deserve special mention. The NHBC minimum for a wall plate is 38 x 100mm, but that size is commercially difficult to source and most builders and building control inspectors prefer 75 x 100mm. The extra depth gives better bearing area for truss or rafter connections and more room for holding-down straps. If your builder or SE hasn't specified a size, 75 x 100mm treated C16 is the safe default.

How to work with treated timber

Cutting and re-treating cut ends

You can cut treated timber with any standard woodworking tool: hand saw, circular saw, mitre saw. The treatment doesn't affect cutting.

But here's what catches people out: factory treatment only penetrates several millimetres into the wood from each face. When you make a site cut, you expose bare, untreated end grain. End grain absorbs moisture up to 250 times faster than face grain. An untreated cut end on a wall plate is a direct route for moisture into the heart of the timber, exactly where it can do the most damage.

This step is routinely skipped on UK building sites. If you see your builder cutting treated timber and not sealing the ends, raise it. It takes 30 seconds per cut and prevents problems that take years to show up and thousands to fix.

Fixings: the corrosion trap

Copper-based preservatives (which is what Tanalith E is) accelerate corrosion of standard zinc-plated fixings. The chemistry is straightforward: copper is more noble than zinc in the galvanic series, so when both are present in a damp environment, the zinc corrodes preferentially. Your bright zinc-plated screws dissolve. Slowly, but they dissolve.

The rule: use hot-dip galvanised or stainless steel fixings with treated timber. Always. Standard zinc-plated (BZP) fixings are not suitable for any application where moisture may be present, which includes all external uses and anything near ground level.

Do not mix stainless steel and galvanised fixings in the same connection. That creates a different galvanic couple and corrodes the galvanised component.

For wall plate fixings specifically, hot-dip galvanised holding-down straps and coach bolts are standard. Stainless steel is overkill for an internal wall plate but correct for external connections.

Storage on site

Treated timber arrives damp from the factory process. It needs time to dry before fine joinery work (not relevant for wall plates, but important if you're using treated timber for window frames or external trim). Stack it flat on bearers off the ground, with spacer sticks between layers to allow air circulation. Cover the top to keep rain off but leave the sides open for ventilation.

Don't store treated timber in direct contact with soil. It's treated, not indestructible. Prolonged ground contact on a muddy site introduces moisture and soil organisms to the end grain before the timber even gets installed.

How to identify treated timber

Treated timber is identifiable by three things:

Colour. Fresh treatment gives a distinctive green tint across all surfaces. The intensity varies by batch, but it's clearly different from untreated timber's pale straw colour. Brown-tinted treatment exists but is less common in structural sizes. Both fade over time.

Treatment stamps. Factory-treated timber should carry an ink stamp showing the treatment plant, the use class achieved, and the BS 8417 compliance reference. These stamps are usually on one face or end. Building control inspectors look for them. If your timber arrives without stamps, ask your supplier for a treatment certificate.

Weight. Freshly treated timber is noticeably heavier than untreated timber of the same size because the preservative is water-based and the wood is wet. A 3-metre length of treated 47x100 feels substantially heavier than the same piece untreated and kiln-dried.

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Cost and where to buy

Treated carcassing timber is stocked by every builders' merchant and timber yard in the country. The price varies by size, grade, and supplier.

For the most commonly used sizes on a domestic extension:

Those ranges reflect typical builders' merchant pricing in 2026. Online specialist timber suppliers sometimes undercut at the bottom end (under £2.5/m for 47x100), while premium suppliers and national chain retail pricing can push above the top end. Regional variation is real; a local timber merchant in the Midlands may be 15-20% cheaper than a national chain in the South East.

For an extension using roughly 20-25 metres of wall plate (75x100 treated C16 at around £5 – £7 per metre) plus 30-40 metres of roofing battens (25x50 at roughly £0.8/m), your total treated timber bill for wall plates and battens sits in the £130 – £200 range. Not a major line item.

Local independent timber merchants consistently offer the best prices for carcassing timber. They cut to length, deliver on smaller lorries that can access tighter sites, and often have better stock of unusual lengths. Travis Perkins, Jewson, and Huws Gray all stock treated carcassing as standard. Wickes carries smaller treated sections but doesn't stock the 47x100 and 47x150 carcassing range that builders need.

Alternatives

Untreated timber is fine for anything internal, dry, and away from masonry. Internal stud walls, ceiling joists in heated spaces, and internal trim don't need treatment. The grade (C16 or C24) matters more than the treatment in these locations. If your timber is never going to get damp, you're paying for protection it doesn't need.

Heat-treated timber (thermal modification at 160-260 degrees C) is a chemical-free alternative used for cladding and decking. It's not suitable for structural applications because heat treatment reduces bending strength by up to 30%. It also costs substantially more: £35++ per square metre for decking versus £15 – £20 for pressure-treated. The only reason to choose it is aesthetics or a preference for chemical-free materials.

Naturally durable hardwoods (oak, sweet chestnut) resist rot without treatment. Building regulations accept naturally durable species as an alternative to preservative treatment. But hardwood costs several times more than treated softwood, is harder to work, and requires different fixings. For structural applications like wall plates, treated softwood is the standard choice.

Disposal: the rule everyone breaks

Treated timber offcuts cannot be burned. Not on a site bonfire, not in a wood burner, not in a firepit. The copper compounds and triazole biocides in Tanalith E produce toxic fumes when burned. The Clean Air Act 1993 prohibits it, and the fines are severe.

Modern tanalised timber (manufactured since around 2004, using Tanalith E rather than the older CCA formulation) is classified as non-hazardous controlled waste. It can go to a licensed wood recycling facility or in a builder's skip.

One complication if you're demolishing or renovating: timber from buildings constructed before 2007 is now automatically categorised as potentially hazardous waste under regulations that took effect in September 2023. If you're ripping out old timbers during your extension work, those need separate disposal as hazardous waste, or testing to confirm they're safe. Your skip company can advise, but don't mix pre-2007 demolition timber with modern offcuts.

Where you'll need this

Treated timber appears in the structural and roofing phases of any extension or renovation project:

• Roof structure - wall plates bedded on blockwork for truss or rafter bearing, typically 20-25 metres for a single-storey rear extension
• Foundations and footings - wall plates and any timber fillets near DPC level
• Drainage - any timber near ground level or in contact with soil
• Skip hire and site setup - scaffold boards and site protection planks

Common mistakes

Using untreated timber for wall plates. This is the classic error, and some experienced builders still argue it's fine because the roof void is dry. LABC guidance is unambiguous: wall plates are UC2D and should be treated. The cost difference is trivial. The risk of rot at the masonry interface is real, especially if there's ever a roof leak or condensation issue in the void. Don't compromise.

Forgetting to seal cut ends. Factory treatment penetrates only a few millimetres from each surface. End grain wicks moisture directly into the heart of the timber. It is the most damaging cut you can leave unsealed. Two coats of end grain preservative take 30 seconds per cut. Skipping it undoes the entire purpose of buying treated timber in the first place.

Using standard zinc-plated fixings. The copper in Tanalith E corrodes ordinary zinc-plated screws, nails, and brackets over time. Hot-dip galvanised or stainless steel fixings are required. The price difference between a box of BZP screws and galvanised ones is a few pounds. The failure mode is invisible until the fixing shears under load.

Specifying UC3 where UC4 is needed. This catches people building decks and raised platforms. Since the 2024 update to BS 8417, deck substructure timber (bearers, joists) must be treated to UC4. UC3 timber in ground contact rots at the interface within a few years. Ask your merchant specifically for UC4-rated timber if any of it will touch or enter the ground.

Burning offcuts. It happens on every other building site. Treated timber in a bonfire or wood burner releases copper compounds as toxic fumes. It's illegal, it's dangerous, and the fines are real. Skip it or take it to a licensed waste facility. No exceptions.