Garage Slab Materials List: Concrete, Base, Vapor Barrier
Part of the project planners: Planning a concrete patio, slab or driveway and Planning a shed, pergola, garage slab or coop.
A 24 × 24 ft garage floor poured 4 inches thick needs 7.82 cubic yards of concrete (with 10% extra, 1 truck load), about 11.9 tons of gravel for a 4 inch base, 676 ft² of vapor barrier, 42 twenty-foot lengths of #3 rebar and 96 ft of control joints. This is the whole list, with a chart for other slabs.
The materials list
| What to buy | How much | How we worked it out |
|---|---|---|
| Concrete | 7.82 yd³ of ready-mix (1 truck load) | Slab calculator: 4 in thick, 10% extra. Order to the next quarter yard |
| Compacted gravel base | 11.9 tons of crushed stone | Gravel calculator: 4 in deep, plus 20% because loose stone compacts |
| Vapor barrier | 676 ft² of 10-mil polyethylene | The slab plus a foot all round, lapped 6 in and taped. A 20 × 100 ft roll is 2,000 ft² |
| Rebar, #3 (3/8 in) | 42 bars of 20 ft | Rebar calculator: a grid at 18 in centers. Or 12 sheets of 5 × 10 ft wire mesh |
| Rebar chairs | About 36 | One for every 4 × 4 ft |
| Form boards | 12 boards of 8 ft (2×6) | The 96 ft perimeter. Skip them if you pour inside a stem wall or footing |
| Stakes | About 39 | One every 2 to 3 feet, plus the corners |
| Control joints | 96 ft to cut or tool | Panels no longer than 10 ft each way, cut a quarter of the slab deep |
| Isolation joint strip | 96 ft | Around the edge where the slab meets walls, footings or columns |
| Sealer or coating | 3 gallons | About 250 ft² a gallon for a sealer, once the concrete has cured for a month |
Materials for other slabs
4 inch slab, 4 inch base, #3 rebar at 18 in centers, 10% extra concrete and 20% extra stone:
| Slab | Area | Concrete | Truck loads | Base stone | Vapor barrier |
|---|---|---|---|---|---|
| 10 × 12 ft | 120 ft² | 1.6 yd³ | 1 | 2.5 tons | 168 ft² |
| 12 × 20 ft | 240 ft² | 3.3 yd³ | 1 | 5.0 tons | 308 ft² |
| 20 × 20 ft | 400 ft² | 5.4 yd³ | 1 | 8.3 tons | 484 ft² |
| 24 × 24 ft | 576 ft² | 7.8 yd³ | 1 | 11.9 tons | 676 ft² |
| 24 × 30 ft | 720 ft² | 9.8 yd³ | 1 | 14.9 tons | 832 ft² |
| 30 × 40 ft | 1200 ft² | 16.3 yd³ | 2 | 24.9 tons | 1,344 ft² |
For other thicknesses or joint spacings, use the garage slab calculator.
What goes under the slab
- Compacted soil. Remove topsoil and organic matter, and compact what is left. Fill soft spots with stone.
- A gravel base. Four inches of crushed stone gives even support and a break against moisture; use 6 inches over soft soil or where it freezes.
- A vapor barrier. Polyethylene sheeting stops ground moisture rising, which matters if you will coat the floor or store things on it.
- Reinforcement. Rebar or wire mesh, held mid-depth on chairs. It holds cracks tight, but it does not prevent them.
Before you order
- Check your local code for the thickness, footing, reinforcement and vapor barrier, since garage slabs are often part of a permitted building.
- Call 811 before you dig.
- Slope the floor toward the door, about 1/8 to 1/4 inch per foot, and keep the surface above the outside ground level.
- Order the pour for a dry day with helpers. A slab this size is too much to mix by hand.
- Plan the finish: a broom finish for grip, or a hard-troweled one if you will coat the floor.
Mistakes that cost people money
- Skipping the vapor barrier under a slab that will be coated or covered.
- Cutting the control joints too late, after cracks have already started.
- Adding water to the truck. It weakens the concrete and encourages cracking.
- Pouring on frozen or soggy ground.
- Forgetting the footing or stem wall that the walls will sit on, which is an extra quantity of concrete.
See also the concrete driveway materials list, the concrete patio materials list and the bags of concrete guide. Estimates only: check your local code, and follow the product labels.
Frequently asked questions
How much concrete do I need for a 24x24 garage slab?
A 24 × 24 ft slab 4 inches thick needs 7.11 cubic yards before waste and 7.82 cubic yards with 10% extra, which is 1 truck load. At 6 inches it is 11.73 cubic yards.
How much concrete for a 12x20 shed slab?
A 12 × 20 ft slab 4 inches thick needs 3.26 cubic yards with 10% extra, on about 5.0 tons of crushed stone.
How thick should a garage floor be?
Four inches is the usual minimum for cars and a workshop. Use 5 or 6 inches for heavy trucks, a car lift or heavy equipment, and follow your local code.
Does a garage slab need a vapor barrier?
Usually yes. A 10-mil polyethylene sheet over the base stops moisture rising through the concrete. A 24 × 24 ft slab needs 676 ft², which is 1 roll of 20 × 100 ft.
How many control joints does a garage slab need?
Panels no longer than about 10 feet each way: 96 ft of joints on a 24 × 24 ft floor, cut a quarter of the slab depth.
More guides
Tools & materials for this project
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Estimates only. Quantities depend on your materials, site and supplier. Confirm important numbers before you order, and follow local building codes for anything structural.