What is Garden Timber?
The Wood Guide · Choosing and comparing
Buying guideGarden timber
Decking, fences, pergolas, planters and furniture. Almost every garden project starts with the same question: which wood can cope here?
- Not a species, but a function
- Class determines service life
- Treatment extends it
Garden timber
Decking, fences, pergolas, planters and furniture. Almost every garden project starts with the same question: which wood can cope here?
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Garden timber in brief
Not a specific wood species, but the collective term for wood that is suitable for outdoor use.
The most important property is durability in the technical sense: withstanding rain, sun, snow and frost — and above all the fungi that live on damp wood. You achieve that in two ways: choosing a species that is naturally durable, or treating ordinary wood so that it lasts longer.
In Belgian and Dutch gardens the second route is by far the most common: pressure-treated spruce, which lasts ten to fifteen years at a fraction of the price of tropical hardwood.
Wood that touches the ground always has a harder time than wood that stands free. A fence post in the ground needs durability class 1 or 2, or pressure-treated wood in the right use class. The same species can last much longer above ground.
- What it is
- wood suitable for outdoor use
- Key factor
- resistance to moisture and fungi
- Naturally durable
- teak, ipé, oak, western red cedar
- Treated
- pressure-treated spruce, thermally modified wood, accoya
- Service life
- 4 years (untreated spruce) to 50 (ipé)
- Biggest mistake
- the right species in the wrong place
To be fair: the most expensive species is not always the right one. Ipé under a planter is money thrown away, and so is teak for a twenty-metre fence. Choose on the basis of load and ground contact, not on the basis of price.
Which wood for which job
| Use | Recommended | Why |
|---|---|---|
| Garden furniture | Pressure-treated spruce, acacia, teak | Strength-to-price ratio, or maintenance-free with teak |
| Decking, heavy foot traffic | Ipé, bangkirai, thermally modified wood | Wear resistance and class 1 to 2 |
| Decking, normal use | Douglas fir, larch, pressure-treated spruce | More than adequate at a fraction of the price |
| Fence, posts in the ground | Pressure-treated softwood, robinia | Ground contact demands the highest use class |
| Pergola and canopy | Douglas fir, larch | Above ground, so class 3 is enough |
| Planters | Pressure-treated spruce with liner | Permanent contact with moisture; the liner separates soil from wood |
| Cladding | Western red cedar, thermally modified wood | Stable, light and durable |
How to stretch the service life
Impregnation
Under pressure, agents are introduced that keep fungi and insects out. The standard route for softwood — see pressure-treated wood.
Thermal modification
Heating to 180 or 212 °C breaks down hemicellulose, which counters rot without chemicals. See thermally modified wood.
Acetylation
Chemical conversion of the cell structure, as with accoya. Delivers class 1 from European softwood.
Oils and stains
Protects the surface against UV and moisture. Does not fundamentally extend the service life, but does extend the appearance — and has to be repeated.
Oil or stain on untreated spruce does not turn it into class 2 wood. Only impregnation, thermal modification or acetylation changes the wood itself. The rest is surface protection, and you have to maintain it.
What garden timber asks of you
- Clean every yearSoft brush, water and soft soap. No pressure washer on soft wood species — it knocks out the soft earlywood fibres and roughens the surface.
- Check the end grainThat is where every problem starts. If you see cracks, seal them with an end-grain coating.
- Oil if you want the colourOnce or twice a year for hardwood, yearly for treated softwood. If you skip it, it turns grey — without any loss of strength.
- Keep it off the groundFurniture legs on tiles or caps rather than straight on the grass. That makes a difference of years.
Frequently asked questions about garden timber
Which garden timber lasts longest?
Can untreated spruce be used in the garden?
Only for a short time. Spruce is in class 4 and lasts four to ten years unprotected, and much less in ground contact. Pressure-treated, it reaches ten to fifteen years.
Do I need to oil garden timber every year?
Only if you want to keep the colour. For service life it is not necessary with durable species — weathering to grey untreated does not affect strength.
What is the difference between durability class and use class?
The durability class tells you how rot-resistant the wood is by nature; the use class tells you how harsh the environment is, from dry indoors (1) to permanently in salt water (5).
Garden timber in perspective
The right species in the right place lasts longer than the most expensive species in the wrong one.
Have a question about your project? Just ask — we're happy to help.
Photos via Wikimedia Commons: Pressure-treated boards with a greenish tint — TS Eriksson, CC BY 3.0 · Wooden planters as raised vegetable beds with tomatoes — Globetrotter19, CC BY-SA 4.0 · Charlecote Park, Warwickshire — Acabashi, CC BY-SA 4.0