Gravel Base Under a Concrete Slab: How Much and Why
A compacted gravel base gives a slab even support and keeps water from collecting under it. The residential code asks for 4 inches under slabs that sit below grade. Here is what the base does, how to build it, and how much gravel to order.
What the base does
- Even support. A level, compacted layer stops the slab bridging over soft spots, which is where cracks start.
- Drainage. Water drains through gravel instead of sitting under the concrete and softening the soil.
- A flat surface to pour on, which keeps the slab thickness even, so you don't pay for extra concrete filling low spots.
Concrete Network notes that most contractors put a compacted gravel subbase under driveways, and that a thicker base supports more weight.
What the code says
The International Residential Code calls for a 4 inch base of clean, graded sand, gravel or crushed stone under slabs that sit below grade, unless a soils report says the ground doesn't need it. Heated floors also need a vapor retarder (at least 6 mil polyethylene) between the base and the slab. Driveways, walks and patios that won't be enclosed are exempt from the vapor retarder. Check your local code for slabs that are part of a building.
How much gravel to order
Gravel volume is slab area × base depth. At 1.4 tons per cubic yard (crushed stone weighs about 1.35–1.4, road base up to 1.5, according to SelectSG):
| Slab | 4 in base (yd³) | 4 in base (tons) | 6 in base (yd³) | 6 in base (tons) |
|---|---|---|---|---|
| 10 × 10 ft | 1.23 | 1.7 | 1.85 | 2.6 |
| 12 × 16 ft | 2.37 | 3.3 | 3.56 | 5.0 |
| 20 × 20 ft | 4.94 | 6.9 | 7.41 | 10.4 |
| 12 × 40 ft | 5.93 | 8.3 | 8.89 | 12.4 |
| 24 × 24 ft | 7.11 | 10.0 | 10.67 | 14.9 |
These are the compacted volumes. Loose gravel settles when it is compacted, so ask your supplier how much extra they suggest for the material you're buying, and order a little over.
Building the base, step by step
- Excavate to the slab thickness plus the base depth, a little wider than the forms.
- Remove roots, topsoil and soft spots, and compact the soil underneath.
- Spread the gravel in layers of a few inches and compact each one with a plate compactor.
- Check the level against your forms with a string line, so the slab comes out the planned thickness everywhere.
- Add the vapor retarder if the slab is a heated floor, lapping the joints.
Which material?
Crushed stone with a mix of sizes (often sold as road base or crusher run) compacts into a firm layer. Clean crushed stone drains faster but needs good compaction. Pea gravel is round and rolls rather than locking together, so it's a poor choice under a slab. Your supplier can tell you what locals use for slab bases.
Sources
- International Residential Code slab requirements, summarized by EVstudio
- IRC R506.2.3 vapor retarder (via Cornell LII)
- Concrete Network: concrete driveway thickness and base
- SelectSG: how much does a yard of crushed stone weigh
Estimates for planning. Follow your plans, your supplier's instructions and your local building code.