Concrete
How thick should a concrete driveway be?
Four inches over a properly compacted base is the usual thickness for a driveway carrying cars, and the base under it decides more than the inches do.
The numbers
- Cars and pickups: commonly four inches, over a base that has been compacted properly.
- Anything heavy that parks, turns or lands on it takes a thicker slab and usually reinforcement.
- The compacted base matters as much as the thickness, and it is what fails first when it is skipped.
- Concrete keeps gaining strength for weeks after the pour. Traffic waits for the curing schedule.
- A slab carrying an RV, a trailer or delivery trucks is a design question rather than a rule of thumb.
Why four inches is the usual number
Four inches over a properly compacted base carries a car and a pickup with room to spare. Adding thickness past that on a residential drive buys very little, because the failure on a normal driveway is almost never the slab being too thin.
What it is instead is the ground under it. A four inch slab on well compacted stone outlasts a six inch slab poured on soft fill, every time.
Where the extra inches earn their money
Anywhere a heavy load lands or sits still. An RV parked in the same spot all summer, a trailer that gets dropped on its landing gear, a delivery truck that swings into the same corner, a dumpster pad taking the weight of a container and the truck that lifts it.
Those spots take a thicker slab and usually reinforcement tied into it, and it is much cheaper to pour them that way once than to replace them. How much thicker is a question about the load and the ground it sits on, so it is worth having somebody look at where the weight actually lands before the forms go in. On a commercial pad it is an engineer's number.
The waiting is part of the job
Concrete gains strength for weeks after it is poured. The slab can take foot traffic long before it can take a vehicle, and putting a car on it early is a known way to crack an otherwise good pour.
The cure schedule comes with the pour, and in northwestern Wisconsin the time of year moves it. A late fall pour needs protection from a hard freeze while it is still green.