What is pressure-treated wood?
The Wood Guide · Calculating with wood
TreatmentPressure-treated wood
The treatment that makes affordable softwood usable outdoors — and the reason our picnic tables last ten to fifteen years instead of three.
- Under pressure, deep into the fibre
- From class 4–5 to 3–4
- Does not protect against UV
Pressure-treated wood in brief
Wood that has undergone a treatment to make it resistant to rot, fungi and insects — and that difference is bigger than most people think.
In pressure treatment, protective substances are forced deep into the wood cells under pressure. That is the essential difference from staining or painting: those sit on the surface, this sits inside. If you damage the outside of pressure-treated wood, the protection is still there.
For softwood that means a jump of two durability classes: from class 4 to 5 (a few years outdoors) to class 3 to 4 (ten to fifteen years, and longer with the right details).
Our KING picnic tables and beer table sets are built from pressure-treated spruce. Without that treatment the same wood would last only a few years outdoors — the treatment is not an extra but the precondition.
- What
- preservative forced into the fibre under pressure
- Effect
- from class 4–5 to class 3–4
- Recognisable by
- light green or brownish tinge
- Protects against
- rot, fungi, insects
- Does not protect against
- UV light and greying
- Maintenance
- cleaning; oil or stain for the colour
To be fair: pressure treatment does not protect against the sun. Your wood will simply turn grey, and if you want to keep the warm colour, you still have to oil or stain it every one to two years.
Click a chapter or use the arrows · drawings simplified, wood cells greatly enlarged.
What happens in the cylinder
- DryingThe moisture content of the wood is first brought down, otherwise there is no room for the preservative in the cells.
- VacuumIn a sealed cylinder the air is drawn out of the wood.
- Filling under pressureThe preservative is forced under high pressure into the space that has been freed up, deep into the fibre.
- FixingThe preservative binds to the wood cells and after that no longer washes out.
Freshly treated wood often has a light green or brownish tint from the preservative. It fades within one season under sun and rain — it is not paint and not a defect, but the sign that the treatment has been done.
What pressure-treated wood is for
Garden furniture
Picnic tables, beer table sets and benches that stand outdoors all year. See our KING picnic tables with 40 mm boards.
Fences and fencing
Posts and boards that face wind and weather every day.
Play equipment
Where structural safety has to remain guaranteed over years.
Decks and decking
An affordable alternative to tropical hardwood, provided you maintain it.
Structures in ground contact
Ground contact needs a heavier treatment class — ask for it explicitly.
Maintaining pressure-treated wood
The treatment removes the risk of rot, not the maintenance for the colour.
Hardly any work
- Clean once a year with lukewarm water and soft soap
- No pressure washer — it eats away the soft fibres
- The wood turns silver grey; structurally nothing changes
Every 1–2 years
- Stain or oil with a UV filter, in spring in dry weather
- Clean first and let it dry completely
- The water test tells you when it is time: if water stays in droplets, the protection is still there
How to do it in practice — with a yearly calendar and the mistakes you are better off avoiding — is in our maintenance guide for softwood.
Frequently asked questions about pressure-treated wood
How do I recognise pressure-treated wood?
By the light green or brownish tinge the preservative leaves behind. It fades within a season under sun and rain; after that the wood slowly turns silver grey if you do nothing.
Do I still need to treat pressure-treated wood?
Not for durability — the pressure treatment takes care of that. But for the colour, yes: the preservative does nothing against UV. If you want to keep it warm in tone, stain or oil it every one to two years.
How long does pressure-treated wood last?
Usually ten to fifteen years above ground, and longer with good construction details. Without the treatment the same spruce would last only a few years.
Can pressure-treated wood go in the ground?
Only with a heavier treatment class intended for that. Ask for it explicitly: ordinary pressure-treated garden timber is meant for use above ground level.
Is pressure-treated wood harmful?
Modern preservatives are a good deal milder than the older generation, but it is still a chemical treatment. Do not saw it without a dust mask, do not burn the offcuts, and take leftovers to the household waste recycling centre.
From treatment to furniture
Pressure-treated spruce is what our picnic tables are made from — the most affordable wood that lasts for years outdoors.
Have a question about your project? Just ask — we're happy to help.
Photo via Wikimedia Commons: Freshly pressure-treated boards with the typical light green tint — TS Eriksson, CC BY 3.0