Greenhouse base preparation: soil and anchors, paving slabs, concrete or sleepers?
You have a patch of lawn, a greenhouse on order and a builders’ merchant ready to sell you concrete. Stop at the lawn. On firm, level soil, both of our greenhouses stand on the ground anchors that come in the box, and the base you need is a level, firmed strip of ground. A hard base earns its place on paving, on soft or made-up ground and on a slope, and each kind has one detail that decides whether the doors still close cleanly.
- Firm, level soil may be all you need. Plot 3.14 and NORDIC come with ground anchors, and neither needs concrete on standard soil.
- A hard base needs brackets. On concrete, slabs or sleepers the legs sit in bolt-down U-brackets, bought separately.
- One continuous, level line. Whatever you build, run it all round, across the doorway too, and let water drain away.
Does a greenhouse need a base?
Not always. On firm, level soil, Plot 3.14 and NORDIC stand on the ground anchors supplied with them. A prepared base is recommended for long-term alignment, and a hard base is the answer on paving, soft ground or a slope.
The Plot 3.14 base plan puts it plainly: a base is recommended, not compulsory, and on suitable firm, level soil the supplied anchors hold the greenhouse as the manual directs. The NORDIC page says a concrete foundation is not required for a standard soil installation, but the site must be firm and level, and a prepared base is recommended for the best long-term alignment.
What the base really does is keep the frame square and level. A frame that settles unevenly racks out of true, and the first sign is a door that no longer closes cleanly. Other greenhouses come with their own instructions, so follow the manual for yours.
Which base suits your garden?
Firm soil: nothing to build. Gravel: a surface over firm soil. Concrete, slabs or sleepers: a hard base with brackets under the legs. A slope: a level platform first.
| Base | When it suits | What you buy | How the frame is held |
|---|---|---|---|
| Firm, level soil | Lawn or bed on firm ground | Nothing | Ground anchors from the box |
| Gravel on top | You want a gravel finish | Gravel | Anchors in firm soil under the gravel; keep soil open at each anchor |
| Concrete strip, or a pad you have | Soft ground, or an existing slab | Concrete and formwork, U-brackets, masonry bolts | A U-bracket under each leg, one bolt through both sides |
| Slabs or bricks under the frame line | A neat edge on firm ground | Slabs or blocks, bedding material, U-brackets, masonry bolts | A U-bracket under each leg, well in from a slab edge |
| Sleepers or a timber ring | A raised edge, or beds lifted inside | 200 mm sleepers, stakes, hardcore, U-brackets, coach screws | A U-bracket coach-screwed under each leg |
| A slope or mixed ground | Any fall across the site | Hardcore | Cut a level platform, then build one of the bases above |
On a hard base the anchors are not used. For NORDIC, the page says loose, made-up, paved or unusually exposed sites may need a different fixing approach, so contact us before ordering if your ground is one of those.
How big should the base be?
Build to the base size on the plan for your model and length: 3.2 m wide for Plot 3.14, 2.8 m for NORDIC. Each plan gives the frame footprint and a diagonal to check.
| Greenhouse | Base, build to this | Frame footprint | Diagonal to check |
|---|---|---|---|
| Plot 3.14, 4 m | 3,200 × 4,250 mm | 3,000 × 4,020 mm | 5,320 mm |
| Plot 3.14, 6 m | 3,200 × 6,250 mm | 3,000 × 6,020 mm | 7,022 mm |
| Plot 3.14, 8 m | 3,200 × 8,250 mm | 3,000 × 8,020 mm | 8,849 mm |
| Plot 3.14, 10 m | 3,200 × 10,250 mm | 3,000 × 10,020 mm | 10,738 mm |
| Plot 3.14, 12 m | 3,200 × 12,250 mm | 3,000 × 12,020 mm | 12,661 mm |
| NORDIC, 2 m (8 × 6 ft) | 2,800 × 2,250 mm | 2,580 × 2,048 mm | 3,294 mm |
| NORDIC, 3 m | 2,800 × 3,250 mm | 2,580 × 3,048 mm | 3,993 mm |
| NORDIC, 4 m | 2,800 × 4,250 mm | 2,580 × 4,048 mm | 4,800 mm |
| NORDIC, 5 m | 2,800 × 5,250 mm | 2,580 × 5,048 mm | 5,669 mm |
| NORDIC, 6 m (8 × 20 ft) | 2,800 × 6,250 mm | 2,580 × 6,048 mm | 6,575 mm |
Peg out the rectangle and check that both diagonals match before you build anything. For Plot 3.14, build to the outer base line and keep raised edging at least 100 mm clear of the frame footprint, because the arch widens above the ground. NORDIC stands on a timber base frame that lies on the ground all round, door included: for a strip of sleepers, slabs or hardcore, centre material at least 200 mm wide on the footprint line; for a full slab or pad, use the base size as the outer edge. Its plan adds a useful caution: don’t cut or pour to a final size until you can check it against the assembled base frame.
The Plot 3.14 manual also asks for 2 m clear of fences and buildings on every side.
How do you prepare soil for ground anchors?
Strip the turf and loose topsoil, firm and level the ground, stand the frame and bury an anchor at each leg the manual marks. You need a string line, pegs, a long spirit level and a spade.
- Peg out the frame footprint for your length and check that both diagonals match.
- Strip the turf and loose topsoil along the line, then firm the soil down and level it.
- Stand the frame on it, slide a kit anchor into each leg the manual marks, fix it with its one screw and bury it as the manual shows.
- Check level all round once more before the first sheet goes on.
These steps are from the Plot 3.14 base plan; NORDIC’s anchors follow its own manual and anchoring schedule. Our steel arch assembly guide takes the build on from there. For a gravel finish, prepare the soil in exactly the same way, keep open soil where each anchor goes, bury the anchors in the soil and spread the gravel round the frame afterwards. Gravel is a surface, not a fixing. If the ground under it is deep hardcore rather than soil, use slabs or a strip instead.
How do you build a greenhouse base with paving slabs?
Lay one continuous, level line of slabs under the frame line on a compacted bed, across the door end too, and bolt a U-bracket under each leg well in from a slab edge.
- Dig a strip along the frame line and compact a firm, level bed.
- Lay the slabs edge to edge in one continuous line under the frame, across the door end as well.
- Bolt a U-bracket under each leg, well in from a slab edge, with masonry bolts.
- Drop the legs in, put one bolt through both sides, then check level and both diagonals.
Mark the frame line before you decide where the joints fall, so that no leg lands on a joint, and lay the line with a long level so the frame doesn’t bridge a dip. Let rain run off the slabs rather than collect where the frame sits. Bricks or blocks work the same way, as long as the line under the frame is continuous and level. An existing patio needs checking before it takes masonry bolts: its bed and condition matter more than its looks.
How do you make a concrete base for a greenhouse?
A strip, not a slab: one continuous strip 200 mm wide to the outer line, with a level top. Inside it the soil stays open for beds.
Cast the strip all round and across the door end, then bolt a U-bracket to the concrete under each leg once it has set. Each leg drops into its bracket and takes one bolt through both sides. Leave no dips where water can stand, because the leg ends are open. The base plan illustrates concrete but asks you to confirm the material and depth for your own site, so ask a groundworker to specify the dig, the sub-base and the concrete for your ground.
With a 200 mm strip, the open soil inside a Plot 3.14 is 2,800 mm wide and 150 mm shorter than the nominal length, which is the space for beds and a path. A slab you already have works too, if it is level and sound.
Can you use sleepers or a timber ring?
Yes. Bed 200 mm sleepers on compacted hardcore, centred on the frame line and joined into one level ring, and coach-screw a U-bracket under each leg.
Pin the ring with stakes and don’t cut the sleeper at the door: the end frame sits on it. The doorway then becomes a step, so backfill inside to the top of the sleeper and lay a slab or a short gravel ramp outside.
What if the ground slopes?
Cut a level platform first. Dig the high side down rather than building the low side up, and use compacted hardcore, never loose soil, for any fill.
Measure the fall by running a level string from a peg at the high end: the gap at the low end is the fall. Dig the high side down to one level platform, build the low side up with hardcore compacted in layers, then check level all round and on both diagonals before building any base on top. For a big fall, send us a photo and your postcode before you dig, because a retaining edge cannot be judged by eye.
Which details do people get wrong?
The doorway and the water. Keep the base continuous under the door, because the end frame’s bottom rail runs across it, and make sure water drains away from the base rather than standing on it.
The bottom rail of each end frame is the threshold. If the strip or slab line stops either side of the door, the frame loses support where it is opened most. Outside the door, bed a slab or a short gravel ramp level with the base, and a wheelbarrow rolls straight in.
Water is the other enemy. Look at the spot after heavy rain and note where water stands before you build over it. On a hard base, let water run off rather than collect where the frame stands; on soil, firm the ground and avoid hollows round the legs. If water runs downhill towards the site, cut a shallow channel on the uphill side.
Quick answers about greenhouse bases
Not for Plot 3.14 or NORDIC on firm, level soil, where the supplied ground anchors hold them. Concrete is one option for soft ground or a permanent hard edge, laid as a strip under the frame line.
Yes, on one continuous, level line of slabs under the frame, with a bolt-down U-bracket under each leg, bought separately.
Plot 3.14 and NORDIC need none on firm, level soil. A plinth of sleepers or blocks is a choice for a raised edge, built as one level ring with brackets under the legs.
Not for the frame line: the base plan lays them on a compacted bed with the turf removed, so they can’t settle unevenly.
No. The anchors are for soil; on concrete, slabs or timber the legs sit in bolt-down U-brackets instead.
What should be ready before delivery?
A level, firmed site or a finished hard base, both diagonals checked, the door line continuous and the brackets bought if you need them.
Six things to check
Tick them off in the week before the pallet arrives. Your ticks stay on this device.
Two greenhouses that stand on soil
Both come with ground anchors and need no concrete on firm, level soil.
- Width
- 3.00 m on the ground, 3.14 m at the widest
- Lengths
- 4, 6, 8, 10 or 12 m
- Ridge
- 2.02 m
- Glazing
- 4 mm twin-wall, 6 mm option
- Air
- two gable doors, upper halves open
- Roof vents
- optional, automatic openers
- Width
- 2.6 m walk-in
- Lengths
- 2 to 6 m, 8 × 6 to 8 × 20 ft
- Ridge
- 2.3 m, eaves 1.51 m
- Frame
- 45 × 45 mm FSC pine, galvanised brackets
- Glazing
- 4 mm twin-wall, 6 mm option
- Air
- opening door top and rear window
From the product pages, 20 October 2026. Staging, watering and the base are arranged separately.
Download the plan for your length
The Plot 3.14 page has the base plan for every length, the six base types and the assembly manual. An expert can look at a photo of your site.
What we checked for this guide
The base sizes and methods come from the published Plot 3.14 and NORDIC base plans and the product pages. Checked 20 October 2026.
- 01WaldenhausPlot 3.14 base plan and foundation guide
- 02WaldenhausNORDIC base plan
Related guides
Written by the Waldenhaus team. We sell outdoor saunas and greenhouses and write these guides from manufacturer manuals, official guidance and our own product pages. Where your equipment manual says something different, follow the manual.