Slab Thickness, Base, Reinforcement, and Joints for the Load You Have

Slab thickness for an interior floor, a garage, a shop, a driveway, an RV pad, and a forklift floor, with the gravel base, the vapor barrier, the wire or fiber or rebar, the joint spacing, and the cubic yards and cost.

Ground-bearing slab lines: a 3.5 in slab on a 4 in gravel base with a poly moisture barrier is the IRC R506.2.3 line for an interior floor with no vehicle load, 5 to 6 in is the garage and driveway line, 6 in is the shop line, and 8 in is the floor that carries a forklift. Thickness and base move together: a 6 in slab on 6 in of compacted base is the standard garage detail, and a 4 in slab on 4 in of base is the interior floor detail.

—Slab thickness and base for the load
—Cubic yards, bags, and installed cost
—Wire, fiber, rebar, and cover
—Base tons, vapor barrier, and prep
—Control joint spacing and cut depth
—Cure, saw window, and weather protection
How the math works
  • Thickness by load: 4 in for an interior floor with no vehicle load (the IRC R506.2.3 line is 3.5 in on a 4 in base), 5 in minimum for a garage and a driveway with 6 in for an RV, a lifted truck, and a trailer pad, 6 in for a shop, and 8 in for a forklift, a pallet rack leg, and a warehouse floor.
  • Base: 4 in of compacted aggregate under an interior slab and a driveway, 6 in under a garage slab and an RV pad, compacted in 4 in lifts to 90 to 95 percent of modified Proctor, with a 10 mil poly vapor barrier over the base, lapped 6 in, taped, and lapped up the wall.
  • Reinforcement: welded wire W1.4 at 0.155 lb per sq ft on chairs in the middle third, synthetic macro fiber at 1.5 lb per cubic yard dosed at the plant, or #3 rebar at 18 in on center each way with 3 in of cover and 12 in lap splices; #4 rebar on the 8 in warehouse slab.
  • Joints: control joints at 2 times the slab thickness in feet, maximum 3 times, sawn to a quarter of the thickness within 4 to 12 hours of finishing, sealed with a non-hardening sealant, with dowels at a joint that transfers a wheel load.
  • Quantity: cubic feet = sq ft times thickness divided by 12, and cubic yards = cubic feet divided by 27, which is sq ft times thickness divided by 324; an 80 lb bag yields 0.6 cu ft, so bags = cubic feet divided by 0.6; base tons = cubic yards of base times 1.4; installed price runs $2.05 per sq ft per inch of thickness with the base, the poly, the reinforcement, the saw, and the finish.
Slab planning tool on the IRC R506.2.3 ground-bearing floor lines with ACI 302.2, ACI 308, ACI 330.1R, ACI 360R, and ACI 318 practice for slab-on-grade, with standard field detail. Soil bearing capacity, expansive and collapsible soil, a high water table, a frost footing, a monolithic pour, a slab on a basement, a slab over radiant tube, a slab with a sump, a slab meeting a slab-on-grade at an addition, a slab in seismic and expansive-soil regions, a slab on a compaction-requirement fill, a warehouse slab with a flatness and levelness spec, and a slab with a coating, a broadcast finish, a hardener, a dry-cast floor, and a power-troweled floor are all calls for the geotechnical report, the drawings, the specifier, and the ACI detail, and a structural slab with a rack leg, a point load, a forklift aisle, and a 50,000 lb truck scale is a slab designed by an engineer and not sized by a calculator. Contractor and finisher pricing by region and pump access moves 25 percent.

How thick should a garage slab be?

6 in on 6 in of compacted base is the standard garage detail. The code floor line for a ground-bearing floor with no vehicle load is 3.5 in on a 4 in gravel base with a moisture barrier (IRC R506.2.3), and 5 in is the minimum most garage details use. An RV, a lifted truck, a trailer on jacks, and a shop with a car lift post are 6 in with reinforcement.

How thick does a driveway need to be?

5 in on 4 in of compacted base for passenger vehicles. The wheel line and the apron are where a 4 in driveway fails, and an RV, a truck, a trailer, a delivery, and a fire lane are 6 in with #3 rebar at 18 in on center. Joints sawn at 2 times the thickness (8 ft on a 4 in slab, 10 to 12 ft on a 6 in slab) with dowels at the apron joint.

Is welded wire or fiber better in a slab?

For crack control in a slab-on-grade, macro fiber at 1.5 lb per cubic yard is the easier, more reliably placed answer, and welded wire W1.4 on chairs in the middle third is the load-crack answer. The failure mode is wire left on the ground, which cracks with the slab. Rebar at 3 in of cover is the answer where the slab carries a point load: a rack leg, a fifth wheel jack, a boat, and a forklift.

How far apart do control joints go?

Two times the slab thickness in feet, with 3 times the thickness as the maximum: 8 ft on a 4 in slab, 10 ft on a 5 in slab, 12 ft on a 6 in slab, 16 ft on an 8 in slab. Saw to a quarter of the thickness within 4 to 12 hours of finishing and seal the joint.

How many cubic yards for a 400 sq ft slab at 6 in?

400 sq ft times 6 in divided by 12 is 200 cubic feet, which is 7.41 cubic yards. Order 8 yards with the spill and slump allowance, about $1,400 in ready-mix at $175 per yard, and about 335 80 lb bags if it is mixed by hand. Installed with the base, poly, wire, saw, and finish, about $16.30 per sq ft, about $6,500 for 400 sq ft.

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