Concrete
What Goes Under a Concrete Slab?
The layer stack under a good slab: compacted subgrade, gravel base depth by use, vapor barrier placement, and reinforcement - with a diagram and quantity math.
Cracked, settled, or heaved slabs almost never fail because of the concrete. They fail because of what's underneath it. The concrete industry has a saying - a slab is only as good as its base - and the base is a stack of layers, each with a distinct job.
The layer stack
Layer 1: Subgrade - the soil itself
Strip all topsoil and organics (they decompose and settle), then compact the exposed soil. If the ground was recently filled, it must be compacted in lifts - uncontrolled fill is the number-one cause of slab settlement. Soft spots that pump underfoot after compaction should be dug out and replaced with stone. On expansive clays, get geotechnical advice before pouring anything you care about.
Layer 2: Gravel base - support, drainage, capillary break
Crushed, angular stone (crusher run / dense-graded base) compacted in lifts does three jobs: spreads loads evenly, drains water away from the underside of the slab, and breaks capillary rise of soil moisture.
| Slab use | Base depth | Notes |
|---|---|---|
| Patio, walkway | 4 in | One compacted lift |
| Driveway | 6 in | Two 3-in lifts; 8 in on soft or frost-prone ground |
| Garage / shop | 6–8 in | Compact each lift to refusal |
Size the base 12 in beyond the slab footprint on all sides, and quantity it by the ton with the gravel calculator - remember it's ordered loose but installed compacted, roughly 15% more loose volume than the finished layer.
Layer 3: Vapor barrier - interior slabs
Any slab under roof or that might be conditioned later gets a polyethylene vapor barrier - 10 or 15 mil sheet, seams lapped 6 in and taped, placed directly under the concrete (current practice, per ACI 302 guidance, rather than buried under a sand blotter layer). It stops soil moisture from migrating through the slab and wrecking flooring and finishes. Open-air patios and walks skip it so they can dry downward too.
Layer 4: Reinforcement - in the slab, not under it
Welded wire mesh or #3/#4 rebar controls crack width. It only works if it sits within the slab - support it on chairs or dobies at the correct height. Mesh laid on the base "to be pulled up during the pour" almost always stays on the bottom, where it adds nothing. Reinforcement does not replace joints: plan sawn or tooled control joints at 24–36× slab thickness in inches (8–12 ft for a 4-in slab) each way.
Worked example: 12 × 16 ft shed slab
- Base area (extend 1 ft each side): 14 × 18 = 252 ft²
- Base: 4 in compacted → 252 × 0.333 = 84 ft³ = 3.1 yd³ compacted → ~3.6 yd³ loose ≈ 4.9 tons of crusher run
- Vapor barrier: 12 × 16 plus laps ≈ 220 ft² of 10-mil poly
- Concrete at 4 in with 10% waste: 2.6 yd³ → order 3 yards
When to call a professional
Slabs over recent fill, on expansive or organic soils, on slopes, or supporting structures need a geotechnical assessment of the subgrade. If a screwdriver pushes easily into your compacted subgrade, stop and get advice before the pour - no base thickness fixes bad ground.
Frequently Asked Questions
How much gravel goes under a concrete slab?
4 inches of compacted crushed stone is the residential standard for patios and walkways; 6-8 inches for driveways and garages, or where soils are soft or frost-prone. Extend the base 12 inches beyond the slab edge on all sides.
Do I need a vapor barrier under a slab?
Under any interior or enclosed slab (garage, basement, shed you'll condition), yes - a 10-15 mil polyethylene sheet directly under the concrete. Open patios and walkways generally skip it.
Can I pour concrete directly on dirt?
On undisturbed, well-draining, compacted soil, small non-critical pads sometimes are. But without a stone base you lose drainage, uniform support, and a capillary break - on clay or recently filled ground, expect cracking and movement.
What kind of gravel is best under concrete?
Crushed, angular stone with fines - often sold as crusher run, road base, or dense-graded aggregate - compacts into a firm, stable layer. Rounded pea gravel doesn't lock together and makes a poor base.
Does rebar go under or in the slab?
In it. Reinforcement belongs in the middle-to-upper-middle of the slab thickness, held on chairs - not resting on the base where it does nothing. Wire mesh pulled up 'by hoe' during the pour usually ends up on the bottom.