How to Build a Shed Base: Concrete, Slabs, Timber or Plastic Grid
Last updated: September 2026
A shed is only as good as the base it sits on. A level, solid base keeps the floor dry, stops the frame twisting (so doors keep closing properly) and makes the shed last years longer. Here are the four main options.
Which base should you choose?
| Base | Best for | Cost | DIY difficulty |
|---|---|---|---|
| Concrete slab | Large sheds, workshops, heavy contents | Higher | Moderate–hard |
| Paving slabs | Most garden sheds | Medium | Moderate |
| Timber frame | Sloping gardens, quick builds | Medium | Easy–moderate |
| Plastic grid | Small–medium sheds, flat ground | Lower | Easy |
Getting the size right
Sheds are still sold in feet in the UK (6×4, 8×6, 10×8 and so on), so convert carefully: 1 foot = 0.305m. Most manufacturers recommend the base is the same size as the shed's floor, or very slightly smaller. That way rainwater running off the walls drips onto the ground, not onto a lip of base where it can soak into the timber. Check your shed's instructions.
Option 1: Concrete base
The strongest, longest-lasting option.
- Mark out the area and dig down about 150–200mm, removing turf and topsoil.
- Build timber formwork around the edge, set level (or with a very slight fall), with its top at the finished height.
- Lay and compact 75–100mm of MOT Type 1.
- Lay a damp-proof membrane (DPM) over the sub-base.
- Pour 75–100mm of concrete, tamp it level with a straight timber off the formwork, and leave it to cure for several days before building on it.
For an 8×6ft shed (about 2.4 × 1.8m), a 100mm slab needs around 0.43m³ of concrete, or about 0.48m³ with 10% extra. That's over 40 bags of pre-mix, so mixing your own or ordering a small ready-mix load often works out better. The Concrete Calculator gives both options.
Option 2: Paving slab base
- Dig out, then lay and compact 75mm of MOT Type 1.
- Lay slabs (usually 600 × 600mm) on a 25–40mm mortar or sharp sand bed.
- Check level across every slab as you go.
- A full mortar bed gives a far firmer base than blobs of mortar under each corner.
Option 3: Timber frame base
A frame of pressure-treated timber, with joists at around 400mm centres, sitting on slabs, blocks or a layer of compacted gravel over a weed membrane. It's great on slopes, because you can build up one end with packers or posts. Keep the timber off the soil and use treated timber rated for ground contact.
Option 4: Plastic grid base
Interlocking recycled plastic grids clip together over a weed membrane on levelled ground, then get filled with gravel. They're quick, cheap and drain well, but the ground underneath must be properly levelled and firm first. They're best for small and medium sheds without heavy contents.
Do I need planning permission for a shed?
Most garden sheds fall under permitted development in England. Typical conditions: it isn't in front of the house; it's no more than 2.5m tall if within 2m of a boundary; and outbuildings don't cover more than half the land around the house. Listed buildings, conservation areas and flats have different rules – check with your local council.
Frequently asked questions
How thick should a concrete shed base be?
75mm is enough for most small sheds. Use 100mm for larger sheds, workshops or heavy contents.
Can I put a shed straight on the grass?
It's not a good idea. The floor will rot, and the shed will twist as the ground moves.
Should the base be bigger than the shed?
Usually no – the same size or slightly smaller helps rainwater drip clear. Follow your shed manufacturer's advice.
How long before I can build on a concrete base?
Wait at least 3–7 days depending on the weather.