Work out the concrete, Type 1, mesh, DPM and formwork for a concrete shed or outbuilding base.
100mm Type 1, then a concrete slab on top
Concrete slab thickness, with mesh reinforcement
Area × Type 1 depth (100mm by default) × 2,100 kg/m³ compacted, plus waste, rounded up.
Area × slab thickness gives the concrete. Ready-mix is rounded up to 0.25 m³. For mixing by hand, we multiply by 1.54 for the dry materials and split by the mix ratio.
A142 mesh sheets lapped 200mm, a polythene DPM with 10% for laps, and formwork board and pegs round the perimeter. Enter length × width for exact edge lengths.
These are estimates for ordering materials, not a structural design. Check site conditions, and anything unusual, with a qualified builder or engineer.
75–100mm of concrete on 75–100mm of Type 1 for a typical garden shed. Use 100mm or more for a workshop, summer house or anything heavy.
The same size as the shed's footprint or very slightly smaller, so rainwater runs off the edge instead of collecting under the walls.
Yes. A polythene damp-proof membrane under the slab stops damp rising into the shed floor and stops the wet concrete losing water into the ground too quickly.
Yes. Slabs on Type 1 and a mortar bed work well for smaller sheds. Use the patio calculator with your shed's size.
You can walk on concrete after a day or two, but leave it about a week before building a shed on it, and longer in cold weather.