How thick should a residential driveway be?

The residential standard is a 4 inch slab, meaning 4 inches of concrete measured after the pour, over a compacted stone base. That thickness carries passenger cars, SUVs and pickups for decades when the base is right and the joints are cut. Most driveways in Cookeville and the Upper Cumberland are poured at 4 inches, and that is what a standard estimate assumes.

Four inches is a minimum, and it is easy to lose. If the base is uneven and the forms are set to a nominal 4 inches, the slab ends up 3 inches in the high spots. Morgan's grades the base to the finished thickness so the slab is the number in the estimate across the whole drive.

Concrete thickness is chosen for the heaviest thing that will sit on the slab, and a driveway sees more than the family car. A loaded utility trailer, a camper, a boat on a trailer, a delivery truck, a propane truck and a dump truck bringing gravel to the back lot all press harder on the slab than a sedan. If any of those park on the drive or cross it regularly, plan on 5 to 6 inches with heavier reinforcement, and say so when you request the estimate so the number reflects it.

Thicker concrete costs more, in concrete and in steel. That extra is small next to replacing a 4 inch slab that cracked under a loaded trailer in its second year. Joe asks what parks on the drive during the site walk for exactly this reason, and the thickness he recommends goes in the written estimate.

• Cars, SUVs and light pickups: 4 inches with reinforcement
• Trailers, campers, boats and delivery trucks: 5 to 6 inches with heavier reinforcement
• Regular dump truck or equipment traffic: thickness and reinforcement sized to the load and stated in the written estimate

Reinforcement does not stop concrete from cracking. It holds the two sides of a crack tight so the slab stays flat and the crack stays hairline. Options are welded wire mesh, rebar on a grid, or fiber mixed into the concrete, and each has a place. Wire and fiber suit a standard 4 inch drive. Rebar on a grid suits thicker slabs that carry real loads.

The reinforcement has to be in the middle of the slab to work. Mesh that gets walked to the bottom of the pour is doing nothing. The crew sets it on chairs or pulls it up as they place the concrete so it sits where the estimate says it sits.

Concrete is strong in compression and weak in bending. A slab on a solid, evenly compacted base is only ever compressed, because the ground carries the load with it. A slab over a soft spot or a void bends across that spot, and concrete cracks when it bends. Adding 2 inches of thickness makes the slab stiffer, and a stiffer slab over a void still bends and still cracks. It just takes a heavier load to do it.

A driveway base is compacted crushed stone, generally 4 inches or more, over subgrade that has been cut to grade and compacted itself. Soft fill, topsoil and old organic material come out first. Drainage is part of the base: water that is allowed to sit under a slab softens the subgrade and washes fines out of the stone, and that is how a good base turns into a bad one. Morgan's does its own excavation and base work, so the crew that pours the slab is the crew that built what it sits on.

A written estimate should name the slab thickness, the reinforcement type, the base depth and material, and the compaction method. If any of those are missing, ask. Two estimates that differ by a large amount are often for two different slabs: one is 4 inches over whatever is there, the other is 4 inches over 4 inches of compacted stone with reinforcement on chairs. The second one costs more and lasts longer, and the difference is in the parts you cannot see after pour day.