Concrete Flatwork Guide: Driveways, Patios, and Slabs

Last updated: August 20, 2026

Flatwork is roughly 80 percent of the residential concrete a homeowner ever buys, and almost every decision that sets its lifespan happens before the finishers pick up a trowel. A driveway gets judged on the surface, which is the last two hours of a three-day job. The dirt under it and the week after it are the rest, and neither one photographs well enough to sell anybody anything.

What counts as concrete flatwork?

Flatwork is any horizontal concrete pour finished level or at a controlled slope. That covers driveways, patios, sidewalks and walkways, slabs and building pads, and garage and shop aprons. The word exists to separate flat horizontal pours from vertical work like walls and footings, which get formed, reinforced, and cured on a different set of rules.

The distinction isn’t academic. A wall stays plumb and happy on a footing that shifts a quarter inch. A driveway that shifts a quarter inch has a trip edge, a crack, and a phone call.

What goes under the slab before any concrete arrives?

Three layers, in this order: a compacted subgrade, 4 to 6 inches of crushed granular base, and on interior slabs a 10 to 15 mil vapor barrier. Leave one out and the concrete becomes the only thing holding the shape of the ground.

Subgrade compaction is the single biggest predictor of whether a slab settles. Soil gets placed and rolled in lifts to roughly 95 percent standard Proctor density. Not dumped, leveled with a bucket, and driven over twice. Loose fill compresses under load, the slab bridges the soft spot for a while, and then it doesn’t.

This is the step that gets skipped, and it gets skipped for a reason nobody says out loud: it’s a day of work that leaves nothing to look at. The homeowner sees the same patch of dirt at quitting time that they saw at breakfast. Every dollar of it shows up years later, or it doesn’t, which is a hard thing to sell and an easy thing to shortcut.

The granular base is 4 to 6 inches of crushed stone, compacted in lifts. It gives the pour a stable platform and gives water a path out instead of a place to sit against the underside of the slab.

The vapor barrier is 10 to 15 mil polyethylene, and it belongs under any slab in a conditioned space or under a floor covering. Ground moisture moves up through concrete constantly, and when it reaches a coating, an adhesive, or a rug pad, you get coating failures, efflorescence, and the musty smell people blame on their HVAC. Exterior flatwork generally skips it, because an open driveway evaporates freely from the top.

Where the barrier does go in, it goes directly against the concrete. Poly buried under a layer of gravel is easier to walk on during the pour and gives up most of what it was there to do, because the gravel above it stays wet against the bottom of the slab.

How thick should concrete flatwork be?

Thickness follows the load, and mix strength follows thickness. Foot traffic is fine at 4 inches. Vehicles above roughly 8,000 pounds need 5 to 6 inches, and that’s not a padding number.

ACI 332, the residential code requirements for structural concrete, treats 4 inches as the floor for residential slabs on grade. Above it, slab strength scales with the square of thickness, so moving from 4 inches to 6 isn’t a 50 percent improvement. It’s closer to double. That’s why “we’ll pour it a little thicker where the RV parks” is the wrong answer to the RV question.

The mix matters as much as the depth. ASTM C94 governs ready-mix delivered to a spec, C150 governs the cement, and C143 governs slump, which is the workability of the load coming down the chute. Water added at the truck to make placement easier buys the crew an easier afternoon and costs you PSI, density, and surface durability for the life of the slab.

What does reinforcement actually do?

The honest truth about reinforcement is that none of it prevents cracks. Concrete shrinks as it cures, and it will crack somewhere. What steel decides is what happens next: whether the crack stays tight and the slab keeps behaving as one piece, or whether the two sides start moving independently until one of them drops.

For most residential flatwork, #4 rebar on a 16 to 24 inch grid, supported on chairs so the steel lands in the middle third of the slab, is the durable choice. Rebar lying on the base is rust with extra steps.

Wire mesh is cheaper and it’s fine for patios and light driveways, on one condition: somebody has to lift it into the concrete as the pour goes. Mesh left flat on the ground sits below everything it was supposed to hold together. Fiber goes into the mix rather than onto the base, helps the surface through its first hours, and does nothing for load transfer across a crack. Different tools, different jobs.

Where do control joints go, and when do they get cut?

Joint spacing runs 2 to 3 times the slab thickness in feet. A 4-inch slab gets joints every 8 to 12 feet, a 6-inch slab every 12 to 18. Saw depth is one quarter of slab depth, and panels stay close to square, with the long side under 1.5 times the short side. Long skinny panels crack across the middle no matter how good the sawing is.

Timing is the part crews miss. Sawcuts go in within 6 to 12 hours of finishing, per ACI 302.1, the guide for concrete floor and slab construction. Cut too early and the edges ravel. Cut too late and shrinkage has already picked its own line through the slab, and your joints are just decoration in a floor that already cracked where it wanted to.

Which finish belongs outside, and which belongs indoors?

Broom finish is the standard for exterior driveways, patios, and sidewalks, because the texture gives you traction when the surface is wet. A smooth steel-trowel finish belongs indoors, where it serves as the base for polishing or a coating. Put that same finish outdoors and you’ve built a skating rink in the rain.

Two decorative options change the look without changing the structure. Exposed aggregate washes the top paste off to reveal the stone in the mix, so the color you end up with is the color of the rock that came on the truck. Stamped concrete presses a pattern into the wet slab before it sets, which means the pattern gets one shot, on one afternoon, in whatever weather shows up.

For an interior slab headed for a polished floor, flatness is governed by the ACI 117 FF and FL tolerances, and those get specified before the truck arrives, not discovered afterward. That’s the whole point of the Pour-4-Polish process on slabs and building pads: pour the slab and refine it to a polished surface in one workflow, with the in-house Slab Boss polish program handling the grinding and densifying.

How do you cure concrete correctly?

Curing is where good concrete is won or lost, and it costs almost nothing. Concrete hits roughly 70 percent of design strength in 7 days and approaches full strength near 28 days, but only if it stays wet that first week. A slab left to dry stops gaining strength the moment the surface goes dry, often within hours on a 95 degree afternoon.

There are two ways to do it right. Run a 7-day moist cure with wet burlap, soaker hoses, or plastic sheeting, or spray a cure-and-seal membrane immediately after finishing to hold the water in. Either one can raise the final surface strength and abrasion resistance by 30 to 50 percent compared to a slab that air-dried in the sun.

Weather changes the plan on both ends. Hot-weather pours follow ACI 305, cold-weather pours follow ACI 306, and both exist because concrete does not care about your schedule.

Is that crack a problem?

Hairline shrinkage cracks under 1/8 inch wide are cosmetic and normal, especially where one runs at or straight into a control joint. That’s the joint doing exactly what it was cut to do.

The cracks worth an honest look are wider than 1/4 inch, offset vertically so one side sits higher than the other, or curving across the slab and ignoring the joints entirely. Those don’t start at the surface. They trace back to the base or to water: poor compaction, no drainage, or expansive clay swelling and lifting the slab from underneath. Grinding and filling that crack without fixing the ground buys you a season, maybe two.

The vertical offset is the tell worth learning, because it’s the one you can check with your foot. Shrinkage pulls a slab apart sideways and leaves both halves at the same height. When one side has dropped or risen, something underneath moved, and the crack is a symptom rather than the problem.

What drives the price of flatwork?

Five things move the number: square footage, thickness, reinforcement, finish, and site access. The one that surprises people is square footage, because small pours cost more per foot, not less. A 100 square foot stoop carries a higher per-foot price than a 1,000 square foot driveway, since the truck, the crew, and the mobilization cost the same either way and get spread over a tenth of the concrete.

Site access is the quiet one. A pour the truck can reach and chute directly is a different job from a pour that has to be wheeled through a gate in buggies, and the difference shows up in labor hours rather than in materials. It’s worth mentioning the gate, the slope, and the overhead wires when you ask for a number, because those are the details that move a quote after the fact if nobody raised them first.

The full breakdown, with ranges by service, is in the concrete cost guide. Pricing on individual pours lives with each scope: driveway work, patio pours, and walkway and sidewalk scope.

Anybody can trowel a slab flat on pour day. Keeping it flat for thirty years is the part you’re actually buying.