How Much Gravel Under a Concrete Slab?
Crushed stone, compacted in lifts. Details below.
Base depth by project
| Project | Compacted base |
|---|---|
| Walkway, patio, shed pad | 4" |
| Driveway, garage slab | 6" |
| Clay or disturbed/filled soil | 8"+ (and consider geotextile fabric) |
Doing it right, in order
Excavate to undisturbed soil and compact it. Lay the stone in 2–3" lifts, compacting each with a plate compactor — one pass over 6 loose inches compacts the top and leaves the bottom soft. Screed the final lift flat: every high spot steals slab thickness, and every low spot wastes concrete. Then vapor barrier if the slab will be enclosed, rebar on chairs, and pour.
The base is the cheapest part of the job and the cause of most slab failures. An extra hour with the compactor outlasts every crack repair product ever sold.
Common questions
How much gravel goes under a concrete slab?
4 inches of compacted gravel is the residential standard under patios, shed pads, and walkways; 6–8 inches under driveways and garages or over clay/soft soils. Compact in 2–3 inch lifts.
What kind of gravel goes under concrete?
Crushed stone with fines — #57 stone topped with crusher run, or road base/#411 — because angular crushed material locks together when compacted. Rounded pea gravel shifts and is the wrong choice under a slab.
Do I really need gravel under a slab?
On well-drained, undisturbed soil, small non-structural pads sometimes skip it. Everywhere else, the base does three jobs: uniform support (prevents differential settling and cracks), drainage (water under a slab + freeze = heave), and a capillary break keeping ground moisture out of the concrete.
Should I put plastic under the concrete?
A 6-mil+ vapor barrier between gravel and concrete is essential under garages, basements, and anything that will be finished, flooring-covered, or climate-controlled. Open patios usually skip it.