Concrete Slab: The Complete Guide

What a concrete slab is

The American Concrete Institute defines a concrete slab as "a molded layer of plain or reinforced concrete, flat, horizontal (or nearly so), usually of uniform but sometimes of variable thickness, and supported by beams, columns, walls, other framework, or on the ground." That last phrase is the fork in the road. A slab supported on the ground is a slab on grade, and the soil carries the load while the concrete spreads it out. A slab that spans between supports is a suspended or structural slab, and it has to resist calculated bending moments on its own. Everything a homeowner pours is the first kind. Everything that needs an engineer is the second.

So at its simplest a slab is concrete placed in a form over a prepared base and finished flat. What changes from one slab to the next is the thickness, the reinforcement inside it, the mix specified, and the base underneath, all matched to the load it will carry and the climate it sits in. A garden path and a garage floor are both slabs, built to very different specifications.

Cement slab or concrete slab?

People search for a "cement slab" nearly as often as a concrete one, so it is worth clearing up, and the answer is not pedantry. Cement is an ingredient of concrete, not a synonym for it.

ACI defines concrete as "a composite material that consists of a mixture of hydraulic cement, aggregates, and water, with or without admixtures, fibers, or other cementitious materials." The National Ready Mixed Concrete Association puts the proportions on it: a typical mix is about 10 to 15 percent cement, 60 to 75 percent aggregates, and 15 to 20 percent water, with entrained air taking up another 5 to 8 percent in many mixes.

In other words, cement is the fine gray powder that reacts with water and glues everything together. It makes up roughly an eighth of what is in your slab. Nobody pours a cement slab, because a slab of neat cement paste would be expensive, weak and would crack apart as it shrank. When a supplier, a contractor or a price list says cement, they almost always mean concrete. The one place the distinction really matters is when you are buying: a bag of portland cement and a bag of concrete mix are different products at different prices, and only one of them contains the sand and stone. Our concrete mix calculator works out the cement, sand and aggregate for a given ratio if you are batching from raw materials.

Types of concrete slab

The word covers a lot of ground. These are the types you will actually run into, in rough order of how likely you are to meet one.

• Slab on grade. Cast directly on prepared soil, continuously supported, carrying no calculated bending moment. The code minimum thickness is 3-1/2 inches. This is the default for patios, garages, sheds, driveways and most single-family floors.
• Monolithic slab. A slab on grade poured thicker at the edges to form an integral footing, and placed all in one go. Fastest and cheapest, and the usual choice where the ground does not freeze or where the structure is small enough to be exempt from frost protection.
• Stem wall, or T-shaped, foundation. Three separate pours: a footing below the frost line, walls on top of it, then the slab. What separates this from a monolithic slab is frost, not strength.
• Floating slab. A slab with a thickened or reinforced perimeter that sits above the frost line and moves with the ground. Minneapolis, for example, permits one "for the foundation support of detached garages on all soils except peat and muck." It is a practical, regional term rather than a standardized one.
• Post-tensioned slab. A slab on grade with steel tendons that are tensioned after the concrete gains strength, used mainly on shrink-swell soils in Texas and the Southwest. A typical residential version is 8 inches thick with tendons at 48 inches on center. The thing to know as a homeowner: never cut or core a post-tensioned slab, because a severed tendon is very difficult to repair. That rules out retrofitting floor drains, saw-cut joints and car lift anchors without an engineer.
• Suspended or structural slab. Spans between supports and resists bending itself, so it always requires engineering. A garage floor over a basement is a suspended slab, not a slab on grade.
• Precast slab and hollow core plank. Cast in a plant and craned into place. Hollow core comes in 6, 8, 10, 12 and 16 inch depths spanning roughly 22, 29, 35, 40 and 50 feet respectively, in 8 foot widths, often with a thin topping. In US practice you meet it in multifamily and high-rise floors, hotels, parking structures and mezzanines rather than in houses.
• One-way and two-way slabs, flat plates, flat slabs and waffle slabs. All structural systems for commercial buildings. A flat plate has no beams and uniform thickness; a flat slab adds drop panels at the columns, which is the distinction people most often get wrong. A waffle slab is a two-way joist system formed with dome pans, which the reinforcing steel institute describes as the strongest of the family with the least deflection.
• Composite slab on metal deck. Concrete placed over profiled steel deck where, per the Steel Deck Institute, "the steel deck acts as positive bending reinforcing for the slab." If the deck is only formwork it is non-composite, or form, deck instead.

Everything from post-tensioned downward on that list needs a designer. Everything above it can be built from the prescriptive requirements in the residential code, which is what the rest of this guide covers.

A slab in section, from the bottom up

Most slab failures are decided below the concrete, so it helps to picture the whole assembly rather than just the pour. From the ground up, a residential slab on grade is:

1. Subgrade. Native soil with all vegetation and topsoil stripped off, per IRC R506.2. Topsoil holds water and compresses. Any fill has to be compacted for uniform support, and the code caps fill depth at 24 inches for clean sand or gravel and 8 inches for earth unless it is specifically approved.
2. Base course. Four inches of clean graded sand, gravel, crushed stone, crushed concrete or crushed slag passing a 2 inch sieve, required where the slab is below grade. Its job is drainage and uniform support, not strength. Size it with the gravel calculator.
3. Vapor retarder. Six mil polyethylene with joints lapped at least 6 inches, placed between the slab and the base. Note the exceptions: garages, utility buildings and other unheated accessory structures, unheated storage rooms under 70 square feet, and driveways, walks and patios are all exempt. So this belongs under a house floor and not under a shed.
4. Reinforcement. Rebar or welded wire mesh, where provided. The code is specific about position and not about quantity: reinforcement "shall be supported to remain in place from the center to upper one-third of the slab for the duration of the concrete placement." Mesh dragged down onto the base during the pour does nothing at all.
5. The concrete. Thickness and strength matched to the job, which is the next two sections.
6. A thickened edge or perimeter footing wherever the slab carries a wall, and joints: control joints to decide where the shrinkage cracks go, isolation joints wherever the slab meets a wall, column or another slab.

On joints, the industry rule is worth memorizing. NRMCA puts maximum control joint spacing at 24 to 36 times the slab thickness, capped at 15 feet, with panels no longer than 1.5 times their width, grooves cut at least a quarter of the slab depth and never less than an inch, within 4 to 12 hours of finishing. For a 4 inch slab that is joints every 8 to 12 feet.

The decisions every slab comes down to

Planning a slab means settling a handful of questions in order.

• How much will it cost? At normal size, $6 to $12 per square foot installed, but the rate is not flat: one contractor publishes $17 per square foot on a 10 by 10 and $6.40 on a 20 by 20. See the curve by size and the minimum charge that causes it.
• How thick should it be? Four inches for patios and sheds, 5 to 6 for driveways and garages. See the right thickness for your project.
• How much concrete do I order? Length times width times thickness, divided by 27 for cubic yards. See the formula and a size chart.
• What goes inside it? Rebar or wire mesh, plus a vapor barrier and gravel base. See sizes, spacing, and placement.
• How do I pour it? Lay out, base, forms, reinforce, pour, finish, cure. See the step-by-step guide.
• Is it a foundation? A slab-on-grade is both floor and foundation. See how slab foundations work.

Slab specifications by project

The same slab built for six different jobs is six different specifications. This table is the short version, with the detail behind each link.

One number that cuts across several of those rows: IRC Table R402.2 groups garage floor slabs with porches, carport slabs and steps exposed to weather, and asks for 2,500 psi where weathering potential is negligible, 3,000 psi where it is moderate and 3,500 psi where it is severe, with total air content between 5 and 7 percent in those last two cases. Interior slabs on grade are a 2,500 psi item. So the mix matters as much as the thickness, and concrete psi covers why air entrainment does more for freeze-thaw durability than strength does.

Slab, pad, or foundation

The words overlap, so it helps to keep them straight. A slab is any flat poured concrete surface. A pad usually means a small freestanding slab, like the base for a shed, hot tub, or air conditioner. A slab foundation is a structural slab that carries a building, built with thickened edges and reinforcement to code. They are all concrete slabs, just sized and specced for the job.

There is one real code consequence to the distinction. Frost protection is not required for "free-standing accessory structures with an area of 600 square feet or less, of light-frame construction, with an eave height of 10 feet or less." That exemption is what lets a shed pad or a detached garage slab float above the frost line in a cold climate, while the house next to it needs footings below it. A 24 by 24 garage at 576 square feet slips under the limit. A 24 by 26 at 624 does not.

What installation involves, and what it costs

A slab pour is a one-day event with several days of preparation in front of it. In order: mark out and check the layout, excavate and strip the topsoil, compact the subgrade, set forms to the finished height and slope, place and compact the base course, lay the vapor retarder if the job calls for one, set reinforcement on chairs, then pour, screed, float, edge, finish and cut the joints before covering for cure. How to pour a concrete slab walks through each step, and concrete cure time covers how long before you can walk on, drive on or build on it.

On cost, be aware that there is no industry body or government figure for any of this. Every number in circulation comes from lead-generation aggregators or contractors, and they disagree with each other. Angi puts a slab at $4 to $8 per square foot generally, then $6 to $12 for a garage slab on a different page. HomeGuide puts a slab at $6 to $12 per square foot installed, with materials alone at $3 to $7. Labor is typically half to two thirds of the total.

Which means the useful thing is not a national average but your own numbers. Run the job through the concrete slab cost calculator for a dollar figure, and the concrete cost calculator to compare bags against a ready-mix delivery. Somewhere between one and two cubic yards, bags stop making sense.

Getting the numbers right

Almost every slab decision comes back to volume, because that sets both your concrete order and a big part of the cost. Measure the length, width, and thickness, convert to the same units, and you can price the job, order the right amount, and check a contractor's quote. The concrete slab calculator does that math for you, returning the cubic yards, bag counts, and a waste allowance from your dimensions, and the concrete slab cost calculator turns it into a dollar estimate. Pouring a flat interior floor rather than a foundation slab? Our concrete floor guide covers finishes, cost, and how thick to pour one.

When a slab goes wrong

Slabs fail in three broad ways, and the fix is different for each.

• Cracking. Hairline shrinkage cracks are normal and are what control joints are for. Cracks wider than about a quarter inch, cracks that step vertically, or cracks that keep opening are telling you something moved. Concrete floor repair covers the cosmetic end.
• Settlement. The slab is intact but has dropped, because the soil under it consolidated or washed away, leaving a void. This is the one case where the slab can often be saved rather than replaced: concrete slab lifting covers mudjacking and polyurethane foam, what each costs, and why ACI is clear that lifting "will level the slab, but it won't correct any underlying poor soil conditions."
• Surface wear. Spalling, flaking and scaling are usually a finishing or freeze-thaw problem rather than a structural one, and lifting will not touch them. Resurfacing is the route there.

One diagnosis worth doing before you call anyone: has the slab settled, or has it heaved upward? A slab that rises in winter and drops back in spring is moving seasonally, and lifting it locks in the wrong elevation permanently. If the floor is flat but at the wrong height, you need lifting. If the height is fine but the surface is dished under a finish floor, you need a topping instead: see self-leveling concrete and underlayment.