How to Build a Raised Garden Bed

What you need

Timber boards, screws, a drill/driver, a tape measure, a spirit level, a saw, soil and compost, and cardboard or weed membrane for the base. Use untreated timber or a naturally rot-resistant wood — treated timber can leach chemicals into soil you plan to grow food in, and old railway sleepers are worse: they are treated with creosote and do not belong in a vegetable bed.

Step 1 — Choose the spot and the size

Pick a spot with at least six hours of sun a day, level ground and water nearby — you will be watering it all summer. A bed 8 ft long and 4 ft wide is a good size: you can reach the middle from both sides without stepping in, and stepping in compacts the soil you just loosened. About 12 in high is enough for most vegetables; deeper is better for root crops.

Step 2 — Build the frame

Cut the boards to length and screw the corners together, two screws per joint, then check the frame is square by measuring the diagonals — they must be equal. Set the frame on the ground and level it with the spirit level, packing soil under the low side. A frame that is not level fills unevenly and dries out on the high side.

Step 3 — Line and fill

Lay cardboard over the grass inside the frame — it smothers the weeds underneath and rots away — and add weed membrane if you want extra protection. Then fill with a mix of topsoil and compost: roughly two parts topsoil to one part compost for vegetables. The garden bed calculator works out the volume and the bags from the bed size, so you can order the soil in one trip instead of guessing.

Step 4 — Water, settle and plant

Water the filled bed well and let it settle for a few days — fresh soil sinks, and a bed filled to the brim on day one is half empty by the weekend. Top it up, rake the surface level, and plant. Water little and often for the first two weeks while roots establish, then water deeply and less often so the roots grow down.

Worked example

Two beds, each 8 ft × 4 ft and 1 ft high. One bed holds 32 ft³ of soil, so two hold 64 ft³ (2.37 yd³). With 10% wastage that is 70.4 ft³ (2.61 yd³, or 1.99 m³), which is 50 bags of 40 L. Those are exactly the numbers the garden bed calculator gives you.

Common mistakes

• A bed too wide to reach. If you have to step into the bed to reach the middle, the soil compacts and the plants suffer. Keep it 4 ft wide or less.
• Using treated timber or old sleepers. Chemicals leach into the soil. Use untreated timber or a naturally rot-resistant wood.
• Filling with poor soil and no compost. The bed is only as good as its soil. Compost is not optional — it feeds the plants and holds water.

Safety

Soil bags are heavy — lift with your knees and work in pairs for a big bed. Wear gloves: soil can hide sharp objects and thorns. If you are digging the ground beneath the bed, check for underground cables and pipes first. Never use creosote-treated timber or old railway sleepers for a bed you will grow food in.

Try these calculators

• Garden Bed Calculator — soil volume and bags for any bed.
• Soil Calculator — soil volume and bags for beds and borders.
• Compost Calculator — compost volume for beds and borders.