Concrete slabs and pads: thickness, base and cost by use

Slab thickness by use

There is no single right thickness. The code sets a floor, and the load decides the rest.

Use	Typical thickness	Source
Interior floor slab (code minimum)	3.5 in	2021 IRC R506.1[1]
Shed slab	About 4 in, usually unreinforced	HomeGuide[3]
Patio or walkway	4 in minimum	HomeGuide[4]
Driveway, cars	4 in minimum, thicker edges	HomeGuide[6]
Driveway or pad for RVs, heavy trucks	5 to 6 in	HomeGuide[6]
Hot tub pad, outdoor kitchen	6 in with extra reinforcement	HomeGuide[4]
Garage floor	4 to 6 in, reinforced	HomeGuide[3]
House slab foundation	Typically 6 in with rebar	HomeGuide[3]

Treat these as starting points. Soft clay, fill soil, heavy point loads (a hot tub full of water, a car lift, a loaded trailer tongue) and frost all push the design thicker or call for an engineer. Full detail is in slab thickness by use.

What goes under a slab

Subgrade. Native soil, stripped of topsoil and roots and compacted. Most slab failures start here: a pad poured over soft or uneven fill settles unevenly and cracks.

Base course. The 2021 IRC calls for a 4-inch base of clean graded sand, gravel or crushed stone under floor slabs that sit below grade, with an exception for well-drained soils^[1]. Contractors use a gravel base under most exterior pads for drainage and even support.

Vapor retarder. Under a heated interior floor, the 2021 IRC requires a 10-mil vapor retarder meeting ASTM E1745 Class A, with joints lapped 6 inches^[1]. It can be left out under garages, carports, unheated accessory buildings, driveways, walks and patios that will not be enclosed and heated^[1]. So a quote that skips the barrier under a driveway is not cutting a code corner, but a shed you may later heat and finish is a different call. See vapor barriers under slabs.

Rebar, wire mesh or fiber

Reinforcement controls how wide cracks open. It does not stop them. NRMCA states plainly that wire mesh will not prevent cracking and that it tends to keep cracks and joints from opening up^[5]. Joints do the main work of controlling where cracks form. NRMCA's rule is joint spacing of no more than 24 to 36 times the slab thickness, with a 15-foot maximum^[5]. Rebar is common in thicker pads, garage slabs and anything carrying heavy loads. Compare the options in rebar vs wire mesh vs fiber.

Strength and curing

The model code sets 2,500 psi as the minimum for interior slabs on grade, and up to 3,500 psi with entrained air for garage floors and slabs exposed to severe weathering^[2]. Whatever the mix, curing finishes the job. NRMCA recommends curing for at least 3 to 7 days and keeping new concrete above 50 F^[8].

What slabs cost

HomeGuide's February 2026 figures^[3]:

Project	Typical size	Installed cost
Shed slab	100 to 400 sq ft	$600 to $4,800
Patio	200 to 400 sq ft	$1,000 to $6,000
Garage floor	400 to 576 sq ft	$2,400 to $6,900
RV pad	200 to 700 sq ft	$1,200 to $8,400
Driveway	400 to 800 sq ft	$2,400 to $13,000

Ready-mix runs about $119 to $172 per cubic yard delivered and poured, per HomeGuide^[7]. HomeGuide also reports that labor makes up 40% to 50% of a slab's total cost^[3]. See concrete slab cost per square foot.

What happens on pour day

Knowing the sequence helps you judge the crew:

1. Layout and forms. Forms are set to grade, with the slope planned so water runs off.
2. Base and compaction. Gravel is spread and compacted in layers, not dumped loose.
3. Reinforcement. Rebar or mesh sits on chairs so it ends up inside the slab, not lying on the ground under it.
4. Placement and screeding. Concrete is placed, struck off level, and floated.
5. Finishing. Edges are tooled, joints are tooled or marked for sawing, and the surface gets its broom or trowel finish.
6. Curing. A curing compound, plastic sheeting or wet covering goes on as soon as the surface allows. NRMCA calls for curing to start right after finishing^[8].
7. Saw cuts. If joints are sawn, they go in within hours, before random cracks form^[5].

Common slab mistakes

• Pouring on topsoil, roots or uncompacted fill.
• Adding water at the truck to make the mix easier to place, which weakens it.
• Leaving out joints, or spacing them too far apart.
• Mesh that ends up at the bottom of the slab, where it does nothing.
• Letting the slab dry in sun and wind with no curing.
• Building a heated room on a slab that has no vapor retarder.

Every slab and pad guide

• Shed slabs
• Hot tub pads
• AC and generator pads
• Garage slabs
• Barndominium and pole barn slabs
• RV and boat pads
• Basketball and sport courts
• Slab thickness by use
• Vapor barriers under slabs
• Rebar vs wire mesh vs fiber
• Slab on grade basics

When to call a pro

• Any slab that will carry a building, a hot tub, vehicles or heavy equipment.
• Slabs on clay, fill or wet ground, or in areas where frost reaches deep: ask whether an engineer should set the design.
• Barndominium, pole barn or garage slabs: these usually need a permit and plans that tie the slab to footings.
• An existing pad that has tilted, cracked through or pulled away from a building.