What to put a greenhouse on: greenhouse bases explained
A greenhouse is only as good as what it stands on. A base that is out of level twists the frame, cracks panes and lets the wind get underneath.
The short answer
For most aluminium greenhouses, the best base is the manufacturer's steel base frame, bolted down to a level row of paving slabs or a concrete strip, with soil borders inside. It raises the greenhouse, squares the frame and anchors it.
Whatever you use, the base must be level, square (equal diagonals) and anchored. Check what your greenhouse's warranty asks for before you build it.
Why the base matters so much
A greenhouse frame is a lightweight structure designed to be held perfectly square. Put it on ground that is out of level, or that settles unevenly, and the frame racks out of shape. Doors stop running, vents jam, glazing clips work loose and glass panes crack under the strain.
The base also does the anchoring. Most greenhouse damage in British storms happens when wind gets underneath or inside an unanchored greenhouse and lifts it. A greenhouse fixed down to a solid base is far harder to move.
The main options
| Base | Best for | Effort | Notes |
|---|---|---|---|
| Steel base frame (from the maker) | Most aluminium greenhouses | Low to medium | Squares and raises the frame; needs fixing down to slabs, concrete or ground anchors |
| Paving slabs on a compacted bed | Small to medium greenhouses | Medium | Lay a row under the walls, or a full floor; must be level |
| Concrete strip or slab | Large greenhouses, exposed sites | High | The firmest anchor; plan drainage |
| Timber base | Small greenhouses, wooden greenhouses | Low to medium | Use pressure-treated timber; it will rot eventually |
| Brick or block dwarf wall | Glass-to-wall designs, a permanent look | High | Raises the greenhouse; the frame bolts to the wall top |
| Ground anchor kit | Light polycarbonate greenhouses on soil | Low | Better than nothing, not a substitute for a level base |
Steel base frames
Most aluminium greenhouse makers sell a matching galvanised steel base. It bolts to the greenhouse and gives it a rigid, square footprint, raises it a few inches (which helps on uneven or wet ground) and makes anchoring straightforward. It still needs something firm underneath: a row of slabs, a concrete strip, or the frame's own ground spikes set in concrete.
If you are buying a new aluminium greenhouse, the matching base is usually the simplest choice and the one most likely to satisfy the warranty.
Paving slabs
Slabs work well for small and medium greenhouses. You can lay:
- a perimeter row under the walls, leaving soil inside for border planting, or
- a full floor, which is cleaner and suits staging and pots, but leaves nowhere to plant directly into the ground.
Lay them on a compacted sub-base, ideally on a sand and cement bed, and check every slab with a spirit level as you go. Fix the greenhouse or its base frame to the slabs with the brackets or bolts supplied.
Concrete
A concrete strip footing or a full slab is the firmest base and the best anchor on an exposed, windy site. It is also the most work and the hardest to undo. Make sure the surface is level and that water can drain away: a slab that holds puddles inside the greenhouse makes it permanently damp.
Timber bases
A frame of pressure-treated timber or sleepers is quick to build and suits small greenhouses and wooden greenhouses. Keep it off direct contact with standing water if you can, check it is square, and expect to replace it eventually. Fix it down, because a timber frame sitting loose on soil gives the wind something to lift.
Dwarf walls
A low brick or block wall raises the greenhouse and gives a traditional look, and some greenhouses are designed specifically to sit on one. It takes the most planning: the wall top has to be level and its outside measurements must match the greenhouse exactly. Build the wall to the manufacturer's dimensions, not your own measurements of the frame.
Getting it level and square
- Choose the spot. Somewhere with as much sun as possible, sheltered from the prevailing wind, on ground that doesn't flood, and away from overhanging trees.
- Mark out the footprint using the base dimensions from the manufacturer's instructions.
- Level the ground. Remove turf and soft soil, then compact the area. On a slope, dig into the high side rather than building up the low side.
- Lay the base and check it with a long spirit level along every side and across the middle.
- Check it is square. Measure both diagonals, corner to corner. If they are equal, the base is square. If not, adjust until they are.
- Anchor it before you start glazing.
Which way should a greenhouse face?
For the most winter light, the usual advice is to run the ridge east to west, so the long side faces south. For mainly summer growing, a north to south ridge gives more even light through the day. In a small garden, the sunniest spot you have matters more than the exact direction.
Questions people ask
Do you need a base for a greenhouse?
Yes. Even small greenhouses need a firm, level, square base to stop the frame twisting and to anchor them against wind. Many manufacturers also make the warranty conditional on a suitable base.
Can I put a greenhouse straight on soil?
Not without a frame of some kind. On bare soil a greenhouse will settle unevenly and can be lifted by the wind. At minimum use a steel or timber base frame fixed with ground anchors.
What is the cheapest greenhouse base?
A pressure-treated timber frame or a single row of paving slabs under the walls is usually the cheapest base that does the job properly. Both still need to be level, square and fixed down.
How do I check a greenhouse base is square?
Measure the two diagonals from corner to corner. If they are exactly the same length, the base is square.
Which direction should a greenhouse face in the UK?
For the most winter light, run the ridge east to west so the long side faces south. In a small garden, choosing the sunniest available spot matters more than the exact direction.