Shed Base Options UK: Concrete vs Slabs vs Plastic Grid vs Timber

The four shed bases compared, for an 8 × 6 ft shed

Whichever you choose, check the shed maker's instructions first: many give the base size and the type of base the guarantee needs. Most want the base the same size as the floor, or a little smaller, so that rain runs off rather than pooling under the walls.

1. A concrete slab

A 100 mm slab of 2.4 × 1.8 m takes 0.48 m³ of concrete with 10% extra, so order 0.50 m³ of ready-mix, or mix it yourself from 7 bags of 25 kg cement, 0.33 tonnes of sharp sand and 0.63 tonnes of 20 mm aggregate. Under it goes 1.05 tonnes of MOT Type 1, 5.2 m² of damp-proof membrane if you use one, and 8.4 m of timber for the shuttering (figures from the shed base calculator).

It is the strongest and longest-lasting base, and the only sensible choice for a brick or concrete building. It is also the most work: a deeper dig, mixing or a ready-mix delivery, and a wait of a few days before you build on it. The full list, with the mix and the method, is in the concrete shed base materials list.

2. Paving slabs

An 8 × 6 ft base is exactly 4 × 3 slabs of 600 × 600 mm when they are butted together, so 12 slabs cover it with no cuts. The paving calculator says 13 with 10 mm joints and 10% spare, which gives you one to replace a broken slab. Under them goes 1.00 tonnes of MOT Type 1 at 100 mm and 0.22 tonnes of sharp sand for a 30 mm bed. For a firmer base, lay the slabs on a full mortar bed instead: 0.32 tonnes of sharp sand and 2 bags of cement.

Slabs suit most timber and metal sheds. The weak point is the ground beneath them: a slab laid on soil, or on a thin sub-base, rocks and sinks unevenly, and the shed door then binds. Compact the sub-base well and check each slab with a level.

3. A plastic grid filled with gravel

Plastic shed base grids clip together into a mat that you lay on levelled ground over a weed membrane and fill with gravel. For 2.4 × 1.8 m you need 4.32 m² of grid, and for a grid 40 mm deep about 0.29 tonnes of 10 to 20 mm gravel to fill it (0.18 m³, or about 12 bags of 25 kg), from the gravel calculator. The plastic walls of the cells take up some of that volume, so this is on the generous side. Grids are sold in different depths, so use the depth of the one you buy.

It is the quickest base to lay, needs no concrete and drains freely, which helps keep the shed floor dry. It is less rigid than a slab, so on soft ground some grid makers ask for a compacted sub-base first: 50 mm of MOT Type 1 for this size is about 0.52 tonnes. Check the load the grid is rated for before you put a heavy shed or a workshop full of tools on it.

4. A timber frame on piers or concrete pads

On sloping ground, a frame of treated timber joists on concrete pads saves digging out a level platform. For 2.4 × 1.8 m with joists at 400 mm centres you need 7 joists, each 1.8 m long, plus the timber round the edge. Six pads of 300 × 300 mm, 400 mm deep take 238 litres of concrete, or 4 bags of 25 kg cement mixed 1:2:4 (from the decking calculator, which works out the same frame and pads).

Use timber treated for ground contact (often labelled UC4 or Use Class 4), and keep it off the soil where you can, so that it does not stay wet. The joist size and the number of pads depend on the weight of the shed and how far the joists span, so check the shed maker's advice. A frame needs care to get square and level, but once it is, the shed floor stays dry with air moving underneath.

Materials for other shed sizes

The main quantity for each type of base, with the same assumptions as above (100 mm concrete with 10% extra, 600 × 600 mm slabs with 10 mm joints and 10% spare on 100 mm of MOT Type 1, gravel to fill a 40 mm grid, joists at 400 mm centres):

A 12 × 8 ft shed is about the size where the effort of a concrete slab starts to pay off: 0.95 m³ of concrete against 26 slabs on 2.00 tonnes of sub-base. For the concrete for one size in detail, see the shed base pages from 6 × 4 and 8 × 6 to 12 × 8 and 14 × 10 ft.

How to choose

• Heavy shed, workshop or log cabin: a concrete slab, or slabs on a mortar bed.
• Ordinary timber or metal shed on level ground: paving slabs on a compacted sub-base.
• Light shed and a quick job: a plastic grid with gravel, on well-levelled, firm ground.
• Sloping or awkward ground: a timber frame on concrete pads, or adjustable supports made for the job.
• Renting, or may move the shed: slabs, a grid or a frame can all be lifted later; a concrete slab cannot.

For the whole job, from choosing the size to the felt on the roof, see the garden, decking, fencing and sheds planner.

Frequently asked questions