Why the British climate is hard on timber

Anyone who has watched a garden bench turn black and spongy within three winters knows that outdoor furniture asks a great deal of its timber. Our climate is a slow, relentless test: damp autumns, mild winters that never quite freeze the moisture out of the wood, and long spells of humid air that keep surfaces above the threshold where fungal decay gets going. Rot in Britain rarely comes from a single soaking. It comes from timber that stays wet for weeks at a time.

So the question is not simply "which wood is hardest?" It is "which wood copes best with repeated wetting and drying, and which one will still be sound in fifteen years?" The answer depends on three things: the natural durability of the species, whether you are using heartwood or sapwood, and how well the piece sheds water.

Oak: the traditional benchmark

English oak and European oak remain the default choice for good reason. The heartwood is dense, rich in tannins, and rated as durable to very durable, meaning it will shrug off fungal attack for decades even when left unfinished. It weathers to a handsome silver-grey, which suits a cottage garden far better than any stain.

Two cautions, though. First, oak must be air-dried or kiln-dried to around 15–18% moisture content for furniture work; green oak moves dramatically as it dries and will split joints that were tight when built. Second, oak's tannins react with iron and steel, leaving black stains and corroding fixings. Use stainless steel or silicon bronze throughout, and never galvanised screws.

Sweet chestnut is a close cousin, often cleft and coppiced, with similar looks and slightly lower durability. It is an excellent, sustainable choice for slatted seats and trestles.

Larch and other homegrown softwoods

You do not need a hardwood to build furniture that lasts. European larch is the quiet hero of British outdoor work: moderately durable, strong, and pleasantly resinous, with a warm colour that fades gently in sun. It is the sensible choice for budget-conscious projects and for painted work, since a good paint system will protect the timber well beyond its natural lifespan.

Douglas fir is stronger but less durable, so it belongs above ground and under cover. Western red cedar is the opposite trade-off: light, exceptionally stable, and naturally resistant to decay and insect attack, though soft enough to dent. It is superb for chairs and table tops that get moved about.

• European larch — good all-rounder, ideal for painted or oiled frames
• Douglas fir — strong and stiff, best for sheltered structures
• Western red cedar — light, stable, excellent for seating
• Scots pine — only with thorough preservative treatment, and never in ground contact

Teak, iroko and the tropical question

Teak earned its reputation honestly. Its natural oils and dense grain make it one of the few timbers that can be left bare for decades, greying gracefully without splitting. The difficulty is provenance: much of what is sold is young plantation timber with far less oil than the old-growth boards of legend, and prices have risen accordingly.

Iroko is the pragmatic alternative, hard, stable and highly durable, though its dust is a known irritant and its colour darkens dramatically on exposure. Both species deserve respect for the forests they come from, so buy only timber with credible certification and ask where it was felled.

The details that cause rot

Most failed outdoor furniture was not let down by the species. It was let down by sapwood, end grain and water traps.

• Sapwood is not durable. In oak, larch and cedar alike, the pale outer rings rot first. Specify all-heartwood and reject boards with wide bands of pale timber.
• End grain drinks water. Seal or cap every cut end, and design so that ends point downwards.
• Joints hold moisture. Use mortise-and-tenon or bridle joints with drainage, and drill a small weep hole in the bottom of any enclosed socket.
• Ground contact is fatal. Put feet on stainless or plastic shoes, or use a rot-resistant species such as iroko or oak for the legs alone.
• Trapped water pools. Crown every top surface very slightly, and avoid flat rebates where rain can sit.

Fixing choice matters as much as timber: stainless steel throughout, countersunk flush, and never two dissimilar metals in contact.

A few practical rules of thumb

For a bench that will live outside all year, oak or iroko for the frame with larch or cedar slats is a hard combination to beat. For a table under a pergola, larch is ample. For anything you intend to paint, Douglas fir and larch take paint beautifully and will outlast their natural rating by a decade or more.

Whichever you choose, buy from a reputable sawmill that can tell you the species, the moisture content and the durability class, and finish with an oil or breathable stain rather than a film-forming varnish that will trap water beneath it. Get those details right, and your furniture will be sitting quietly in the garden long after the cheap imports have gone to the bonfire.