How thick should a concrete driveway be, and what mesh does it need?

How thick a driveway needs to be is not a number you guess, and it is not the same on every block. It comes off the soil. A slab that is thick enough for one site cracks on another, because the ground underneath moves differently. Here is how the thickness, the mesh and the reo are actually decided.

The short answer

A residential driveway for cars is normally 100 mm thick on a compacted base with SL72 mesh. We step that up to 125 to 150 mm with SL82 or added bars where the soil is reactive, where heavier vehicles use it, or where the slab is structural. That is the starting point, not the finish: the exact spec comes off the site, not a flat rate.

What the soil classification sets

House and shed slabs, and driveways on reactive ground, are set out to a site classification under AS 2870. The soil is tested and graded Class A through E by how much it swells and shrinks with moisture. That class decides three things at once: the slab thickness, the footing depth, and the reo in the slab. Skip the classification and you are guessing at all three, which is exactly how a slab ends up too thin for the ground it sits on.

• Stable ground. Rock or sand moves little, so a standard 100 mm slab with SL72 mesh does the job for a car driveway.
• Reactive clay. Clay swells and shrinks with the seasons, so the slab goes thicker, the reo steps up, and the edges go deeper to ride the movement.
• Heavier loads. A caravan, boat, truck or a shed slab calls for more thickness and heavier mesh or bars regardless of the soil.

The thickness and the mesh are not where you save money. They are the line a cheap quote leaves vague, because "a standard slab" with no thickness and no mesh named is the one that cracks across the middle when the ground moves under it.

Why the reo has to sit in the slab

Mesh only does its job if it sits inside the concrete, not on the ground. We set the reo on chairs so the steel ends up in the slab where it carries the load. A mesh thrown on the base and poured over is mesh you paid for that does nothing. That detail, like the base and the control joints, is the part you cannot see once the slab is down, which is exactly why it is the first thing a cheap pour shrinks.

How we spec it at Slabline

We classify the site on the measure and size the slab to it, then write the thickness, the mesh and the MPa on the quote in plain numbers. So you can compare us to the next quote on the part that actually decides whether the slab holds, not on a round number for "a slab".