How do I build a raised bed?

Sizing, siting and filling a raised bed, including the timber choice and the depth that actually matters.

Difficulty: intermediate
Time: 4 hr
Read: 2 min

Short answer

Make it no more than 1.2m wide so you can reach the middle from both sides, at least 25–30cm deep, and site it in the sunniest spot with easy access to water. Fill with a mix of topsoil and compost rather than compost alone, which sinks and dries out.

A raised bed gives you control over soil quality and drainage, which is why it works so well on heavy clay or poor ground. The two decisions that matter are width — because you must never stand on the soil — and what you fill it with.

What you'll need

Timber boards (untreated or naturally durable)
Corner posts or brackets
Screws and a drill
Topsoil and compost
Spirit level
Cardboard for the base (optional)

Step by step

Site it in the sunniest spot with water within easy reach.Most vegetables want six hours of sun. If watering means carrying cans across the garden, it will not happen in July.
Make it no more than 1.2m wide.So you can reach the centre from both sides. Standing on the soil compacts it, which defeats the point of a raised bed.
Make it at least 25–30cm deep.Deeper is better for root crops. Shallower beds dry out fast and limit what you can grow.
Choose durable untreated timber, or a treatment safe for growing food.Larch, cedar and oak last well untreated. Avoid old railway sleepers, which may contain creosote.
Build the frame, checking it is square and level.Screws rather than nails, and corner posts or brackets to stop the sides bowing outward under the soil load.
Lay cardboard on the ground inside before filling.It suppresses grass and weeds and rots away within a season, letting roots and worms through.
Fill with roughly 60% topsoil and 40% compost.Compost alone sinks dramatically and dries out. The soil gives it body and holds moisture.
Top up with compost each year rather than digging.The level drops as organic matter breaks down. Adding a mulch annually is all the maintenance it needs.

Tips

Leave at least 45cm between beds so you can get a wheelbarrow through.
Beds against a fence should be narrower — 60cm — because you can only reach from one side.
A bed filled with pure bought compost will sink by a third in the first year. Mixing in topsoil avoids that.

Common mistakes

Making the bed too wide — You end up standing on the soil to reach the middle, compacting it and undoing the main benefit.
Filling with compost alone — It sinks substantially, dries out fast and has too little body to hold nutrients well.
Using old railway sleepers — Many are treated with creosote, which is not suitable near food crops.

Questions people ask

How deep should a raised bed be?

At least 25–30cm for general vegetables, more for root crops like carrots and parsnips. Shallower beds dry out quickly and limit what will grow well.

What should I fill a raised bed with?

Roughly 60% topsoil to 40% compost. Compost alone looks ideal but sinks dramatically in the first year and dries out much faster.