How thick does a concrete slab need to be?

People ask how thick a slab should be as though the concrete is what carries the load. It is not, quite. A ground-bearing slab spreads load onto the ground, and it is the ground and the sub-base that carry it. Doubling the concrete over soft, uncompacted, organic ground buys you a thicker thing to crack. Getting the base right and using a modest slab thickness gives a far better result for less money. The section drawing shows every layer in order, and how much of the build-up sits below the concrete.

Safety

Step by step

1. Establish what the slab has to carry.A shed or a garden store, foot traffic and a bin, a greenhouse, a hot tub full of water, a car, a workshop with machinery, or the wall of a building. These are wildly different, and the last two are not the same category of problem as the first three.
2. Stop here if it is structural.If it will carry a building, a load-bearing wall, retained ground, or a vehicle regularly, it needs a designed thickness, a designed reinforcement and normally building regulations approval. Get a structural engineer. It is not an expensive piece of advice compared to a failed base under a finished building.
3. Dig out to firm ground.Remove all topsoil, roots, made ground and anything soft. Topsoil is organic and will keep decomposing and settling under a slab for years. Dig until you are on natural, undisturbed material that does not deflect under a heel.
4. Put in a properly compacted sub-base.MOT type 1 crushed stone, laid and whacked with a plate compactor in layers of no more than about 100mm each, not in one deep lift. This is the layer doing most of the work and the one people skimp. It must be compacted, not just tipped and raked.
5. Blind the sub-base if a membrane is going on.A thin layer of sand or fine material over the sharp stone so it does not puncture the polythene above it.
6. Decide whether a damp proof membrane is needed.Anything that will be floored, used as a workshop, or built on wants a membrane, lapped at the joints and turned up at the edges. An open garden base or a bin standing does not.
7. Choose a working thickness for a light base.For a garden building base on a good compacted sub-base, around 100mm of concrete is the common domestic figure. Increasing the thickness makes far less difference than increasing the sub-base compaction, so put your effort there.
8. Put in movement joints on anything large.Concrete shrinks as it cures and it will crack somewhere. A movement joint gives it a controlled line to crack along. Long or large slabs, slabs that change direction, and slabs abutting a building all want joints.
9. Isolate the slab from the building.Never cast a new slab hard up against an existing wall or across a damp proof course. Use a compressible joint filler at the junction so the two can move independently, and keep the concrete level below the building's damp proof course.
10. Read the drainage before you pour.A new slab is an impermeable surface, and water has to go somewhere other than against a wall. Fall it away from the building, and think about where the run-off goes — a slab that drains towards the house is a permanent damp problem.

Tips

• If you can push a screwdriver into the sub-base by hand, it is not compacted. Hire the plate compactor; it is a half-day hire and it is the difference between a slab that lasts and one that dishes.
• Casting new concrete over old is possible but it will not bond usefully unless the old surface is prepared and primed, and a thin unbonded topping over old concrete cracks and lifts. If the old slab is sound, prepare and bond to it; if it is not, take it out.
• Getting the levels right on paper before digging saves an enormous amount of concrete. Every 10mm of over-dig across a base is 10 litres of concrete per square metre.
• Check for buried services before you dig. A slab is a permanent thing to build over a drain run or a gas supply.

Common mistakes

• Putting a thick slab on a thin or uncompacted base — The slab spans nothing; it spreads load. A poorly compacted base settles unevenly and the slab cracks over the soft spot regardless of how much concrete is in it.
• Casting a slab tight against the house wall — It bridges the damp proof course, transfers movement between the two, and gives water a path to the wall. Always separate with a joint and keep below the damp proof course.
• Treating a shed base and a garage base as the same job — A vehicle imposes concentrated wheel loads and needs a designed thickness and reinforcement. A shed does not. Getting the two confused in either direction wastes money or produces a failure.

Questions people ask