Do I need to treat the cut ends of pressure-treated timber?
How far preservative actually penetrates, why an untreated end grain is the fastest route to rot, and what to brush on.
- Difficulty
- beginner
- Time
- 15 min
- Read
- 3 min
Short answer
Yes, always. Pressure treatment penetrates the outer layer, not the core, so every cut, notch and drilled hole exposes untreated timber. Brush end-grain preservative generously onto every cut face before fixing — end grain absorbs water many times faster than a face, so it is where rot starts.
Pressure treatment is not a bulk property of the timber. The preservative is driven into the outer zone under pressure, and how deep it goes depends on the species and the treatment class. The middle is usually untreated.
What you'll need
- end-grain preservative
- brush
- gloves
Step by step
- Assume the core is untreated.The green or brown colouring stops a short way in. Cut a treated post and look at the end — the treated zone is usually visible as a coloured band around a paler core.
- Treat every cut, notch and hole.Any operation that exposes fresh timber. That includes ends, rip cuts, housings, and the holes for bolts and screws.
- Use a proper end-grain preservative.Products sold specifically for cut ends of treated timber. Ordinary wood stain, paint or creosote substitute is not the same thing and does not penetrate.
- Apply generously and let it soak.End grain drinks it. Flood the cut face, let it absorb, and apply again. On a post end, standing it in a tray of preservative for a few minutes is better still.
- Treat before assembly.Once the joint is closed, the cut face is inaccessible. Treat every cut as you make it, before the piece goes into position.
- Pay particular attention to ends in ground contact.A fence post's buried end is the highest-risk part of the whole structure, and it is nearly always a cut face.
- Check the treatment class matches the use.Timber treated for above-ground use will not last in ground contact. The treatment class, often quoted as UC3 or UC4, is on the label or the delivery note.
- Allow treated timber to dry before finishing.Freshly treated timber is wet and often still holds treatment solution. Paint or stain applied over it will not adhere. Wait weeks, not days.
- Use appropriate fixings.Modern copper-based treatments are corrosive to ordinary steel. Use hot-dip galvanised or stainless fixings in treated timber, or the fixings fail long before the timber does.
Tips
- Standing a cut post end in a tray of preservative for five minutes does more than ten minutes of brushing.
- Treat cuts as you make them. Once the piece is fitted, the cut face is the one you cannot reach.
- Ordinary steel fixings corrode quickly in modern treated timber. Hot-dip galvanised or stainless, always.
Common mistakes
- Assuming treated means treated all the way through — Pressure treatment penetrates the outer zone. Every cut opens a path straight into untreated heartwood, and that is where rot begins in almost every failed fence and deck.
- Using ordinary steel screws in treated timber — Copper-based preservatives corrode plain and bright-zinc steel rapidly. The fixing rusts away, stains the timber and eventually fails.
If it doesn't work
Fence post rotted at ground level
Cause: Untreated cut end in ground contact, or timber treated only for above-ground use — Fix: Replace with UC4 ground-contact treated timber, treat the cut end thoroughly, and improve drainage around the post base.
Black staining around screw heads
Cause: Steel fixings reacting with the preservative and tannins — Fix: Replace with stainless or hot-dip galvanised fixings. The existing staining will not clean off easily.
Paint peeling from new treated timber
Cause: Applied before the timber had dried out — Fix: Strip, let the timber dry for several weeks or months, and use a coating specified for treated timber.
Decking boards rotting from the underside
Cause: Cut ends and undersides untreated and unventilated — Fix: Treat cut ends on installation and ensure air can circulate beneath the deck.
Questions people ask
What is the difference between UC3 and UC4?
Use class 3 is for timber above ground but exposed to weather. Use class 4 is for ground or fresh water contact and carries a much heavier treatment. Posts and sleepers need UC4.
Does end-grain treatment really make a difference?
A large one. End grain absorbs water at many times the rate of a face, so an untreated end in a wet position is the fastest failure route in the whole structure.
Can I paint treated timber?
Once it has fully dried, yes, with a coating suited to treated timber. Fresh treated stock is too wet and too alkaline for most finishes to adhere.