The bulk of the residential driveways we pour are 100mm deep, stepping up to 125 or 150mm when something heavier will be using them. Thickness is only half the story, though. A deep slab on poorly prepared ground cracks nearly as readily as a thin one, and around Wollongong, with reactive clay, coastal sand, old fill, shale and sandstone all in the mix, the ground underneath is often the bigger worry.
Below we run through the depths we normally use, why the base gets so much of our attention, and the two parts of a driveway where some extra concrete is money well spent.
What slab depth actually does
More depth lets the slab share a vehicle's weight across a bigger area and keeps it stiffer under load. Make it too thin for the job and it bends, and concrete can't bend without cracking. The weight then passes into the crushed rock and on into the soil, and each of those layers needs to cope. So the real question is never only how thick the concrete is, but what it's sitting on.
Common depths for Wollongong homes
Treat these figures as a baseline. We adjust them after we've seen your ground and heard what you'll be parking.
| Use | Thickness | Reinforcement |
|---|---|---|
| Everyday cars on firm ground | 100mm | Standard mesh lifted on bar chairs |
| Caravans, trailered boats, the odd heavy truck | 125 to 150mm | Heavier mesh or bar over a deeper base |
| Shared or commercial driveways | 150mm or more | Engineer-designed, heavier steel, set-out joints |
All of that presumes good, stable ground. On soft, filled or poorly draining land, each line tends to move up a notch. If a boat, van or caravan will be a regular visitor, mention it when you ask us to quote. Designing for it up front costs far less than fixing a slab that was never meant to take it, and our concrete driveway quotes always allow for it.
What goes underneath the concrete
A suspiciously cheap quote usually saves its money here. Topsoil, roots and dumped fill that was never compacted keep settling for years, and the slab above eventually cracks under its own weight regardless of depth. We excavate the poor material down to whatever your site calls for, then place crushed rock and compact it in a series of thin layers instead of one deep one. On clay or soft ground that can mean digging further than you'd guess, and since an East Coast Low can unload a lot of water quickly, often straight down a sloping block, we always work out where the runoff will go. Our excavation crew gets all of this done before the concrete is ordered.
Setting the steel at the right height
Steel keeps the two sides of any crack locked together so neither half can drop or drift. It can only do that from the right spot: lifted on plastic chairs to the specified height, with concrete above and below to keep the damp out. Mesh flat on the gravel is money spent for very little benefit.
Driveway edges and the crossover
Edges are supported on one side only and take the full force of tyres as cars turn on and off, so we deepen them, usually with an extra bar, to keep them from breaking away. The crossover carries every car in and out, mostly mid-turn, and it can get knocked about where kerb, gutter and footpath come together. Councils may have their own standard for the section between road and boundary, so it's worth a check with yours before we start.
What to ask any concreter
Find out what depth they're proposing and their reasoning, what ground they're expecting and how far down they'll dig, how the base gets compacted, what height the mesh will sit at, and whether the edges are being thickened. One depth for every driveway means they haven't assessed your block. If soil, drainage and vehicles never come up before the price does, treat that price as a guess.