Which timber lasts outdoors without being treated?
The species with genuine natural durability, the sapwood caveat that ruins most of them, and why the detailing matters more than the timber you pick.
- Difficulty
- beginner
- Time
- 10 min
- Read
- 4 min
Short answer
Oak heartwood, sweet chestnut, iroko and western red cedar are durable enough to use outdoors untreated above ground. European larch and Douglas fir are moderately durable and will do if they can dry between wettings. Pine, spruce and whitewood — most of what a merchant stocks — are perishable and need pressure treatment. And in every case durability belongs to the heartwood only: the pale sapwood of every one of them rots.
Natural durability is a measured property — resistance to fungal decay — and it is ranked in classes from very durable to perishable. It matters because a durable timber left unfinished simply weathers to silver-grey and carries on, while a perishable one turns to compost. But the rankings are for heartwood, and the way a piece is detailed changes its life more than the species does.
Step by step
- Separate durability from every other property.Durability is decay resistance, and it has nothing to do with hardness, strength or price. Cedar is soft and durable; beech is hard and perishable. Do not infer one from the other.
- Learn the durable end.Oak heartwood, sweet chestnut, iroko, western red cedar and true teak are the ones you will actually encounter. Oak and sweet chestnut are the two commonly available British-grown options. Cedar is durable but soft and dents easily. Iroko is the usual choice for external joinery.
- Learn the middle.European larch and Douglas fir are moderately durable. Both are used untreated for cladding, and both will give a long life above ground where they can shed water and dry, and a short one in ground contact or trapped against a wet surface.
- Accept that the merchant's standard stock is perishable.Pine, spruce and whitewood have essentially no natural durability. Anything made from them outdoors must be pressure treated to a use class matched to the exposure, and every cut end re-treated.
- Check for sapwood on every board.Durability is a heartwood property. The pale outer band of sapwood on an oak or chestnut board is as perishable as pine. A post with a wide sapwood band will rot at that band while the rest is sound, which is why durable-species posts still fail at the ground line.
- Detail so the timber can dry.Decay needs sustained moisture. Slope surfaces so water runs off, avoid horizontal ledges and open upward joints, keep end grain out of standing water, leave a gap behind cladding so air circulates, and keep everything clear of soil and splashback. Good detailing outlasts a better species.
- Use treated softwood where anything goes into the ground.Even durable species have a limited life in permanent ground contact. For posts and anything buried, pressure-treated softwood to the ground-contact use class, or a post support that keeps timber above ground entirely, is the more sensible answer.
- Expect the colour to go.Every untreated timber outdoors weathers to silver-grey within a year or two as the surface degrades in UV. That is a surface effect and does not affect durability. If you want the colour kept, that is a finishing decision and a maintenance commitment.
Tips
- Sweet chestnut is often overlooked and is one of the best-value durable British timbers — very similar to oak in durability, cheaper, and without oak's tendency to stain everything it touches.
- Western red cedar's durability is genuine, but its softness means it marks and dents easily and it is not a good choice anywhere it will be knocked or walked on.
- Second-growth and fast-grown timber of a durable species is generally less durable than old slow-grown stock of the same species, because there is proportionally more sapwood and wider growth rings.
Common mistakes
- Buying "oak" fence posts and burying them — The sapwood band rots at the soil line within a few years, and the post snaps there while the rest looks perfect. Ground contact wants treated softwood to the right use class, or a metal post support.
- Assuming a durable species does not need any detailing — A flat top on an oak gate post collects water and holds it in the end grain. Even a very durable timber decays where water sits and cannot escape. Slope the top or cap it.
- Applying a film finish to durable timber outdoors and forgetting it — A varnish or a paint that cracks lets water in and then stops it getting out, which is worse than leaving the timber bare. If a durable timber is doing the work, either leave it to grey or commit to a maintainable penetrating finish.
Questions people ask
Is green oak the same as ordinary oak?
Green oak is oak used unseasoned, straight from the log. It is the same species with the same durability, but it is wet and heavy, it will shrink and split as it dries, and it stains anything ferrous within reach. It is used deliberately for structural frames and garden structures, where the movement is designed in and stainless or oak fixings are used throughout.
What about accoya and thermally modified timber?
Both are real answers to this question: fast-grown softwood chemically or thermally modified so that it becomes highly durable and very stable. They cost considerably more than treated softwood and are mostly seen in external joinery and cladding, where the stability is worth as much as the durability.
Does a durable timber still need a preservative?
No, and applying one to durable heartwood achieves very little because it will not penetrate. What durable timber sometimes gets is a UV-blocking oil or a pigmented finish, which is about appearance rather than decay.