How to plan a concrete shed base
A concrete base gives a shed a level, dry floor that lasts. The materials are simple, but the quantities add up fast, so it pays to work them out before the delivery is booked.
- Mark out and dig about 200mm deep, removing all turf and topsoil.
- Set the formwork: timber boards around the edge, level (or with a slight fall), held with pegs.
- Lay the sub-base: 100mm of MOT Type 1, compacted in two layers.
- Pour the concrete to the top of the formwork, tamp it level with a straight board, and keep it damp for a few days while it cures.
Mixing it yourself? A 3m × 2.5m slab at 100mm needs about 0.75m³ of concrete, which is a lot of mixer loads. For bigger bases, compare the price of a ready-mix delivery.
What this base needs: a 3m × 2.5m example
Digging 200mm deep over 7.5m² takes out about 1.5m³ of soil, nearer 2m³ once it's loose and close to 3 tonnes. That's a small skip, or bags if you can spread some of it in the garden. The sub-base is about 1.7 tonnes of MOT Type 1 (2 bulk bags). The slab is about 0.79m³ of concrete with waste, which is 11 bags of cement and 2 bulk bags of ballast in a 1:5 mix. Each step above opens with these sizes filled in, so you can check the details and save them to one job.
Concrete, slabs or a plastic grid?
- Concrete is the strongest and most level, and the best choice for garden rooms, workshops and heavy sheds.
- Paving slabs on a compacted sub-base work well for small and medium sheds and are easier to lift later. The patio calculator covers them.
- Plastic grid bases filled with gravel are quick and drain well for light sheds, but need a firm, level sub-base underneath.
Garden rooms and offices
If the building will be heated or have electrics, plan for it before the pour. Lay a polythene damp-proof membrane under the concrete, and bed a ducting pipe in the sub-base for the power cable (and data or water) so it can be pulled through later. Larger or habitable outbuildings can bring in planning and Building Regulations rules, so check before you build.
Common mistakes
- A base bigger than the shed. Rain runs off the walls and pools on the exposed edge, rotting the floor. Keep the base the same size or slightly smaller.
- Leaving topsoil underneath. Remove all turf and soft soil down to firm ground before the MOT Type 1 goes in.
- Building on it too soon. Give the concrete at least a few days, longer in cold weather, before putting a heavy building on it.
Our sub-base depth guide covers soft ground and drainage in more detail.