Aggregate Base for a Concrete Slab
A concrete slab needs a compacted granular base underneath — typically Granular A or a similar 3/4" crushed stone with fines — usually 4-6 inches thick for residential slabs, more for driveways carrying vehicle loads. This base layer provides drainage under the slab and a stable, evenly-compactable surface to pour on.
Pouring concrete directly on native soil, or on an improperly compacted base, is one of the most common causes of slab cracking and settling, since uneven support under the slab creates stress points as the ground shifts seasonally.
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Choosing the Right Base
A slab base has two jobs: spread the load evenly and break capillary rise of moisture from the ground. Well-graded crushed aggregate compacted in lifts does both. The angular faces lock together and the range of particle sizes compacts to a dense, stable platform. Rounded stone will not lock and should not be used as a structural base.
Typical residential slabs sit on four to six inches of compacted granular over a prepared subgrade, with more depth where soils are poor. Compaction in lifts of a few inches matters more than total depth: a thick, poorly compacted base performs worse than a thinner, properly compacted one.
Frequently Asked Questions
How thick should the base under a concrete slab be?
Four to six inches of compacted granular is typical for residential slabs, over a prepared and compacted subgrade. Poor or wet soils call for more depth and sometimes a separation fabric beneath.
Can I use rounded gravel under a slab?
No. Rounded stone does not interlock, so it will not compact into a stable platform and can shift under load. Use a well-graded angular crushed material with a range of particle sizes so it locks together when compacted.
Does a slab base need a vapour barrier?
For any interior slab, or a slab that will be finished or heated, yes. The granular base breaks capillary rise but does not stop vapour. A poly sheet over the compacted base is standard practice for habitable and finished spaces.
How do I know the base is compacted enough?
Compact in lifts of a few inches with a plate compactor or roller sized to the job. The surface should not rut or spring under a loaded wheelbarrow, and footprints should barely mark it. On larger or structural pours, compaction testing is worth the cost.