Concrete Shed Base: How Much Concrete and Materials Do I Need?
A concrete base for an 8 × 6 ft shed (about 2.4 × 1.8 m) takes 0.48 m³ of concrete at 100 mm thick with 10% extra. If you mix it yourself at 1:2:4 that is 7 bags of 25 kg cement, 0.33 tonnes of sharp sand and 0.63 tonnes of aggregate, on top of 1.05 tonnes of MOT Type 1 for the sub-base.
The materials list
| What to buy | How much | How we worked it out |
|---|---|---|
| Concrete, ready-mixed | 0.50 m³ | Concrete calculator: 4.3 m² at 100 mm, 10% extra, rounded up to the next quarter m³ |
| Or: cement for DIY mixing | 7 bags of 25 kg | For a 1:2:4 mix by volume. Allow for a little extra: the first batch is often a trial |
| Sharp sand | 0.33 tonnes | A bulk bag of 0.8 tonnes is nearly enough, so order the next size up |
| 20 mm aggregate (gravel) | 0.63 tonnes | Washed or "all-in" ballast is also sold ready mixed: ask your merchant |
| MOT Type 1 sub-base | 1.05 tonnes | Gravel calculator: 100 mm compacted, with 10% extra |
| Damp-proof membrane | 6 m² | Optional: a polythene sheet over the sub-base, with an overlap, keeps moisture out of the underside of the concrete |
| Timber for the shuttering | 8.4 m of board | The perimeter, in 100 to 150 mm boards, held by pegs |
Materials for other shed sizes
A 100 mm base on a 100 mm sub-base, 10% waste, 1:2:4 mix:
| Shed size | Concrete | Bags of cement (25 kg) | Sharp sand | 20 mm aggregate | MOT Type 1 |
|---|---|---|---|---|---|
| 6 × 4 ft (1.8 × 1.2 m) | 0.24 m³ | 4 | 0.17 tonnes | 0.31 tonnes | 0.52 tonnes |
| 8 × 6 ft (2.4 × 1.8 m) | 0.48 m³ | 7 | 0.33 tonnes | 0.63 tonnes | 1.05 tonnes |
| 10 × 8 ft (3 × 2.4 m) | 0.79 m³ | 11 | 0.56 tonnes | 1.05 tonnes | 1.74 tonnes |
| 12 × 8 ft (3.6 × 2.4 m) | 0.95 m³ | 13 | 0.67 tonnes | 1.25 tonnes | 2.09 tonnes |
Before you order
- Check for buried services before you dig. The free LSBUD (LinesearchbeforeUdig) service shows the gas, water, electricity and telecoms assets near your address.
- Check the shed maker's instructions for the base size. Most want a base the same size as the floor, or marginally smaller, so that rain does not collect on it.
- Check the rules. Most garden sheds are permitted development, but there are limits on height and on how much of your garden outbuildings can cover, and extra limits close to a boundary, in a conservation area or on a listed property.
- Mix or order? Under about a cubic metre it is practical to mix by hand or with a mixer, but allow for the work. Ready-mix is easier above that, but check the minimum load and the delivery charge first.
How the job goes
- Mark out the base, clear the vegetation and topsoil, and dig down about 150 to 200 mm.
- Lay and compact 100 mm of MOT Type 1, and make sure it is level.
- Fit the shuttering, held with pegs, to the height of the slab, and check it is level.
- Pour the concrete, rake it out, and tamp it level with a straight board. Finish the surface with a float.
- Cover it and keep it damp for about a week, so that it cures slowly. Wait a few days before you build the shed on it.
Mistakes that cost people money
- Skipping the sub-base. A slab on soft ground will crack and settle.
- Ordering exactly the volume you calculated. Uneven ground eats concrete: always add 5 to 10%.
- Pouring in frost or heavy rain, or letting the surface dry out too quickly in hot weather.
- Not leaving the concrete to harden before assembling the shed.
Frequently asked questions
How much concrete do I need for an 8x6 ft shed base?
An 8 × 6 ft shed is about 2.4 × 1.8 m. A 100 mm base is 0.43 m³ before waste and 0.48 m³ with 10% extra, so order 0.50 m³ of ready-mix. For that size, many people mix it themselves.
How thick should a shed base be?
A 100 mm thick slab on a compacted sub-base is the usual choice for a garden shed. Go thicker for a large, heavy shed or a workshop, or on poor ground, and follow the shed maker's instructions.
How many bags of cement for a shed base?
For the 8 × 6 ft base mixed 1:2:4 you need 7 bags of 25 kg cement, 0.33 tonnes of sharp sand and 0.63 tonnes of 20 mm aggregate, with 10% waste.
Do I need a sub-base under a concrete shed base?
It is good practice: about 100 mm of compacted MOT Type 1 stops the slab settling and lets water drain. For the 8 × 6 ft base that is about 1.05 tonnes.
More guides
Tools & materials for this project
As an Amazon Associate we earn from qualifying purchases. These links do not change what you pay.
Estimates only. Quantities depend on your materials, site and supplier. Confirm important numbers before you order, and check the Building Regulations and any planning rules for anything structural or close to a boundary.