Pressure treatment explained

Why treated timber lasts, and where it stops

Pressure treatment forces preservative into the timber under vacuum rather than painting it on the outside. That is the whole difference: a saw cut through treated timber is still protected some way in, where a cut through dipped timber goes straight past the coating into bare wood.

Our fencing timber is pressure treated as standard — seal any cut end before it goes in the ground.

Shop treated timber Reading a grade stamp

The one thing that shortens its life

Cutting a post to length and standing the fresh cut in the ground. The treatment does not reach the very centre of a thick section, and a cut end is the one face water sits against.

Brush the cut with an end-grain preservative before it goes in. It takes a minute and it is the difference between a post that rots at the base in six years and one that does not.

How it works

Vacuum, preservative, pressure

The process is the same whoever does it. What varies is the use class the timber is treated to, which is what decides where you can safely put it.

The timber goes in dry Treatment is drawn into the cell structure, so the wood has to be kiln dried first. Wet timber will not take it up properly.

A vacuum pulls the air out Air is removed from the cells so there is somewhere for the preservative to go. This is the step a dipping tank cannot do.

Preservative is forced in under pressure The cylinder is flooded and pressurised, driving preservative into the timber rather than onto it. A final vacuum draws off the excess.

It is graded by use class UC3 is for timber above ground. UC4 is for ground contact. Posts, gravel boards and sleepers need UC4 — a UC3 post in the ground is a rot problem waiting to happen.

Quick answers

The things people ask us most — you might not need to message at all.

What does UC4 actually mean?

Use Class 4 is the specification for timber in permanent contact with soil or fresh water. It carries a higher preservative retention than UC3, which is rated for use above ground only.

Do I still need to treat it myself?

Not to protect it — that is done. The only essential job is sealing any cut you make, because a cut exposes less-treated timber. Everything beyond that is about appearance.

Will it stay green?

No. The green tint fades over roughly six to twelve months to a silver-grey. That is the pigment weathering, not the treatment failing — the protection is unaffected.

Can I paint or stain it?

Yes, but let it dry out first — six months is a fair guide. Use a spirit-based preservative or an oil rather than a film-forming paint, which traps moisture behind it.

How long should it last?

Manufacturers rate UC4 ground contact timber in decades rather than years, but the real answer is decided by drainage and by whether cut ends were sealed. Soil piled against a panel will finish a fence faster than the weather will.

Is treated timber safe around plants and pets?

Modern treatments are copper-based and are approved for garden use, including raised beds. They are not the old creosote or CCA. Wear gloves when cutting and do not burn the offcuts.