What You Should Put Under a Concrete Base

Under a concrete base you should put a compacted sub-base — typically 100–150mm of crushed stone such as MOT Type 1 hardcore — topped with a damp-proof membrane. The sub-base gives the slab a firm, free-draining platform; the membrane keeps ground moisture out. Pour concrete straight onto soil and the slab will settle and crack.

The sub-base is the layer of compacted material between the ground and the concrete. Its job is to spread the load of the slab (and everything on it) evenly into the ground, and to provide drainage so water isn't sitting directly beneath the concrete.

It's the least glamorous part of any base — and the most important. Our base teams will tell you that almost every cracked, sunken slab they're asked to look at failed underneath first. Get the sub-base right and the concrete above it has an easy life.

• Crushed stone — the go-to for garage bases. It compacts into a highly stable, strong layer, which is exactly what a slab carrying a building and a vehicle needs. • Gravel — offers excellent drainage, which also helps prevent frost heave in cold weather. • Sand — suits smaller jobs, and has a place where the water table is high.

For a garage or shed base, compacted MOT Type 1 hardcore (a graded crushed stone) is the standard choice.

1. Clear the area. Debris, vegetation and any obstructions all come out first — organic material left under a slab rots and causes settlement.

2. Level the ground. Never lay a base over grass or mulch; strip back to firm soil and get it level.

3. Dig out. Excavate to around 100–150mm to make room for the sub-base layers.

4. Install the sub-base. Lay MOT Type 1 hardcore and compact it thoroughly — after compaction you want 100–150mm of sub-base, depending on the soil conditions.

5. Add the membrane. A damp-proof membrane over the compacted sub-base stops moisture seeping up into the concrete.

Use a mechanical vibrating plate (a wacker plate). Treading hardcore down by boot is nowhere near enough — the material must be mechanically compacted into a dense, solid layer, or it will continue settling under the slab for years. Work across the whole area methodically until the surface is tight and stable.

Some ground needs more than the standard recipe:

• Loose or sandy soil needs thorough compaction, and often the addition of gravel, before it can carry a sub-base. • Expansive clay swells and shrinks with moisture — it can call for a specialised base design or lime treatment to stabilise it. • Organic soil (topsoil, peat) can't stay: excavate it and replace it with sound material. • A high water table is best handled with a drainage system, so water is managed rather than left to sit under the slab.

If you're not sure what you're digging into, ask before you pour — ground problems are cheap to fix at this stage and expensive ever after.