A typical home driveway is poured at 100mm, stepping up to 125 or 150mm when heavier vehicles will use it. That number tells only half the story, though. A deep slab over poorly prepared ground breaks just as readily as a thin one, and around Cairns, where you can strike reactive clay, soft delta soils, leftover fill or shallow rock on a hillside block, the ground beneath is usually the bigger challenge.
Here are the depths worth working to, why the base deserves as much attention, and the two zones where a little extra concrete pays off.
The job thickness does
Depth controls how widely a slab spreads the weight of a vehicle and how stiffly it resists bending. Make it too shallow for the vehicles using it and it bends, and concrete cannot flex without cracking. Every load passes from the slab into the crushed rock and then into the soil, and each layer has to cope. So the useful question is always how thick, and sitting on what.
Typical depths for a Cairns home
Treat these as a starting point, to be adjusted once someone has looked at the soil and the vehicles involved.
| Use | Thickness | Reinforcement |
|---|---|---|
| Everyday cars on firm soil | 100mm | Regular mesh lifted on bar chairs |
| Caravans, boat trailers, the odd heavy truck | 125 to 150mm | Stronger mesh or bar over a thicker base |
| Shared driveways or business access | 150mm or more | Engineer-designed, heavier steel, set joint layout |
Those figures assume firm, stable soil. Soft, filled or reactive ground nudges every row upwards. If a boat, caravan or delivery van regularly comes up the drive, mention it when you ask for a quote. Designing for that weight at the start costs far less than fixing a slab that was never meant to carry it. Our concrete driveway quotes always ask about it.
The groundwork under the slab
This is usually where a suspiciously cheap price saves its money. Concrete laid over topsoil, grass roots or fill that was dumped and left loose gives way as it settles, and the slab then cracks under its own load no matter how deep it is. Good preparation means removing unsuitable soil to whatever depth the site actually needs, then spreading and compacting crushed rock in a series of shallow lifts instead of a single deep one. On clay or soft ground that can mean digging deeper than owners expect, and with tropical storms dumping a lot of rain in a short time, the water always needs a planned way off and away. Our excavation crew handles that stage before any concrete is ordered.
Getting the mesh to the right height
Steel keeps the slab acting as one piece if a crack does open, so neither side can drop or shift on its own. It can only do that from the correct position: raised on plastic chairs to the specified depth inside the concrete, with a generous layer of concrete shielding it from damp. Mesh left flat on the gravel adds cost without adding much strength.
Edges and the street entry
A slab edge has soil support along one side only, and it takes the full weight of tyres as cars turn in and out, so thickening it, often with added bar, keeps it from cracking off. The entry from the road carries every vehicle that uses the drive, frequently while turning, and on a sloping block in the hills it may be climbing at the same time. Council may also have standards covering the stretch between the road and your front boundary, so check with Cairns Regional Council before work starts.
Questions to put to your contractor
Ask what depth they recommend and the reason for it, what they expect to find underground and how far down they will dig, how they will compact the base, how high the steel will be set and if the edges get extra depth. A single depth offered for every job means the site has not been assessed. If soil type, drainage and the vehicles using the drive never come up before the quote, treat that figure as a guess.