A greenhouse frame is only as good as what it stands on. The base does three jobs at once: it holds the structure level, it keeps it square, and it stops the ground moving underneath it. Get any of those wrong and the frame will tell you within a season.
The symptoms are always the same. Doors that bind or will not close. Glazing clips working loose. Panes cracking one at a time with no obvious cause. By the time the pattern is clear, the repairs have usually cost more than doing the base properly would have.
We do not supply greenhouse bases, and for most gardens you do not need to buy one. A base is groundwork rather than a product, and it is built from materials available at any builders merchant. This guide covers how to do it yourself, what each type costs you in effort, and where the job most often goes wrong.
What a Base Has to Do
- Level. Out of level and the frame twists, which loads the glazing unevenly.
- Square. A frame assembled on a base that is not square will not accept its panels cleanly.
- Drained. Standing water rots timber, lifts paving through frost and undermines whatever sits on it.
- Firm. Ground that settles after the frame is up takes the frame with it.
Note what is not on that list: strength. A greenhouse is light. Almost no domestic base fails because it could not carry the weight. They fail because the ground moved or water had nowhere to go.
The Main Types of Greenhouse Base
| Base type | Suits | Effort | Watch out for |
|---|---|---|---|
| Manufacturer base frame | Most kit greenhouses | Low | Still needs level, firm ground beneath |
| Paving slabs | Small and medium houses | Medium | Slabs settle individually if not bedded properly |
| Concrete perimeter | Medium and large houses | Medium | Needs a soil or gravel floor inside |
| Full concrete slab | Large houses, permanent siting | High | No ground beds possible afterwards |
| Brick plinth | Traditional and timber frames | High | Needs a footing beneath and a damp course |
| Compacted gravel | Polytunnels and light frames | Low | Not suitable for heavy glazed structures |
| Timber sleepers | Allotments and temporary siting | Low | Timber will need replacing eventually |
Choosing Between Them
Manufacturer base frame
Many kit greenhouses offer a steel or aluminium base frame made for that model, usually as an optional extra from the manufacturer. It bolts to the frame, spreads the load along the whole footprint and gives a fixed reference for squaring the structure.
If your model offers one, it is worth considering, with one caveat that catches people out: a base frame is not a foundation. It still has to sit on ground that is level, firm and drained. Placed straight onto soft soil, it will sink at one corner within a year.
Paving slabs
Slabs laid on a compacted sub base with sharp sand or mortar give a solid, level platform and a clean floor to work on. They suit small and medium houses and can be lifted and relaid if the frame moves later.
Bed each slab properly rather than laying them dry on soil. Slabs laid loose settle individually, and a base that starts level ends up as a series of small steps.
Concrete perimeter
A concrete strip under the frame line only, leaving soil in the middle. This is the arrangement most growers with a medium or large house end up with, because it gives a stable perimeter while keeping the ground open for planting directly into.
If you want beds inside, this is the base to choose. A full slab closes that option permanently unless you build raised beds on top.
Full concrete slab
The most permanent and the most work. It suits large structures, sites that will never be replanted, and anyone who wants a hard floor throughout for barrows and staging.
Lay a fall of a few millimetres per metre towards a door or a drainage point, or water from watering and condensation will pool.
Brick base
A brick base greenhouse sits on a low wall of two or three courses, with the frame bolted to the top. It raises the glazing above splash level, adds thermal mass that holds warmth overnight, and suits traditional and timber framed houses visually.
It is the most demanding option and the one most worth getting a bricklayer for if you are not confident. A brick plinth needs a concrete footing beneath it, a damp proof course to stop moisture rising, and accurate work, because the frame bolts directly to what you build. A greenhouse on brick base done badly is harder to correct than any other type.
Building on Uneven Ground
Very few gardens are flat. There are three ways to deal with a slope, and the right one depends on how much fall you have across the footprint.
Slight fall, under about 10 cm
Level by adjusting the sub base. Dig out the high side rather than building up the low side, since compacted original ground is more stable than anything you add on top.
Moderate fall, 10 to 30 cm
Cut and fill. Remove material from the high side and use it to build up the low side, then compact the filled section thoroughly in layers of no more than about 10 cm. Filled ground that is not compacted in layers will settle for years.
Significant fall, over 30 cm
Build a retaining edge on the low side, using sleepers, blocks or a concrete kerb, and fill behind it. At this point the base becomes a small piece of groundwork rather than a levelling job, and drainage behind the retaining edge matters as much as the level surface on top.
Whichever applies, check the level across both diagonals as well as the sides. A base can read level along each edge and still be twisted.
Getting It Square
Level is easy to check. Square is the one people skip, and it is the one that causes glazing problems.
Use the 3, 4, 5 method. Measure 3 units along one side and 4 units along the adjoining side. If the corner is a true right angle, the diagonal between those two points measures exactly 5 units. Metres work well for a greenhouse footprint.
Then check both diagonals of the whole base. On a rectangle they must be identical. If they differ by more than a few millimetres, the base is a parallelogram rather than a rectangle, and the frame will not sit on it correctly.
Drainage
Water has to leave. A greenhouse concentrates rainwater along its eaves lines, and without guttering that water goes straight into the ground at the base of the frame, which is exactly where you do not want it.
Fit guttering into a water butt during assembly. It solves the drainage problem and gives you rainwater for the plants, which most crops prefer to tap water.
Inside, a soil or gravel floor drains on its own. A solid floor needs a fall towards a door or a drain, or it will hold every drop of condensation and spilt water.
Anchoring Is Not the Same as the Base
This is the single most common misunderstanding, and it is the one part of this job we can help with directly. A base holds the greenhouse level. Anchoring holds it down. A frame sitting on a beautifully laid slab with no fixings will still be moved by wind, because nothing connects the two.
The right anchor depends on what is underneath:
- Soil and grass: spiral or auger anchors that twist in and grip below the surface.
- Gravel: augers long enough to pass through the loose layer into firm ground.
- Paving and concrete: expansion bolts through the base frame into the slab.
- Timber and decking: fixings that reach the joists rather than the boards.
Fit them during assembly. Once the glazing is in, the frame is heavy, fragile and awkward to lift, and reaching the base rail means working around staging. Our anchor kit range covers every surface type.
Five Mistakes Worth Avoiding
- Building the base to the frame's outside dimensions. Check whether your model needs the base to match the frame footprint or sit slightly inside it. Manufacturers differ.
- Laying slabs dry on soil. They will settle unevenly within a year.
- Filling a slope without compacting in layers. Loose fill settles for years afterwards.
- Skipping the diagonal check. Level and square are different measurements.
- Treating anchoring as optional. It is what keeps the structure where you built it.
Common Questions
Does a greenhouse need a base?
Yes. Even a light frame needs level, firm ground and a way to fix down. The question is which type of base, not whether to have one.
Can I build a greenhouse base myself?
For slabs, gravel, sleepers and a concrete perimeter, yes. The work is measuring, digging and compacting rather than skilled trade work. A brick plinth is the exception and is worth getting a bricklayer for.
Can I put a greenhouse straight on grass?
Only for temporary, lightweight structures, and even then it needs anchoring. Grass over soft soil settles unevenly and holds water against the frame.
How thick should a concrete greenhouse base be?
For a domestic greenhouse, around 75 to 100 mm over a compacted sub base is typical. Depth matters less than the ground preparation underneath it.
Can I build a greenhouse base on a slope?
Yes. Under about 10 cm of fall, level by cutting into the high side. Above that, cut and fill with proper compaction, and over about 30 cm build a retaining edge.
What is a greenhouse plinth?
A low wall, usually brick or blockwork, that the frame sits on. It raises the glazing clear of splash, adds thermal mass and suits traditional frames. It needs a footing and a damp proof course beneath it.
Next Steps
Work out the footprint before you order, since the base has to be built to the model. The size guide sets out what each floor area allows, so the groundwork and the frame can be planned together.
Once the base is laid, the anchor kit range covers the fixings, and the full greenhouse range can be filtered by size, shape and material.
