Thickness by job
| Job | Typical thickness | Usual grade and steel |
|---|---|---|
| Garden path or patio, foot traffic only | 75mm minimum | N20, SL62 or SL72 |
| Driveway for cars and small utes (under 3 tonnes) | 100mm minimum | N25, SL72 |
| Garage floor or carport | 100mm | N25, SL72 or SL82 |
| Driveway for vehicles of 3 to 10 tonnes | 150mm minimum | N25 to N32, heavier mesh |
| Council footpath crossing | Set by council | Per council specification |
| Garden shed base | 75mm to 100mm | N20 or N25, SL72 |
| Shed or workshop you drive into | 100mm to 125mm | N25, SL72 or SL82 |
| House slab and footings | Engineer's design under AS 2870 | As specified |
Council crossings often call for more than the driveway behind them. Campbelltown, for example, specifies at least 125mm.
Why thicker isn't always better
Extra thickness helps a slab carry heavier wheel loads. It does little about what actually ruins most Sydney driveways: soft spots in the ground, reactive clay, water getting underneath and missing joints. A 150mm slab poured on topsoil will still crack and sink, while a 100mm slab on compacted road base, with mesh at the right height and joints in the right places, will outlast it.
Consistency matters more. CCAA points out that the minimum thickness has to hold across the whole slab, so the base needs to be trimmed accurately. A driveway specified at 100mm that's only 70mm over a high spot will crack there first.
The base under the slab
Under the concrete there's usually a layer of crushed rock road base, compacted with a plate compactor or roller after the topsoil has been stripped. On reactive clay, CCAA recommends a non-reactive sub-base, typically 100mm. The base adds to the digging: a new driveway that has to finish at the same level as the old one often means removing 150mm to 200mm of ground.
Working out the volume
Concrete is ordered by the cubic metre. Multiply the area by the thickness in metres, then add a margin for uneven ground and spillage:
- A 40m² driveway at 100mm: 40 × 0.1 = 4.0m³
- Plus 10% for wastage: about 4.4m³
- The same driveway at 150mm: 40 × 0.15 = 6.0m³, or 6.6m³ with wastage
Our calculator adds 10% wastage to every estimate. Ordering short is far worse than ordering slightly over: a second truck means a wait, a cold joint where fresh concrete meets concrete that has started to set, and another delivery.
What extra thickness costs
| 40m² broom driveway, level site | Total incl. GST |
|---|---|
| 100mm | $8,100–$8,800 |
| 125mm | $8,550–$9,250 |
| 150mm | $8,950–$9,650 |
Going from 100mm to 150mm adds about $850 on a 40m² driveway. If heavy vehicles will use it, that's money well spent. If they won't, it's better spent on a deeper base or a better finish.
What would your job cost?
Enter your size, finish and site conditions for an instant estimate, built from the same rates as the figures in this guide.
Common questions
Is 75mm thick enough for a driveway?
No. 75mm is the minimum for foot traffic. Anything cars drive on should be at least 100mm, and council crossings usually need more.
How thick should a slab for a caravan be?
A caravan or motorhome over 3 tonnes needs at least 150mm on a well-compacted base. For the heaviest vans, have an engineer design the slab and reinforcement.
Related services
Sources
Prices in this guide come from our concrete price calculator and update when its rates are reviewed. They are estimates, not fixed quotes: the final price is confirmed at a free site inspection. Council rules were checked against each council's published policy and can change, so confirm with your council before you build.
