Greenhouse concrete-base guide

Decide whether concrete solves the right problem

Concrete offers a durable, continuous surface for pots, staging and cleaning. It can simplify a level floor and provide a suitable substrate for approved anchors. It is harder to reverse than paving, however, and it can increase runoff if the surrounding garden has poor drainage. A greenhouse intended for in-ground beds may be better on a designed perimeter foundation.

Choose concrete for the way you will use the greenhouse, not because it sounds strongest. Our base comparison sets it beside paving and perimeter options.

Obtain the actual base drawing

Do not mark out from a “6 × 8” shop category. Get the exact model's base dimensions, anchor layout, door position and any required proprietary frame. The roof or gutters can extend beyond the support line; plan surrounding clearance separately. Ask whether the manufacturer expects a slab to extend beyond the frame and whether a particular edge distance is needed for anchors.

Draw the proposed slab, greenhouse and working path on the plot. Measure diagonals to check it is square. Add the approach to the door before setting the slab height. A high slab can create a step even when the greenhouse has a low threshold.

Investigate ground and services

Remove turf and organic topsoil to competent ground. The required excavation, sub-base, reinforcement and slab design depend on soil, local water conditions and the greenhouse or staging loads. There is no single mix, thickness or reinforcement schedule suitable for every site.

Locate buried electricity, water, gas and drainage before digging. Persistent wet ground, made ground, major roots, slopes or nearby retaining walls merit local professional input. A slab poured over unstable fill can crack or tilt; the greenhouse doors and glazing may then stop fitting correctly.

Plan levels and water flow

The frame needs a flat, level support line within its installation tolerance. Surface water should not be trapped against the base rail or directed toward a neighbour. Where the wider site falls, a groundworker can design the approach and surrounding drainage without building a pronounced slope through the greenhouse footprint.

Consider where rain from the roof will go and whether a water butt or drain connection will fit. Keep inspection space around any gutter. If a solid floor is intended, decide now whether you need a floor drain, removable matting or a cleanable finish. These decisions are difficult to retrofit.

Pour, cure and check

Set formwork to the planned dimensions and finished height. A competent contractor should specify preparation, concrete placement, finishing and curing for the ground and expected loads. Do not fix the greenhouse while the surface is too green to accept the chosen anchors.

After curing, use a long straightedge and level to check the entire support line, not just individual corners. Recheck width, length, diagonals and threshold height. Small out-of-square errors can propagate through a modular frame. Keep photographs and the final base measurements with the manual.

Anchor the approved way

Set the frame in place before marking holes through its designated fixing points. Use only an anchor system approved for the concrete and greenhouse. Edge distance, embedment, drill size and torque are system-specific: guessing can split concrete or distort aluminium. Our anchoring guide explains the sequence and why a secure foundation still needs a properly connected frame.

If you want to preserve soil beds, read the perimeter-base guide before pouring. It is usually easier to choose the right base now than cut a finished slab later.

The Halls foundation guidance is a useful manufacturer example, not a universal concrete design. Ground conditions, the exact frame and the approved anchor system must determine the specification for your site.