How do I stop a wooden raised bed rotting?

The boards are rarely the first thing to fail. Corners loosen because the fixings corrode, and the bottom board goes because its lower edge and cut ends stay permanently saturated.

What you'll need

• Damp-proof membrane or liner
• Stainless or hot-dip galvanised screws
• End-grain preservative
• Gravel (optional)

Step by step

1. Choose the timber deliberately.Untreated softwood gives about three to five years. Tanalised softwood gives ten or more. Larch and sweet chestnut last well untreated. Oak is the longest lasting and the most expensive.
2. Line the inner faces.Damp-proof membrane stapled to the inside of the boards keeps saturated compost off the wood. It is the single biggest change to lifespan and costs very little.
3. Treat the cut ends.Cutting tanalised timber exposes untreated end grain, which is exactly where water enters. Brush end-grain preservative on every cut before assembly.
4. Use corrosion-resistant fixings.Stainless steel or hot-dip galvanised. Ordinary zinc-plated screws corrode within a couple of seasons in wet treated timber, and the bed falls apart at the corners while the boards are still sound.
5. Keep the bottom edge off wet ground where you can.Setting the boards on a strip of gravel, or on bricks or paving at the corners, stops the lower edge sitting permanently in mud.
6. Support long sides.Beds over about 2m long bow outwards under the weight of wet soil. A stake driven inside at the midpoint, or a cross-tie across the top, holds them.
7. Set corner posts proud of the boards.Screwing boards to internal corner posts is far stronger than screwing board ends to each other, and the posts can be replaced without dismantling the bed.
8. Design for board replacement.If the bottom board is a separate piece rather than one tall board, you can replace just that when it goes and keep the rest.
9. Do not paint the inside.Paint traps moisture in the timber and peels. A liner is the right barrier; paint or preservative belongs on the outer faces.
10. Consider alternatives for permanence.Sleepers, gabions, recycled plastic boards, corrugated steel or brick all last far longer than softwood, at more cost and more build effort.

Tips

• It is nearly always the fixings and the bottom board that fail, not the boards you can see.
• End-grain preservative on every cut is a five-minute job that adds years.
• A liner on the inner faces roughly doubles the life of softwood boards.

Common mistakes

• Using ordinary screws in treated timber — The preservatives in tanalised timber corrode zinc-plated steel quickly. Corners loosen within two or three years while the wood is still perfectly sound.
• Painting or oiling the inside of the boards — It seals moisture in rather than out, and it can leach into the growing medium. Use a physical liner inside and put any coating on the outside only.

If it doesn't work

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