One hundred millimetres is the normal minimum for a domestic driveway, rising to one hundred and twenty-five or one hundred and fifty where heavier vehicles are involved. But thickness on its own is close to meaningless. A thick slab on badly prepared ground fails the same way a thin one does, and in Hobart — with reactive clay, old fill and dolerite rock all common, often on a slope — the ground is usually the harder problem.
Below is what the figures should be, why the base matters at least as much, and the two places where extra depth genuinely earns its keep.
What thickness is actually doing
Thickness governs how well a slab spreads load and how much it resists bending. Too thin for the traffic and it flexes, and concrete does not tolerate being flexed — it cracks. The load has to travel down through the slab into the base and then into the ground, and every one of those layers has to be up to the job. Which is why the question is never just how thick, but how thick over what.
The figures worth working to
For a typical Hobart home, these are the starting points before the ground and the traffic are assessed.
| Use | Thickness | Reinforcement |
|---|---|---|
| Cars only, normal ground | 100mm | Standard mesh, chaired into the slab |
| Campervans, trailers, occasional truck | 125 to 150mm | Heavier mesh or bar; deeper base |
| Shared or commercial access | 150mm and above | Engineered design, heavier steel, planned joints |
These are working figures for normal ground. Soft, filled or reactive ground pushes each of them upward. If you regularly park a boat trailer or caravan, or a delivery vehicle comes up your drive, tell whoever is quoting. It changes the specification, and it is far cheaper to build for it than to repair a slab that was not.
Why the base matters at least as much
This is the part that gets skipped when a price looks too good. Concrete sitting on topsoil, vegetation or fill that was tipped in and never compacted will lose support as that material consolidates, and the slab cracks under its own weight regardless of how deep it is. Proper preparation means digging out unsuitable material to a depth that suits what is actually there, then laying and compacting crushed rock in layers rather than in one deep lift. On Hobart's reactive clays it often means going deeper than people expect, and with our wet winters it always means giving water somewhere to go.
Where the steel has to sit
Reinforcement holds the slab together across any crack that forms, so the two sides cannot move independently. It only does that if it is in the right place: inside the slab, propped on bar chairs at the height the design calls for, with enough concrete cover to protect it from moisture. Mesh lying on the crushed rock is buried steel doing essentially nothing.
The two places that need extra
Edges and the entry from the street. A slab's edge has ground support on one side only and is exactly where wheels run when turning in and out, so thickened edges, often with extra steel, stop it breaking away. The entry takes every vehicle, often turning under load and on a grade on a hillside block. Your council may also have its own requirements for the crossover between the kerb and your boundary.
What to ask whoever is quoting
Ask what thickness they are proposing and why, what is under the slab and how deep they will dig, how the base will be compacted, where the reinforcement will sit and whether the edges are thickened. A contractor who quotes the same figure regardless of the ground has not assessed it. If soil, drainage and what vehicles will use the drive do not come up before the price, the price is a guess.