What base does a greenhouse need?

Why a greenhouse base has to be level and square rather than merely flat, the four base options, and getting the anchoring right at the same time.

Difficulty: intermediate
Time: 6 hr
Read: 5 min
Safety: warning

Short answer

The base must be level, square and rigid — a greenhouse frame will not go together properly on anything else, and it is the base rather than the glazing that decides whether panes stay in. A concrete or slab perimeter on a compacted sub-base is the standard, with the frame bolted down to it. Check the diagonals before anything is fixed.

A shed tolerates a base that is slightly out. A greenhouse does not. The frame is a light aluminium or timber assembly relying on being square for every pane to fit its opening, and once it is racked the glass sits at an angle in the glazing bars and starts working loose. Everything in the base is therefore about level and square, and about giving the anchors something solid to grip.

Safety

A greenhouse frame is unforgiving about being out of square: an aluminium frame assembled on an uneven base puts the glass under stress and panes crack or fall. Greenhouse glass is often horticultural float glass, not toughened, and it breaks into sharp shards — wear gloves and eye protection when handling it. Anchor the greenhouse down: an unanchored greenhouse is one of the structures most commonly destroyed in a gale.

What you'll need

MOT type 1 for the sub-base
Paving slabs, or concrete and formwork, or the manufacturer's base kit
Plate compactor or a hand tamper
Long spirit level and a straightedge
Anchor bolts suited to the base
String line and pegs
Tape measure for the diagonals

Step by step

Get the manufacturer's base dimensions first.The base size is not the greenhouse's nominal size. Get the exact external base dimension from the instructions, and build to that. Building to the advertised size and finding the frame does not fit is a common and expensive error.
Choose the site for light and shelter.Open to the sky, away from overhanging trees, and out of the worst of the prevailing wind. A greenhouse in a wind funnel between two buildings takes far more punishment than the same greenhouse ten metres away.
Excavate and compact a sub-base.Strip topsoil and all vegetation, then MOT type 1 in thin layers, each compacted. This is what stops the base settling, and a greenhouse base that settles goes out of square.
Build the perimeter dead level.Whether it is slabs, a concrete strip or a manufacturer's base kit, the surface the frame sits on must be level all round, checked with a long level in both directions and across the diagonals. Not nearly level. Level.
Check the diagonals before you commit anything.Measure both diagonals of the finished base. They must be equal. A base out of square by even 10mm shows up as a frame that will not close and panes that do not seat.
Decide what goes inside the perimeter.A full slab floor is easy to keep clean and good for staging. Soil borders inside the perimeter let you plant directly and are what many growers want. Gravel over membrane is a compromise that drains well and holds humidity. Decide before you build, because the perimeter detail differs.
Build in the anchor fixings.Cast anchor bolts into a concrete base or use expansion anchors into slabs at the positions the manufacturer specifies. A greenhouse must be positively fixed down, not just standing on its base. This is the step that decides whether it survives a gale.
Deal with drainage around the base.Water running off the glass lands in a line at the eaves. A gravel margin around the base takes it away, keeps splashing off the lower panes and stops mud being kicked up onto the glass.
Assemble the frame on the base without forcing anything.If the frame does not go together easily and squarely, the base is wrong. Correcting the base is far quicker than fighting the frame, and a forced frame will never hold its glass.

Common mistakes

Building the base to the nominal greenhouse size — The base dimension is specific and rarely matches the advertised size. Get it from the instructions before ordering slabs.
Nearly level — A greenhouse frame is a precision assembly by garden standards. A few millimetres of twist across the base translates into panes that will not sit and a door that will not slide.
Not anchoring it down — Greenhouses are light, have an enormous surface area and are destroyed in gales every winter. Anchoring is not optional and it is trivially easy at build time.
Building it on soil or on grass — It settles unevenly within a season, the frame racks, and the glass starts to work loose. There is no way to correct it afterwards without dismantling.

Alternatives

Paving slabs on a compacted sub-base: The most common answer. A full slab floor or a perimeter of slabs, bedded and levelled on compacted MOT type 1. Easy to adjust while building, easy to anchor into, and it gives a clean floor.
Concrete strip perimeter with soil inside: A concrete footing under the frame line only, with the interior left as soil for direct planting. The preferred base for anyone who wants to grow in the ground, and it gives an excellent anchor fixing.
The manufacturer's steel or aluminium base kit: A bolt-together frame sold with the greenhouse, sitting on a level surface. It guarantees the right dimensions and squareness, but it still needs a level compacted surface under it and it still needs anchoring down — the kit is a base frame, not a foundation.
Timber sleeper or treated frame on compacted stone: A perimeter of sleepers or heavy treated timber, levelled and pegged, with soil or gravel inside. Quick and inexpensive and easy to adjust. Timber moves, so check the level and squareness annually for the first couple of years.

Questions people ask

Can I use a plastic grid base for a greenhouse?

For a small aluminium greenhouse on well-prepared and level ground, with the frame bolted down to something solid at the corners, it can work. For anything larger the frame really wants a continuous, rigid, level perimeter, and a gravel-filled grid does not give the anchors much to grip.

Should the greenhouse have a solid floor or soil?

It depends how you grow. Soil borders let you plant tomatoes and cucumbers directly and hold humidity well, at the cost of soil-borne disease building up over years. A solid floor is cleaner, better for staging and pots, and easier to disinfect. A perimeter footing with soil inside gives you both options.

How do I anchor a greenhouse down?

Bolt the frame to the base at the positions the manufacturer specifies — cast-in anchors in a concrete footing, or expansion anchors into slabs. Where there is no solid base, ground anchors or auger anchors with the frame strapped down are the retrofit answer, and they are far better than nothing.