concrete

How to Lay a Concrete Shed Base: Shuttering, Mix and Levelling

Gabriel

09 Nov 2025 — 3 min read

A properly constructed concrete base is the foundation of a shed that will last decades rather than years. A shed sitting on uneven ground or poor foundations will rack (go out of square), let in damp, and develop door problems within a few seasons. Getting the base right is the most important part of a shed installation, and it's a genuinely manageable DIY project.

Planning and Size

The base should be 75–100mm larger than the shed footprint on all sides — this keeps the shed wall plates clear of ground moisture. For most garden sheds, a 100mm concrete slab on 75–100mm hardcore is the standard specification. For larger structures or soft ground, increase the slab depth to 150mm.

Does this work need planning permission? Shed bases don't typically require it, but the shed itself may under Permitted Development if it exceeds 15m² floor area, is close to the boundary, or is in a Conservation Area. Check with your local planning authority for your specific situation.

Tools and Materials

Spade and wheelbarrow
Timber for shuttering (25 × 150mm PAR or 25mm planks)
Hardcore (MOT Type 1) or crushed concrete — typically 0.75 tonnes for a standard 2.4 × 3m base
Sharp sand and cement, or pre-mixed ballast — a 2.4 × 3m base to 100mm requires roughly 0.7m³ of concrete
Polythene DPM (damp proof membrane)
A2 reinforcing mesh (optional but recommended for larger bases)
Tamping board (a straight length of timber to screed the surface)
Plate compactor for the hardcore (hire: £60–80/day)

Step by Step

Mark out and excavate: Dig out the area to a depth of 175–225mm below finished ground level (100mm slab + 100mm hardcore + a little margin). Remove all topsoil and organic material — topsoil compresses over time and will cause settlement.
Level the sub-base: The bottom of the excavation should be reasonably level — check with a spirit level on a long straight edge. Fill any low points with hardcore.
Lay and compact hardcore: Spread MOT Type 1 to approximately 100mm depth and compact in layers with the plate compactor. Don't compact in one deep layer — 50mm passes give better compaction.
Build the shuttering: Knock timber stakes into the ground at the perimeter and screw or nail the shuttering boards to them. Check the shuttering is level with a spirit level — this is what determines your finished slab level. Take time to get this right.
Lay the DPM: Lay a sheet of 1000-gauge polythene over the hardcore, overlapping 150mm where sheets join. Turn edges up inside the shuttering. This prevents damp rising into the slab.
Mix and pour concrete: A mix of 1 part cement : 2 parts sharp sand : 4 parts coarse aggregate (or equivalent ready-mix C25) is correct for a shed base. Mix in a hired mixer (around £40/day) or order ready-mix for larger areas. Pour in from one end, raking level.
Tamp and level: Draw the tamping board across the shuttering in a chopping motion, working backward. Any low spots will be obvious — fill and re-tamp. The surface should be flat but not polished — a rough tamped finish is less slippery and provides a better key for a shed's DPC.
Cure: Cover with polythene sheeting to retain moisture and allow to cure for at least 72 hours before loading. Full strength takes 28 days.

Ordering Ready-Mix vs Mixing Yourself

For bases over about 2m³, ready-mix concrete delivered by lorry is usually more economical than mixing on site. A cubic metre of ready-mix C25 typically costs £90–130 delivered. You'll need to be ready to place it quickly as the driver won't wait more than 30–45 minutes.

For smaller bases, hire a cement mixer and buy bags of ready-mixed ballast — a 25kg bag at around £4.50 makes roughly 12 litres of concrete. A 2.4 × 3m base at 100mm requires approximately 36 bags plus cement.