Ask ten contractors how thick a driveway should be and you will get "four inches" ten times. That is a reasonable default for passenger cars on decent subgrade. It is not a universal answer, and treating it as one is why some driveways crack under a boat trailer in their second summer.
When four inches is right
- Passenger cars and light SUVs only
- Firm, well-compacted subgrade
- Reinforcement present and properly positioned
- No regular delivery trucks, dumpsters or RVs
When to go to five or six inches
Weight concentrated into small contact areas is what breaks slabs. An RV's leveling jacks, a loaded work truck's rear axle, a dumpster dropped on two rails - all of those put far more stress into the concrete than a sedan ever will.
- RVs, boats, trailers or work trucks parked regularly
- Soft or expansive clay subgrade that cannot be improved cheaply
- Frequent heavy delivery access
- Wide turning areas where vehicles cut corners onto slab edges
Thickness is only one of three variables
A six-inch slab over uncompacted fill will fail faster than a four-inch slab over a properly prepared base. Thickness, reinforcement and subgrade are a system, and a contractor who only talks about one of them is only looking at one third of the problem.
Reinforcement matters specifically because of what it does after cracking: it holds the pieces in plane so you do not get vertical offsets and trip edges. Steel needs to be tied on chairs in the lower-middle third. Mesh thrown on the dirt and pulled up with a rake during the pour ends up wherever it ends up, which is usually the bottom.
Edges deserve extra attention
Most driveway cracking starts at the edges, because that is where vehicles run off and where the slab is least supported. Thickened edges cost very little at the time of the pour and eliminate the most common failure point.
To talk through the right section for your driveway and how it will be used, call 877-889-1187.