How Thick Should a Concrete Driveway Be?

Thickness is the first technical question most people ask about a concrete driveway, and it is also the easiest place for a quote to be quietly trimmed. Fifteen millimetres off the depth across a whole driveway is a real saving for the contractor and invisible to the customer until the cracks appear.

Out through Melton, Cobblebank, Aintree and the surrounding estates it matters more than it does in most of Melbourne, because of what the ground underneath is doing.

Why Melton ground changes the answer

Much of the western corridor sits on reactive clay. Reactive clay swells when it takes up water and shrinks when it dries out, and it does that through every season. The ground under a driveway is not a stable platform; it moves.

That movement is what cracks driveways, not the weight of the car. A slab has to be able to span across ground that is no longer uniformly supporting it, and thickness and reinforcement are what give it the capacity to do that. We have written separately about why Melton driveways crack, and reactive clay is the common thread through nearly all of it.

Thickness for a standard domestic driveway

For ordinary domestic use, meaning cars and the occasional small van, 100mm is the common residential standard and it is what most quotes will be based on.

That figure assumes a properly prepared base underneath. A 100mm slab poured onto poorly compacted fill, or straight onto soft clay with no base course, will fail regardless of the number. Thickness is only half the specification; what is under it is the other half.

On reactive clay, going to 125mm is a genuinely worthwhile upgrade for a modest cost increase, because it gives the slab more capacity to bridge across areas where the ground has moved away underneath it.

When you need more

Heavier vehicles change the calculation substantially, and this is where a lot of driveways in the newer estates are underspecified for what the owner actually does.

Caravans, boats on trailers and larger dual-cab utes concentrate load. A caravan sitting in the same position for months at a time applies sustained point loading through its jockey wheel and legs, which is a different problem to a car driving over. Where a caravan or boat will be parked, a thicker slab in that area with additional reinforcement is the sensible approach, and we cover the council side of that in our piece on caravan side access.

Trucks, commercial vehicles or regular heavy delivery access push the requirement higher again, and at that point it is properly an engineered slab rather than a standard domestic one.

Reinforcement, and where it has to sit

Reinforcing mesh is what holds a slab together once it does crack, and concrete on reactive clay will eventually crack somewhere. The mesh does not stop cracking; it stops the crack becoming a displacement, where one side lifts relative to the other.

The part that matters and gets done badly is position. Mesh has to sit within the slab, held up on bar chairs at the correct height. Mesh laid flat on the ground and then poured over does essentially nothing, because it is sitting in the wrong part of the section to resist the forces that open a crack.

If you are on site during a pour, this is the single easiest thing to check. The mesh should be visibly held up off the base on chairs before any concrete goes in. If it is lying on the ground, it is not going to do its job.

Joints

Control joints are the other half of crack management. Concrete shrinks as it cures, and it will crack to relieve that. A control joint is a deliberate weak line that decides where the crack happens, so it goes somewhere straight and tidy rather than wandering across the middle of the driveway.

Joints need to be cut at the right depth and at sensible spacing relative to the slab width. Cut too shallow, too late, or too far apart, and the slab cracks where it wants to instead.

The base underneath

None of the above works over a bad base. The subgrade needs to be properly prepared, with soft spots removed rather than filled over, then a compacted crushed rock base course to a suitable depth, compacted in layers rather than dumped and rolled once.

On new estate blocks this deserves particular attention, because the ground around a new house often contains builders fill that was never compacted for a slab. Our new estate handover checklist covers what to look at before committing.

What to ask a contractor

Ask for the slab thickness, the reinforcement type, the base course depth, and the joint spacing, in writing. Any contractor doing this properly will answer without hesitation. A quote that only gives a square metre rate with no specification is not comparable to one that does, and the difference between them is usually exactly the things listed above.

Also worth asking what happens in the area where you intend to park a caravan or trailer, if that applies, because it is commonly missed.

The short version

100mm is the standard domestic figure, 125mm is a sensible upgrade on Melton's reactive clay, and anything carrying caravans, boats or commercial vehicles needs more than either along with extra reinforcement. Get the mesh up on chairs, get the joints cut properly, and get the base right, because thickness alone will not save a slab poured on unprepared ground.