Raised beds in a greenhouse work best as two runs flanking a central path: 25 to 30 cm deep, no wider than you can reach from the path, with at least 60 cm left to walk on. Fill them with topsoil, organic matter and a little sharp sand, and leave the frame breathing room.
The exact numbers shift with the width of your frame, and that is where most layouts go wrong. A 2.6 m timber house takes a bed either side comfortably. A 3.14 m arch takes two wider beds and a barrow-friendly path. At 4 m you get choices: three beds, or a double path. This guide works through each, with the maths shown.
One caution before the timber goes on the shopping list. Beds are optional. If your greenhouse sits on decent loam, the open border will grow tomatoes perfectly well for a season or two while you learn how the light falls. Build beds when you know where you want them.
Raised beds, border soil or pots: choosing your growing setup
Inside a greenhouse the three options behave differently from the way they do outdoors, mainly because rain never reaches any of them. Every drop of water arrives from your can or your irrigation line, so drainage and soil volume matter more, and natural re-wetting never bails you out.
| Raised beds | Border soil | Pots and grow-bags | |
|---|---|---|---|
| Spring warmth | Warms earlier than the border; University of Minnesota Extension lists earlier planting as the first benefit | Slowest to wake, especially clay, which the RHS notes drains poorly and warms slowly in spring | Fastest to warm, fastest to chill overnight |
| Drainage | You build it: open base plus a gritty mix | Whatever your subsoil gives you | Free-draining but dries out in hours on a hot day |
| Soil control | Full: swap the top 30 cm when tomato root troubles build up | Limited; replacing border soil means a barrow and a sore back | Total, though the volume is small and compost is an annual cost |
| Cost to start | Timber plus roughly 1 m³ of fill per 3 m bed | Nothing | Smallest outlay in year one, repeated every year |
Verdict: grow-bags earn their keep in a first season and beds win from year two. The border makes a fair compromise on light soil, but after a few summers of tomatoes in the same spot, soil-borne root problems accumulate, and a raised bed lets you lift and replace 30 cm of growing medium in an afternoon rather than digging out the whole floor.
Bed heights and widths that keep the paths workable
Reach sets the width. The RHS advises keeping beds under 1.5 m where you can work from both sides; the University of Minnesota Extension puts the one-sided limit at 2.5 ft, about 75 cm. In a greenhouse nearly every bed sits against a wall, so the one-sided figure governs. Anything much past 75 cm and the back row becomes a stretch, which in practice means it gets weeded less and watered worse.
Height matters for your back more than for the plants. A 20 to 30 cm bed suits most crops and most budgets. At 45 cm you kneel less. At about 69 cm (the 27 in figure Minnesota Extension cites) a wheelchair user or a seated gardener can work without leaning. Taller beds cost more to fill, though there is a trick for that in the filling section below.
Paths carry everything else. The RHS minimum for a wheelbarrow is 45 cm, and 1 m accommodates a wheelchair. Between those, 60 cm feels fine walking forwards and tight when you turn with a full watering can.
Whatever you choose, never step on the beds themselves: compacted soil loses the open structure that makes a raised bed worth building, which is the same reason field growers in AHDB's soil programmes keep traffic to fixed wheelings.
Materials: boards, sleepers and the treated-timber rule
Scaffold boards, gravel boards, new softwood sleepers, brick, recycled plastic: all of them make sound bed walls. Oak and larch heartwood last for years untreated, and a standard 225 mm scaffold board stood on edge gives you a bed wall of about 22 cm in a single course.
The rule that catches people out concerns reclaimed railway sleepers. The Creosote (Prohibition on Use and Marketing) (No. 2) Regulations 2003 ban creosote-treated wood inside buildings, in gardens where frequent skin contact is likely, and in containers intended for growing. A greenhouse raised bed manages all three at once. If a salvaged sleeper smells of tar on a warm afternoon, use it for a retaining wall by the drive, never for food crops under glass or polycarbonate.
Modern pressure-treated softwood is a different case. Today's preservatives are copper-based, the RHS lists treated softwood sleepers as an economical choice that lasts many years, and Minnesota Extension's advice is to pick timber rated for ground contact. Cautious growers staple a polythene liner down the inside faces, sides only, base open, so soil never rests against the treated surface. That liner also protects the boards themselves, so it earns its place either way.
One frame-specific note. In a timber greenhouse such as our NORDIC, with its 45 × 45 mm FSC Swedish pine frame, leave a 5 cm air gap between bed walls and frame posts. Damp soil held against any timber shortens its life, whatever the treatment says.
Depth and filling: how much soil you actually need
Match depth to roots. The RHS guidance runs 30 cm for salad leaves and strawberries, 45 cm or more for most other plants, and deeper still rewards parsnips and long carrots. On an open-ground base, roots carry on past the bed into the soil below, so 25 to 30 cm of built depth does the job for tomatoes and cucumbers. On slabs or concrete the bed becomes a large container: go to 45 cm and leave weep gaps in the lowest course. How the base itself gets built, and which one to pick, is covered in our greenhouse base preparation guide.
Volume sneaks up on people. A single bed of 3.2 m by 1.1 m by 0.3 m holds just over 1 m³, so a two-bed layout needs around 2 m³ of fill, which arrives as roughly three bulk bags. The RHS suggests a general mixture of topsoil, organic matter and sharp sand for most beds; two barrows of topsoil to one of compost is a sensible ratio to start with. For beds deeper than 50 cm, the RHS trick is to fill the bottom with rubble or subsoil, which improves drainage and roughly halves the topsoil bill.
Then comes water. Under cover, beds dry from the top down with no rain to reset them, so a finger test 5 cm below the surface twice a week tells you more than any schedule. Beds hold moisture far longer than grow-bags, which is most of the argument for them by July.
Greenhouse layouts by width, with real numbers
Width decides everything, length just repeats the pattern. Below are layouts for the widths we build plus the two commonest UK frame sizes. If you are still choosing between sizes, our greenhouse size calculator turns a crop list into a floor area first.
| Frame width | Layout | Beds | Paths |
|---|---|---|---|
| 1.83 m (6 ft) | Single bed | 1.1 m | 0.7 m |
| 2.44 m (8 ft) | Bed each side | 2 × 0.8 m | 0.8 m |
| 2.6 m (NORDIC) | Two beds, wide path | 2 × 0.8 m | 1.0 m |
| 3.14 m (SteelRoot) | Two full-width beds | 2 × 1.1 m | 0.9 m |
| 4.0 m (SteelRoot Wide) | Three beds | 0.95 + 0.9 + 0.95 m | 2 × 0.6 m |
| 4.0 m, double path | Wide centre bed | 0.8 + 1.2 + 0.8 m | 2 × 0.6 m |
3.14 m greenhouse layout: two beds and a working path
Our SteelRoot Plot 3.14 spans 3.14 m with 2.02 m of standing height at the ridge, in five lengths from 4 m (12.56 m²) to 12 m (37.80 m²), priced from £899 to £1,899. The layout that fits it: a 1.1 m bed down each side and a 0.9 m central path, which sums to 3.1 m and leaves a few centimetres of tolerance against the galvanised arches.
The arch works in the layout's favour. Headroom shrinks towards the walls, exactly where the beds sit, and stays full-height over the path where you actually stand. A 1.1 m bed does exceed the 75 cm comfortable-reach rule, so plant deliberately: cordon tomatoes and peppers at the front where the daily work happens, and the far 30 cm under the curve for garlic and other sow-and-forget crops. The 0.9 m path takes a wheelbarrow with room to spare, double the RHS minimum.
4 m wide greenhouse layout: three beds or a double path
The SteelRoot Wide 4.00 measures 4.00 m across and 2.30 m to the ridge, again in five lengths, 4 m (16 m²) up to 12 m (48 m²), from £1,149 to £2,149. Two layouts divide that width honestly.
Three beds: 0.95 m against each wall, a 0.9 m bed down the centre, and two 0.6 m paths between them. The centre bed earns its keep because you reach it from both sides, well within the RHS both-sides rule, and it becomes the natural home for anything fussy. Alternatively, run the double-path version, 0.8 m side beds with a 1.2 m centre bed, when you want one deep block for a soil-hungry crop such as melons on a hotbed. Either way the paths still clear the wheelbarrow minimum, and 4 m of width means a helper can pass behind you without anyone stepping on soil.
2.6 m timber greenhouse: a bed either side
Our NORDIC timber greenhouse is 2.6 m wide inside its pine frame, in five sizes from 8 × 6 ft at £999 to 8 × 20 ft at £1,549. Two 0.8 m beds and a 1.0 m path split the width exactly, and 0.8 m beds sit within one-armed reach, so nothing at the back gets neglected. With the stable door at one end and the opening rear window at the other, the path doubles as the ventilation corridor; keep it clear and the through-draught does its work in a heatwave.
Half-and-half suits many growers better: one bed for planted crops, and where the second bed would go, a run of benching for trays and cuttings. Heights, tiers and what to put where are covered in our greenhouse staging and shelving guide.
Layouts for 6 ft and 8 ft greenhouses
The same arithmetic serves the classic frame sizes. A 6 ft house (1.83 m) has room for one 1.1 m bed and a 0.7 m walkway, with staging opposite the bed rather than a second run of soil. An 8 ft house (2.44 m) takes a 0.8 m bed each side of a 0.8 m path, a squeeze for a barrow but fine for a trug. In both, resist the urge to widen the beds at the path's expense; a path you cannot kneel in turns every low job into a balancing act.
Wheelchair and seated access
An honest note for anyone planning wheeled or seated access. The RHS calls for paths of at least 1 m; a straight run at that width works in the 3.14 m frame with slightly narrowed beds (1.05 m each), though turning happens at the door rather than mid-house. The 4 m frame gives real comfort: 1.2 m beds either side of a 1.6 m central path leave room to turn a chair between them. Build bed walls to about 69 cm for seated working, and check the door width of any greenhouse against your chair before ordering, because the door, never the path, is the narrowest point.
Whichever width you land on, mark the layout with string and pegs on a dry afternoon before any timber arrives. A plan that reads 1.1 m, 0.9 m, 1.1 m on paper feels different underfoot, and string costs nothing to move.
Frequently asked questions
How wide should the path between raised beds in a greenhouse be?
At minimum, 45 cm, which is the RHS figure for wheelbarrow access. In practice 60 cm walks comfortably, 90 cm lets you kneel and work sideways, and wheelchair access needs 1 m or more. Take path width from your frame first and let the beds have whatever remains.
How deep should raised beds in a greenhouse be?
On an open-soil base, 25 to 30 cm suits most crops because roots continue into the ground below. On slabs or concrete, build to 45 cm with drainage gaps in the bottom course, and give parsnips 45 cm anywhere.
Can I use old railway sleepers for greenhouse beds?
Reclaimed creosote-treated sleepers, no. The 2003 creosote regulations prohibit them in gardens with frequent skin contact and in growing containers, and a food bed under cover fails on both counts. New pressure-treated softwood sleepers use copper-based preservatives and are the standard choice; add a side-only polythene liner if you want a barrier.
Are grow-bags better than raised beds in a greenhouse?
For a first season, often yes: they cost little, warm quickly and carry no commitment. Their weakness is water, a shallow layer of compost that can need soaking twice a day in July. A 30 cm bed holds many times the moisture, so from the second year most growers move over and keep grow-bags for overflow.
Do the same layouts work in a polytunnel?
Yes, with one adjustment. A polytunnel's film curves lower at the edges than a rigid arch does, so pull the beds 10 to 15 cm in from the skirt to avoid trapping damp air against the cover, and keep the central path at 90 cm where the full headroom runs.
Sources & further reading
- RHS: Raised beds, the bed widths, depths, path sizes and fill mixes referenced throughout.
- University of Minnesota Extension: Raised bed gardens, reach rules, wheelchair working heights and treated-wood guidance.
- legislation.gov.uk: The Creosote (Prohibition on Use and Marketing) (No. 2) Regulations 2003, the law behind the railway-sleeper rule.
- AHDB: GREATsoils, soil structure and assessment resources behind the keep-off-the-beds advice.
