Greenhouse Bases
A greenhouse base that is dead level and drains
Glass is unforgiving. IBRAN grids give a firm, perfectly level base that drains freely and anchors properly, so the frame stays square, the panes stay in and the door slides all summer.
Why the base matters more here than anywhere
A greenhouse is the least tolerant garden building there is. A shed on a slightly uneven base is a shed with a sticky door. A greenhouse on a slightly uneven base is a frame that has twisted, and once an aluminium frame is out of square the panes stop sitting properly in their clips, the door jams or drops off its runner, and glass starts cracking at the corners.
Then there is water. A greenhouse generates a surprising amount of it: watering, condensation running down the glass, splash from the hose, spills from staging. On an impermeable slab that water has nowhere to go, so it sits on the floor, keeps humidity high and gives algae and moss a permanent home.
And there is wind. A greenhouse is a large light box with a lot of surface area, and one gust under an unanchored frame will move it. Anchoring is not optional, and it has to have something solid to anchor into.
A grid base addresses all three. Confined stone gives a rigid, precisely level platform that will not settle unevenly, the surface stays fully permeable so water drains straight through the floor, and the compacted build-up gives a firm medium to fix a frame down into.
Where this works well
- Aluminium and timber framed greenhouses
- Polytunnel bases and entrances
- Cold frames and grow houses
- Potting sheds and glasshouse porches
- Staging and standing-out areas
- Paths between beds under glass
- Allotment structures
- Replacing a failed slab or paved base
At a glance
| Sub-base | 75-100mm compacted MOT for a typical domestic greenhouse |
| Infill | 6-14mm graded blend, angular and clean, filled level with the cell tops |
| Total dig | Around 110-140mm, depending on grid depth and sub-base |
| Tolerance | Level and square are critical. Check in both directions and on the diagonals |
| Anchoring | Frame anchors fix through the base into the compacted sub-base beneath |
| Drainage | Fully permeable. Watering, condensation and spills drain straight through |
| Design life | 25 years, subject to documented installation |
| Material | 100% recycled polypropylene, made in the UK |
Level, square, and why both matter
Greenhouse manufacturers specify a base that is level and square because the frame is assembled to fixed dimensions and the glass has no give in it at all.
Level means flat in both directions across the whole footprint, not just at the corners. A high spot in the middle lifts the frame off its line and twists it, and the twist appears at the far end as a door that will not close.
Square means the diagonals match. An aluminium greenhouse assembled on a base that is a centimetre out of square will fight you the entire way, and the last pane will not go in.
Check both before you lay a single grid, then again after the blinding layer is screeded, then again before the frame goes down. It is the cheapest quality control on the whole job, and the only stage at which any of it can still be corrected.
Drainage under glass
A permeable floor changes how a greenhouse behaves day to day.
Water from watering cans, hoses and condensation drains through the base rather than standing on it, so the floor stays walkable and dries quickly. That reduces the standing damp that grows algae on the floor and moss in the corners, and it makes the space more pleasant to work in.
It also matters for what is growing. Where you have soil borders inside the greenhouse, a free-draining surrounding base means excess water moves away rather than sitting against the beds. Where everything is in pots on staging, the drips simply disappear.
A gravel surface has one more benefit under glass: damp it down on a hot day and it holds humidity gently as it evaporates, which is a traditional glasshouse technique and considerably harder to do on a sealed floor.
Which grid depth?
The number on a grid is a decision about how deep the confined stone layer is. Greenhouse loads are light, so this is about level accuracy and dig depth rather than strength.
- IBRAN®-X30 for most domestic greenhouses. Shallow dig, minimal spoil, and easy to get precisely level, which is what this application actually needs.
- IBRAN®-X40 for larger structures, commercial glasshouses, or where the base extends into a wheelbarrow route or standing-out area.
- IBRAN®-X50 where a vehicle or heavy plant will use the same surface, such as a nursery access apron.
Choosing your stone
Use a 6-14mm graded blend, angular and clean. A blend packs tighter than a single size because the smaller stone fills the gaps between the larger, and here that density does double duty: it holds the level precisely and it gives a comfortable surface to stand and kneel on while you work.
Fill level with the cell tops rather than proud, so the floor is flat underfoot and nothing rolls when you move staging or a heavy pot.
Avoid rounded pea shingle, which shifts underfoot, and anything carrying fines or dust, which binds the surface, holds water and gives moss somewhere to establish. A clean washed blend keeps the floor draining and stays cleaner year to year.
How it goes in
- Check the greenhouse manufacturer's base dimensions before marking anything out. Frames are built to fixed sizes and the base needs to match them, not the other way round.
- Mark out the base larger than the frame footprint by at least 100mm on every side, so runoff from the glass lands on stone rather than soil.
- Clear turf, roots and soft soil down to firm ground, and check that ground falls away from the greenhouse position rather than towards it.
- Excavate to your total depth: sub-base plus grid depth, typically 110-140mm.
- Lay a woven geotextile across the dig, lapping joins by at least 300mm and taking it up the sides.
- Add and compact the sub-base. 75-100mm of MOT is ample, compacted properly rather than tipped and raked, because this is what your anchors will ultimately bite into.
- Blind with a thin layer of grit sand and screed it dead level. Check with a straight edge and spirit level in both directions and along both diagonals, and check the diagonals of the marked area match.
- Push-fit the grids together, cutting the perimeter to line with a hand saw.
- Fill the cells with a 6-14mm graded blend, level with the cell tops, brushing in so no cell is left hollow.
- Set out the frame, check square and level again, then anchor it down before glazing.
Full step-by-step detail is in our installation guides.
Anchoring the frame
Do not skip this and do not leave it until later. An unanchored greenhouse in a gale is a large glass sail, and the damage is rarely limited to the greenhouse.
Most aluminium greenhouses come with anchor plates or ground anchors designed to fix at the corners and at intervals along the base rails. These fix through the grid base into the compacted sub-base beneath, which is why compacting that layer properly matters more here than on a shed base.
Where the manufacturer specifies concrete anchor pads, you can set small pads at the fixing points within the grid layout, keeping the rest of the base permeable. It is the anchor points that need mass, not the whole floor.
Fix the frame down before you glaze. Glazing a frame that then has to be shifted to line up an anchor point is a job nobody enjoys twice.
Soil borders and staging
Most greenhouses end up as some combination of beds and benches, and the base can accommodate both.
For soil borders, leave the border areas out of the grid layout and run the base as paths and standing areas around them. The permeable base beside a border drains water away from the bed rather than into it, and the border keeps direct contact with the ground beneath, which is what you want for deep-rooted crops.
For staging and benching, a fully gridded floor gives an even, firm surface that legs will not sink into, and drips fall straight through rather than pooling under the bench.
If you are not sure yet, grid the whole floor. Removing grids later to open up a border is straightforward; adding a base under an established bench is not.
Polytunnels
Polytunnels have a different failure point: the ground anchors and the trench where the polythene is buried, both of which sit at the perimeter.
A grid base works well for the internal paths and standing areas while leaving the perimeter free for anchoring and skirting. It is particularly worth doing at the doorway, which is the muddiest square metre of any polytunnel and the one place traffic concentrates.
Get it right first time
The one thing worth knowing: check level and square three times.
Every other application on this site tolerates a few millimetres of variation. This one does not, because glass has no give and an aluminium frame transmits any twist straight to the panes.
Check when you mark out, check after screeding the blinding, and check before the frame is fixed. Straight edge in both directions, spirit level across the middle as well as the edges, tape across both diagonals. It takes ten minutes each time and it is the difference between a greenhouse that goes together in an afternoon and one that fights you all weekend and cracks a pane on the way.
Ask our engineer
Questions about depths, anchoring, borders or getting the base square? Ask below, or get in touch directly.
Common Questions
Can you put a greenhouse on a gravel base?
Yes, provided the gravel is confined in a grid rather than loose. Loose gravel shifts underfoot and will not hold a level, but stone confined in cells gives a rigid, precisely level platform that also drains freely.
That combination suits a greenhouse better than a slab, because water from watering, condensation and spills drains straight through the floor.
Does a greenhouse base need to be perfectly level?
Yes, and square as well. A greenhouse frame is assembled to fixed dimensions and glass has no give in it. A high spot lifts the frame off its line and twists it, which shows up at the far end as a door that will not close or panes that will not sit in their clips.
Check level in both directions and check the diagonals match, before laying grids and again before fixing the frame.
How do you anchor a greenhouse to a grid base?
Most aluminium greenhouses come with anchor plates or ground anchors that fix at the corners and along the base rails. These fix through the grid base into the compacted sub-base beneath, which is why compacting that layer properly matters.
Where the manufacturer specifies concrete anchor pads, set small pads at the fixing points within the grid layout and keep the rest of the base permeable. Anchor the frame before glazing.
How deep should a greenhouse base be?
Around 110mm to 140mm in total for a typical domestic greenhouse: 75mm to 100mm of compacted MOT sub-base plus the grid depth.
Compact the sub-base properly rather than tipping and raking it, because it is both your level and the medium your anchors bite into.
What gravel is best for a greenhouse floor?
A 6mm to 14mm graded blend, angular and clean. A blend packs tighter than a single size and holds the level precisely, while giving a comfortable surface to stand and kneel on.
Avoid rounded pea shingle, which shifts underfoot, and anything carrying fines, which binds, holds water and gives moss somewhere to establish.
Can I have soil borders inside if the floor is gravel?
Yes. Leave the border areas out of the grid layout and run the base as paths and standing areas around them. The permeable base beside a border drains water away from the bed rather than into it, and the border keeps direct contact with the ground beneath for deep-rooted crops.
If you are undecided, grid the whole floor: removing grids later to open a border is straightforward.
Will a permeable floor make the greenhouse too damp?
The opposite. Water drains through rather than standing on the floor, so the surface dries quickly and there is less standing damp for algae and moss.
On a hot day the gravel can be damped down deliberately to raise humidity gently as it evaporates, which is a traditional glasshouse technique and much harder to do on a sealed floor.
Which grid depth do I need for a greenhouse?
The 30mm grid suits most domestic greenhouses: shallow dig, minimal spoil and easy to get precisely level, which is what this application actually needs.
Use 40mm for larger structures, commercial glasshouses, or where the base extends into a wheelbarrow route or standing-out area.
How big should the base be compared to the greenhouse?
Larger than the frame footprint by at least 100mm on every side, so runoff from the glass lands on free-draining stone rather than on soil that will splash back against the frame.
Check the manufacturer's stated base dimensions first, since frames are built to fixed sizes and the base has to match them.
Does this work for polytunnels?
Yes, for internal paths and standing areas, leaving the perimeter free for ground anchors and the trench where the polythene is buried.
It is particularly worth doing at the doorway, which is the muddiest square metre of any polytunnel and where traffic concentrates.
Ask our engineer
Questions about depths, sub-base, edging or which system fits your project? Ask below.
