Treated Isn't Finished: Reading Use Classes Before the Wood Goes In

There's a moment at the counter that plays out most Saturdays. Someone points at a stack, asks whether it's treated, gets a "yep, it's tanalised" and walks out happy. Then it goes in the ground, and eighteen months later they're back with a photo of a soft post and a face like thunder.

Treatment isn't a badge you can tick. It's a specification — a set of numbers agreed between the sawmill, the treating plant and the job the timber is actually going to do. Get those numbers wrong, or never ask for them, and "treated" becomes a word that only means something was done, not that it was enough.

Here's the plain-English version, from someone who loads the stuff.

"Tanalised" is a brand, not a standard

Worth clearing up, because it causes half the confusion. Tanalised is a trade name, in the way Hoover became a verb. It's shorthand for a family of copper-based preservatives — copper azole being the common one now that CCA has been off the menu for general use. Nothing wrong with it at all. But the word tells you which chemical family you're in, not how much went in or how deep it went.

Two posts can both be "tanalised" and be nowhere near equivalent.

Use classes, in something close to English

The system runs on BS EN 335, and the UK has its own code of practice in BS 8417 that marries use class to how long you want the thing to last. Crucially, the classes describe the environment the timber will sit in — not the timber itself.

• Use Class 1 — interior, dry, heated. Loft timbers, furniture frames, internal studwork.
• Use Class 2 — interior, but occasionally damp. Roof voids, some flooring, anywhere condensation gets a look in.
• Use Class 3 — exterior, above ground. Split into 3a (coated, so painted or stained, water runs off) and 3b (uncoated, such as cladding, fascia, fence rails). Most garden projects belong here, and plenty of them get put in a class too low.
• Use Class 4 — in permanent contact with the ground, or constantly wet. Fence posts, retaining timbers, pergola legs going into soil, deck joists sitting low.
• Use Class 5 — in fresh or salt water. Marine work, jetties. Rare on a domestic order, routine on a harbour contract.

The important bit: UC3 is often bought where UC4 is needed, because UC4 is dearer and sometimes awkward to get in the exact size. A UC3 post dropped straight into a hole is a post with a countdown running on it.

Incising: those little slits aren't damage

If you order UC4 softwood and it turns up covered in small, evenly spaced slits, don't ring up to complain. That's incising, and it's a good sign. Spruce doesn't take preservative deep into the heart as readily as pine, so the surface gets opened up to let the chemical penetrate rather than sit in a skin on the outside.

Which leads to the next distinction, and it matters more than most buyers realise.

Retention versus penetration

Retention is how much preservative is in the timber, usually quoted in kilograms per cubic metre of active ingredient. Penetration is how deep it got. Both are specified, both are tested, and "treated to Use Class 4" without a certificate showing either is just a phrase.

A board with a good envelope but a dry heart decays from the inside out once that envelope is broken — and it will be broken, because you're going to saw it.

The cut end is where the promise quietly ends

This is the single biggest cause of early failure on domestic timber, and it's almost always a site problem rather than a supplier one.

Every cross-cut exposes untreated end grain, and end grain is the thirstiest part of the board. On a fence post the cut end is usually at the bottom, in the wettest place, doing the most work. That's a perfect storm, and it's why so many "twenty-year" posts are rotten inside three.

Two things help. Buy a brush-on end grain preservative when you buy the timber, and use it liberally on every cut, notch and drill hole. Then detail the job so water can leave: post shoes or saddles keeping the base clear of soil, a slight chamfer on the top of the post, and nothing banked up against it.

Read the guarantee on any treated timber before assuming it covers you. Cut ends and ground contact are excluded on most of them, and no amount of arguing at the counter changes what's printed.

Your fixings need to be in on the plan

Copper-based preservatives and cheap zinc coatings don't get along. Standard bright or electro-galvanised screws and brackets corrode faster in treated timber than in bare wood, particularly outside and particularly wet. Hot-dip galvanised to EN ISO 1461, or stainless A2/A4, is safe ground. If the fixing matters — a ledger board, a post base, anything carrying weight — stainless is cheap insurance.

Worth asking, too, whether any galvanised steelwork is compatible with the treatment system being used. Some newer formulations are kinder than others, and whoever sold you the timber should know.

Is it wet, and does that matter?

Treated softwood often leaves the plant wet. For fencing, decking carcassing and general outdoor work that's normal — factor shrinkage into your spacing and let it dry in place. For tighter tolerances, ask for kiln-dried-after-treatment (KDAT). It costs more and earns it when gaps matter.

What treatment can't do

It won't turn a lower strength grade into a structural one; grading and treatment are separate conversations entirely. It won't rescue poor detailing. It won't match the natural durability of oak, sweet chestnut or western red cedar in the right application, and it won't compensate for timber that sits permanently wet with no airflow, however many stars are on the label.

Six questions for the counter

Ask these and you'll get a straighter answer than "it's treated, mate":

1. What use class is this actually specified to, and can I see the treatment certificate?
2. Is it incised, and what's the penetration — through-and-through or envelope?
3. What species and strength grade is it, if it's going structural?
4. What do you sell for re-treating cut ends?
5. Which fixings do you recommend with this treatment system?
6. Has it been dried after treatment, or is it going out wet?

Six questions, two minutes. Cheaper than rebuilding a deck.

Come and ask us any of them at the yard — that's what we're here for.