How thick should a concrete driveway be?

A passenger car puts roughly a thousand pounds on each tire. A full-size pickup, loaded, maybe twice that. A 4-inch slab of 3,000 psi concrete on a compacted base carries that load with room to spare, which is why 4 inches has been the residential standard across North Texas for decades. Going thicker for a driveway that only ever sees cars buys you very little.

What 4 inches does not do is forgive a bad base. Concrete is strong in compression and weak in bending. A slab on firm, even support is only ever in compression under a tire. A slab bridging a soft spot or a void is in bending, and that is where it cracks, regardless of whether it is 4 inches or 6. Most failed driveways we tear out in Arlington failed under the slab, not in it.

Our driveway pricing is built on a 4-inch broom-finish slab, and the calculator on this site lets you see what a thicker pour adds before you call.

Step up to 5 or 6 inches when the driveway will regularly carry something heavier than a pickup: a loaded work truck or dually, a boat or equipment trailer, an RV, a delivery box truck, or a roll-off dumpster during a remodel. A 6-inch slab has more than twice the bending strength of a 4-inch slab, and that margin is what keeps a corner from breaking off under a trailer jack or a dumpster rail.

It also makes sense to go thicker where the slab takes a concentrated load. An RV pad, the bay in front of a shop door, and a pad that will carry steel posts all qualify. If a carport or gate post is going on the slab, tell us before we form it. We pour the pad and the thickened footings, and CJM Metals builds the steel on it.

Thicker costs more, but not as much as people expect. On our price bands a 5-inch slab runs about 8 percent over a 4-inch slab and a 6-inch slab about 18 percent, because the extra is mostly mix and a little more excavation. The crew, forms and finishing are the same either way. See concrete driveways for what a full job includes.

Most of Arlington and the south side of the Metroplex sits on clay that swells when it is wet and shrinks when it is dry. A slab poured straight on top of it rides that movement like a boat. The fix is not more concrete. It is a properly prepared subgrade: strip the topsoil and organics, cut to grade, compact the native soil, then place and compact a crushed-stone or flex base so the slab sits on something that does not change volume with the weather.

Grade and drainage belong in the same conversation. The slab should fall away from the house and the garage, and water should have somewhere to go at the low end instead of soaking the edge of the base. Water that sits against a slab edge softens the subgrade right where the load is greatest, and an edge on soft ground is the first thing to crack and settle.

A thicker slab on a bad base is still a slab on a bad base. When you compare quotes, ask what the base is, how thick it is and whether it is compacted. That answer tells you more about how the driveway will hold up than the slab thickness does.

Steel does not stop concrete from cracking. It holds the cracks tight so the two sides stay level and keep carrying load together. For a 4-inch driveway we run #3 or #4 rebar on a grid, 18 inches on center both ways as a baseline, set on chairs so it sits in the middle of the slab. Steel lying on the dirt does nothing; it has to be in the concrete. On 5- and 6-inch slabs and anything carrying trucks we tighten the grid to 12 inches and use #4 bar throughout.

Welded wire mesh is the cheaper option. It works when it is actually lifted into the slab, but in practice it usually ends up walked to the bottom during the pour, and at that point the driveway has no steel at all. We pour rebar on every driveway, and on our price bands it adds roughly 60 cents to $1.10 per square foot. On a 600 square foot driveway that is a small part of the bill and the difference between a hairline that stays a hairline and a crack you can drop a key into.

For the mix, order 3,000 psi minimum for a residential driveway and 4,000 psi for anything carrying trucks or where the slab is exposed to a lot of water. Ask for air entrainment and keep the slump low. Extra water makes a pour easier to push around and makes the finished slab weaker and more prone to shrinkage cracks, which is why we do not let the truck add water on site.

The edge of a slab is its weak point: it has support on only one side and it is where tires drop off the driveway onto the lawn. We form a thickened edge, typically 8 to 12 inches deep and about a foot wide, along the street side and anywhere a heavy load will cross the edge. It acts as a small footing and keeps the corners from breaking off. It also runs along the garage approach and wherever the driveway meets a slab or pad.

Concrete shrinks as it cures and it will crack somewhere. Control joints decide where. The rule is joint spacing in feet of two to two and a half times the slab thickness in inches, so a 4-inch driveway gets joints every 8 to 10 feet, cut to a quarter of the slab depth, within a day of the pour. A 6-inch slab can go 12 to 15 feet. Skip the joints or cut them too shallow and the driveway picks its own crack lines, usually in the wrong place. More on that in why concrete cracks.

Finally, thickness does not shorten the cure. Keep cars off a 4-inch driveway for 7 days and a 6-inch slab for 10, and keep heavy trucks off for 14 to 28 days. Concrete reaches most of its design strength at 28 days, and a slab loaded early is a slab that cracks early. Our curing time guide has the full schedule. In Arlington, the approach between your property line and the street is in the public right-of-way, so confirm with the City of Arlington (One Start Development Center, 101 W. Abram St, 817-459-6502) whether a permit, inspection or a City specification applies; see permits in Arlington. Call (214) 603-9142 and we will look at what you park and tell you what thickness the driveway needs.