How to Build a Raised Bed with Topsoil

Why Raised Beds?

Raised beds solve several problems at once: poor drainage, bad soil quality, limited mobility, and the need for intensive growing in a small space. They're particularly valuable if your garden soil is heavy clay, contaminated with builder's rubble (common in new builds), or just too poor to grow well in.

The trade-off is cost — filling a raised bed requires a significant volume of topsoil and compost. Getting the fill mix right first time is important because replacing the contents of a filled bed is a miserable job.

Construction Basics

Materials

• Timber: The most popular choice. Use untreated hardwood (oak, larch) or softwood sleepers. For food growing, check that any treated timber is safe — modern tanalised timber uses copper-based preservatives that are considered safe for food contact, but avoid older CCA-treated (arsenic) timber
• Brick or block: Permanent, attractive, but more expensive. No lining needed
• Galvanised steel: Increasingly popular, durable, and modern-looking. Warms soil quickly in spring
• Stone or gabion: Attractive in the right setting but expensive

Dimensions

• Width: 1.2m maximum so you can reach the centre from either side without stepping on the soil
• Length: Whatever suits your space — 2.4m is popular (standard timber length)
• Height: Minimum 200mm for shallow-rooted plants, 300-450mm for vegetables, 600mm+ for accessibility (wheelchair or standing height)

Base

Raised beds can sit directly on existing soil, gravel, or even paving:

• On soil: The ideal setup. Roots can grow down into the ground below, and drainage is natural. Fork over the base to break up compaction
• On paving or concrete: Drill drainage holes in the paving if possible, or line the base with a layer of coarse gravel (50-75mm) before adding topsoil
• On gravel: Good drainage but may need a membrane at the base to stop soil washing into the gravel

Filling the Bed: The Mix

This is where most people go wrong. Pure topsoil settles, compacts, and doesn't provide the organic matter that intensive growing demands.

The Standard Mix

For most purposes — vegetables, flowers, herbs, shrubs — the recommended fill is:

• 60-70% screened topsoil (general-purpose or premium grade)
• 30-40% composted organic matter (composted green waste, mushroom compost, or well-rotted farmyard manure)

This gives you good structure, adequate drainage, and the fertility needed for productive growing. For more on topsoil for food growing, see our vegetable garden guide.

Alternative Mixes

• For Mediterranean or herb beds (lavender, rosemary, thyme): 50% topsoil, 30% sharp grit, 20% compost — these plants need excellent drainage
• For acid-loving plants (blueberries, cranberries): 40% topsoil, 40% ericaceous compost, 20% composted bark — check the pH
• For cut flower beds: 60% topsoil, 40% compost — flowers are hungry plants

What NOT to Use

• Pure compost: Too rich, too light, dries out, and subsides dramatically. Also lacks the mineral content plants need long-term
• Subsoil or unscreened fill: No nutrient value, poor structure, likely full of stones
• Topsoil alone: Compacts, becomes hard, doesn't sustain intensive growing

How Much Fill Do You Need?

Calculate the internal volume: Length × Width × Height.

Example: A bed 2.4m × 1.2m × 0.45m = 1.3m³

That's roughly 3 bulk bags of topsoil and 3-4 large bags (around 0.5m³ each) of compost, mixed together.

Add 15% to your calculation for settlement. Fresh fill drops noticeably in the first few weeks as it's watered and rained on.

Cost Estimate

For a 2.4m × 1.2m × 0.45m bed:

• Topsoil (3 bulk bags at £60-£90 each): £180-£270
• Compost (equivalent volume): £80-£150
• Total fill cost: roughly £260-£420

This is a significant investment, which is why getting the mix right matters.

Filling Technique

Layer by Layer

Don't dump all the topsoil in and all the compost on top. Mix as you go:

1. If the bed is over 300mm deep, you can fill the bottom 100mm with less premium material — composted green waste, semi-composted garden material, or even well-rotted logs (a technique called Hügelkultur). This reduces cost without significantly affecting the growing zone
2. Add the topsoil-compost mix in 100-150mm layers, lightly mixing each layer as you fill
3. Water each layer before adding the next — this helps settlement happen now rather than after planting
4. Fill to 25-50mm above the bed rim to account for settlement

After Filling

Water thoroughly and leave for at least a week before planting. The mix will settle. Top up any low spots and re-level.

If filling in autumn or winter, leave until spring before planting — the freeze-thaw cycle will help break up the topsoil and create a beautiful tilth by planting time. This is one of the advantages of preparing beds in the off-season.

Maintaining the Soil

Raised bed soil needs annual replenishment:

• Each autumn: Add a 25-50mm layer of compost or well-rotted manure on top. Don't dig it in — worms will incorporate it over winter
• Don't walk on the soil. This is the whole point of raised beds — uncompacted soil means healthy roots and better drainage
• Rotate crops if growing vegetables, to prevent nutrient depletion and disease build-up
• Top up topsoil every year or two as the level drops from decomposition and harvest removal

Common Mistakes

• Filling with whatever's cheapest. Economy topsoil and unscreened material saves money upfront but gives you a bed full of stones, weeds, and clay lumps
• No drainage. If the bed sits on solid ground with no drainage path, it becomes a container that floods in winter. Ensure water can escape from the base
• Too narrow. A 600mm-wide bed looks neat but gives you a 300mm growing strip after the edges. 1m minimum width is practical
• Too tall without support. A 450mm raised bed filled with wet soil exerts significant lateral pressure. Timber walls need corner posts and intermediate supports to prevent bowing