Choosing the Right Cedar for a Garden Room

Why cedar still earns its place

Walk around any well-kept British garden and you will spot cedar sooner or later. Western red cedar, properly called Thuja plicata, has been the timber of choice for garden buildings, greenhouses, summerhouses and sheds for well over a century. Not because it is fashionable, but because it does something remarkable: it resists decay without any chemical treatment, holds its shape through freeze and thaw, and fades from a warm biscuit-brown to a soft silver-grey that sits beautifully against brick, flint and hedgerow.

The reason lies in the wood itself. Cedar contains natural extractives called thujic acid and other oils that fungi and insects find deeply unappealing. Its heartwood is rated as durability Class 2, which means a realistic service life of thirty to forty years or more in above-ground use, and considerably longer if air can move around it. Compare that with untreated softwood cladding, which may struggle to reach ten. For a garden room that you intend to sit in with a cup of tea for decades, that difference is worth paying for.

Not all cedar is the same cedar

This is where a great many projects go wrong. In Britain, "cedar" is used loosely, and the boards you are offered may be one of several quite different timbers.

• Western red cedar (Thuja plicata) — the genuine article, usually imported from British Columbia or the Pacific Northwest, and the standard against which the rest are judged. Light, stable, superb weathering.
• Japanese cedar (Cryptomeria japonica) — softer and noticeably less durable, closer to Class 3 or 4. It can work for sheltered, painted applications, but it is not a substitute for western red cedar on an exposed elevation.
• British-grown western red cedar — increasingly available and a sound choice, though the growth rings tend to be wider and knots more frequent, so the boards are more variable in appearance and stability.
• "Cedar" cladding of no stated species — treat with polite suspicion. Ask for the botanical name, not the marketing name.

A reputable supplier will tell you the species, the origin and the grade without hesitation. If they cannot, walk away.

Reading a board before you buy it

Once you have the right species, the quality of the individual board matters just as much. There are three things worth checking with your own hands.

Heartwood versus sapwood. The pale, creamy strip along the edge of a board is sapwood, and it has almost none of the rot resistance you are paying for. On a good board the sapwood is minimal or absent. On a cheap one, it can be a third of the width, and it will be the first part to go grey, split and soften.

Growth rings. Count them across the end grain. Slow-grown timber with tight rings — perhaps fifteen to twenty-five rings across an inch — is denser, more dimensionally stable and far less likely to cup or twist. Wide, spongy rings mean fast-grown wood that will move.

Moisture content. For cladding and garden room boards, aim for 12 to 15 per cent. Kiln-dried stock from a covered yard is ideal. Wet boards will shrink as they dry, opening up joints you carefully set tight on a warm Saturday in June.

Which profile, which thickness

For a garden room you are heating, insulating or simply using a great deal, 19mm shiplap is the sensible default. It overlaps, sheds water well and gives a clean horizontal or vertical line. A 19 x 144mm board typically covers about 133mm once the overlap is taken up, so a run of four metres takes roughly thirty boards per course — worth calculating before you order.

• Shiplap, 19mm — the workhorse for walls and gable ends.
• Tongue and groove, 19mm — tidier, more furniture-like, good for sheltered elevations and interior linings.
• Feather edge, 12–16mm — fine for a simple shed, but too thin for a building you care about.
• 25mm boards — worth considering on a fully exposed west or coastal elevation, where driving rain is relentless.

Whatever you choose, keep boards clear of the ground by at least 150mm and let air circulate behind the cladding.

Fixings, finishes and letting it weather

Cedar is acidic and will corrode ordinary galvanised nails and screws, leaving black streaks down your new cladding within a season or two. Use stainless steel fixings throughout — A2 is acceptable, A4 is better near the coast. Silicon bronze is traditional and lovely, but expensive.

You have two honest choices on finish. Leave it bare and let it silver gracefully, accepting that the first two years of weathering are uneven and slightly patchy. Or treat it with a quality oil or water-based stain containing UV inhibitors, which preserves the colour and slows the greying. What you should not do is varnish it. Varnish traps moisture, cracks in the sun and is a misery to remove. Re-oil every two to three years, ideally in early autumn.

Buying well and storing it properly

Order ten per cent more than you measure, so you can cut around the worst knots and still finish the job. Ask for boards from a single batch where possible, so the colour and grain match across the building. When the timber arrives, do not lean it against the wall in the rain. Stack it flat on bearers, face to face, with spacers between the layers, under a breathable cover, and let it acclimatise for a week before you cut. Cedar rewards that small patience with a garden room that will still be standing, and still looking well, long after the fence panels beside it have been replaced twice.