Start with a safety check on the timber

Reclaimed timber is one of the quiet pleasures of a British garden. Boards pulled from a skip, a neighbour's shed clear-out or a salvage yard carry their own history, cost very little, and often outlast the flimsy stuff sold in bundles at the garden centre. Before any of that, though, you need to know what has been done to them, because a raised bed is a place where soil, roots and eventually your dinner all sit against the wood.

Run through these checks before you cut anything:

• Creosote and coal tar. If a board is dark, oily and smells strongly of tar, leave it. Creosote has been banned for domestic use in the UK since 2003, and old railway sleepers are the usual culprit. Not worth the risk around vegetables.
• Old lead paint. Flaking paint on pre-1960s timber can contain lead. Scraping it creates dust, and the residue sits in your soil. Use it elsewhere or pass it on.
• Chromated copper arsenate. Older imported pressure-treated timber can contain arsenic-based preservatives. If you cannot trace where a board came from and it has a greenish tinge, be cautious.
• Modern pressure-treated wood. Today's tanalised timber uses copper-based preservatives and is widely considered acceptable for growing beds. Lining it, as described below, gives extra peace of mind.
• Pallets. Look for an HT stamp, which means heat-treated and safe. Avoid MB, which indicates methyl bromide fumigation, and steer clear of any pallet with chemical staining or a strong odour.

Untreated hardwood such as oak, sweet chestnut or larch is the gold standard. It will give you ten to twenty years without any treatment at all. Untreated softwood, by contrast, may only manage five to seven.

Choosing and preparing the boards

The ideal reclaimed board for a raised bed is about 200–225mm deep and at least 38mm thick. Reclaimed scaffold boards are perfect if you can find them: sturdy, uniform and usually cheap. Thin cladding or old floorboards will bow under the weight of wet soil.

Give each board a quick going-over before it goes anywhere near your plot. Hammer flat any protruding nails, then pull them out with pincers. Run your hand along the edges and sand down splinters. Old timber is often very dry and splits easily, so pre-drill every screw hole. Use galvanised or stainless steel screws — 75mm for joining board to board, longer if you are fixing into posts.

Sizing it for a small space

In a courtyard, balcony or narrow back garden, one well-made bed beats three cramped ones. A 1.2m x 1.2m square is the classic small-space size: you can reach the middle from any side without treading on the soil, and it holds a surprising amount. If you have room for a rectangle, 1.2m x 2.4m is generous without becoming a trek.

Keep the width at or under 1.2m for the same reason. Two boards deep gives you roughly 450mm of growing depth, enough for carrots, parsnips and maincrop potatoes, and kind to your back. Leave 60–75cm between beds if you are adding more than one, so a wheelbarrow and a kneeling gardener can both fit.

Place it in full sun — six hours or more for tomatoes, beans and courgettes — and within easy reach of a water butt or tap. On paving or a patio, a permeable membrane underneath will stop weeds and save your slabs from staining.

Building the frame and lining it

Cut two boards to length and two to width for each layer. Butt the corners together and fix through the side boards into the ends, or set 100mm x 100mm corner posts inside the frame and screw each board to them. Corner posts make a far more rigid bed and are the trick that stops reclaimed timber twisting as it dries.

When the frame is up, line the inside faces with heavy-duty polythene, damp-proof course membrane or butyl pond liner. Staple it to the top edge of the boards and let it hang down the inside, trimming it just above the soil line. This creates a barrier between your growing medium and any treatment in the wood. Do not line the base — you want water to drain freely, and open ground below lets worms and roots move up.

Filling and planting up

For a 1.2m x 2.4m bed at 450mm deep, you need roughly 1.3 cubic metres of material. A good blend is around 60 per cent topsoil, 30 per cent garden compost, and 10 per cent horticultural grit for drainage on heavy clay. Mix it on a tarpaulin beside the bed rather than in it, then fork it in loosely — do not trample it down.

Water thoroughly and let the bed settle for a week before planting. Salad leaves, radishes and herbs will crop within weeks. In the first season, keep taller crops such as climbing beans and cordon tomatoes at the northern end so they do not shade the rest.

Looking after reclaimed timber

Reclaimed boards move as they age, so check the fixings each spring and tighten any that have worked loose. Topping up the soil with compost every year keeps the level and feeds the bed at the same time. Avoid painting or staining the inside faces, and never treat timber that is already in contact with growing soil.

Looked after like this, a bed built from somebody else's discarded boards will quietly outlast most things you could have bought new — and it will have cost you an afternoon rather than a fortune.