My parents had a concrete driveway laid by a local contractor about fifteen years ago. It was quoted as 100mm thick. For the first five years it looked perfect. Then the cracks started appearing - one ran from the edge almost to the centre, another appeared near the dropped kerb where the car wheels always land.
When we lifted a cracked section to investigate, we found the concrete was more like 70-75mm in places. The sub-base beneath it was barely 50mm of what looked like mixed hardcore - nothing like the 100-150mm of compacted MOT Type 1 that should have been there.
The contractor had cut corners on depth and sub-base, knowing that nobody would check until it was too late. That is not a story about shoddy builders - it is a story about knowing what you are supposed to get.
The full build-up: not just about the concrete
Thickness decisions cannot be made in isolation from the sub-base decision. A 150mm concrete slab on a poor sub-base will crack. A 100mm slab on a properly prepared 150mm MOT Type 1 sub-base will last for decades.
From bottom to top, a proper concrete driveway:
- Subgrade (natural ground, well compacted)
- Geotextile membrane (recommended on clay)
- Compacted MOT Type 1 sub-base: 100-150mm
- DPM (damp-proof membrane)
- Concrete slab: 100-150mm
Total excavation depth: typically 230-310mm below finished surface level.
A lot of DIY concrete driveways fail not because the concrete was under-specified, but because the sub-base was skimped or skipped entirely.
Recommended slab thickness by use
| Use case | Minimum | Recommended |
|---|---|---|
| Standard cars (up to 2t) | 100mm | 125-150mm |
| SUVs and large estate cars | 100mm | 150mm |
| Occasional van / light commercial | 125mm | 150mm |
| Regular van or commercial use | 150mm | 175-200mm |
| Heavy vehicles, HGV occasional | 200mm | Seek specialist advice |
What affects the thickness you need?
Ground conditions. On firm, stable soil (gravel, chalk, sandy ground), 100mm is more acceptable. On soft clay, made ground, or ground with tree roots or old drainage runs below it, go thicker and invest in the sub-base depth.
Vehicle weight. A standard family hatchback weighs 1.2-1.5 tonnes. A large SUV or people carrier can be 2.5 tonnes. A fully loaded transit van can be 3.5 tonnes. Every step up in vehicle weight is an argument for more slab thickness.
Slab size. Larger slabs with fewer expansion joints are more susceptible to differential movement underneath them. For large areas (over 15-20m2 in one continuous pour), use 150mm or include regular expansion joints.
The dropped kerb edge. The point where the car drives up from the road onto the driveway bears a significant concentrated load. Consider extra thickness (or a thickened edge beam) at this point.
Reinforcement mesh: when do you need it?
Reinforcement mesh does not prevent cracking. What it does is hold cracks together when they occur, preventing them from widening and keeping the slab structurally intact.
Consider mesh if:
- The slab is larger than about 15-20m2
- Ground conditions are suspect
- Heavy vehicles will use the driveway
- You want extra insurance
A142 mesh (6mm wire at 200mm centres) is the standard domestic spec. It should be supported at mid-depth using plastic spacer chairs - laying it flat on the ground before pouring means it ends up at the bottom of the slab where it does almost nothing.
Concrete specification
| Grade | Common name | Use |
|---|---|---|
| C20 / GEN3 | ST2 | Paths, light-duty footings |
| C25 / GEN4 | ST4 | Residential driveways (minimum) |
| C30 | RC30 | Better abrasion and freeze-thaw resistance |
C25 is the minimum for a concrete driveway. C30 is worth the small extra cost per m3 for better durability on an exposed driveway surface, particularly in areas where de-icing salt is used or where the driveway faces north and gets limited sun.
Ask your ready-mix supplier for C25 or C30, slump class S2 or S3 for a standard driveway pour.
Volume needed by driveway size
At 150mm depth with 10% wastage, rounded to nearest 0.25 m3:
| Driveway area | Volume to order |
|---|---|
| 15m2 | 2.5 m3 |
| 20m2 | 3.5 m3 |
| 25m2 | 4.25 m3 |
| 30m2 | 5.0 m3 |
| 40m2 | 6.75 m3 |
| 50m2 | 8.5 m3 |
At 100mm depth:
| Driveway area | Volume to order |
|---|---|
| 15m2 | 1.75 m3 |
| 20m2 | 2.25 m3 |
| 30m2 | 3.5 m3 |
| 50m2 | 5.5 m3 |
My tips for a concrete driveway
Fifteen years of occasional concrete work and a healthy mistrust of shortcuts:
Get the sub-base right before worrying about the concrete spec. The concrete spec matters, but the sub-base matters more. A week spent on excavation, membrane and 150mm of properly compacted MOT Type 1 is better value than spending the extra money on going from C25 to C30 on a poorly prepared base.
Plan your expansion joints. Concrete will crack - the question is whether it cracks where you want it to or randomly across the surface. Saw or form expansion joints at 3-4m intervals and around any fixed obstacles (drain covers, piers, gate posts). Joints should be a minimum of 25-30% of the slab thickness deep.
Include a drainage fall. A completely flat concrete driveway will pool water. Aim for a fall of at least 1:60 (roughly 16mm per metre) away from the house. Your eye will barely notice this gradient but puddles will not form.
Do not use calcium chloride accelerators in winter. If you are pouring in cold weather, it is tempting to add an accelerator to speed up setting. Some accelerators are chloride-based and can cause long-term corrosion of any reinforcement mesh. Use a non-chloride accelerator if you need one, or better yet, avoid pouring in cold weather if possible.
Use the Concrete Base Calculator to confirm your volume before calling the ready-mix supplier.