How Thick Should a Concrete Driveway Be?

A concrete driveway for ordinary family cars should be at least 100mm thick, increased to 150mm on clay or poor ground, laid on at least 100mm of compacted crushed stone sub-base. Those are the minimums in the LABC Warranty technical manual, and pattern imprinted concrete follows the same figure: trade guidance gives 100mm as the minimum for driveways and parking areas, 75mm for paths. Vans, motorhomes and anything heavier need more.

There's no legal thickness for a private driveway. These are construction standards, and they're what a decent contractor should be quoting against.

Concrete driveway thickness at a glance

• Paths and patios (foot traffic only): 75mm minimum concrete on a 75mm sub-base.

• Driveway, family cars, good ground: 100mm minimum concrete on a 100mm sub-base.

• Driveway on clay or poor ground: 150mm concrete. The sub-base may need to be deeper too.

• Medium duty (occasional small lorries such as refuse or fuel deliveries): 150mm sub-base, and the slab should be designed thicker for the load.

• Weak or soft ground: LABC says the sub-base should be designed by an engineer.

Add the two layers together and a car driveway on decent ground is dug out to at least 200mm below the finished level, more where a thicker slab, a blinding layer or a membrane is needed.

Why does a thin concrete slab crack?

Concrete is very strong when you squash it and weak when you bend it. A car wheel pushes down on the slab; if the ground underneath gives a little, the slab bends over that soft spot, and the underside goes into tension. A thin slab has very little depth to resist that bending, so it cracks.

Thickness works as a multiplier. Going from 100mm to 150mm doesn't make a slab one and a half times stiffer; it makes it more than three times stiffer, because bending stiffness rises roughly with the cube of the depth. That's why the 150mm figure for clay matters: clay shrinks in dry summers and swells in wet winters, and a stiffer slab bridges that movement instead of following it.

The usual causes of a cracked concrete drive are almost all hidden: a slab poured at 70-80mm to save concrete, a sub-base that wasn't compacted, no joints, or soft ground underneath. Our pattern imprinted concrete driveways are laid to full depth over a compacted base with mesh throughout, because those are the shortcuts that show up within a year or two.

The sub-base matters as much as the slab

The concrete is only as good as what it sits on. The sub-base is a layer of well-graded crushed stone (MOT Type 1 is the usual material), compacted in layers, that gives the slab even support and keeps it off soft soil.

• Depth: LABC minimums are 75mm for a path, 100mm for a light-duty drive and 150mm for medium duty.

• Preparation: all topsoil and organic material stripped first. Old service trenches backfilled and compacted in layers no deeper than 150mm.

• Level: pavingexpert's guidance for imprinted concrete is that the sub-base should be levelled to within plus or minus 12mm so the slab is a consistent thickness. A sub-base that dips by 30mm means a slab that's 30mm thinner in that spot.

• Membrane: LABC recommends casting onto a damp proof membrane where the sub-base is well drained, so the fresh concrete doesn't lose water into the stone below.

Mesh or fibres?

Both are used, and they do different jobs.

Steel mesh sits in the slab and holds cracks tight if they form, so they don't open up and step. It's standard on heavier-duty slabs and on poor ground. Sheets have to be lapped and supported at the right height; mesh lying on the sub-base does very little.

Fibres (polypropylene or similar) are mixed into the concrete. Pavingexpert notes that mesh is not usually specified for residential imprinted concrete, and that the usual ready-mixed imprint mixes carry fibres instead, to reduce fine surface cracking.

We put mesh in our imprinted driveways. It costs more, but it's cheap insurance on West Midlands clay.

Joints: where the cracks are allowed to go

Every concrete slab shrinks as it cures. Joints give that shrinkage somewhere tidy to happen.

• Control joints are sawn into the slab at a quarter to a third of its depth. Pavingexpert's rule of thumb is a joint roughly every 30 times the slab thickness: about every 3m on a 100mm slab and 4.5m on a 150mm slab.

• Bay size: LABC guidance is to lay in-situ concrete in areas of no more than 20 square metres, with a maximum bay length of 6m.

• Isolation joints: a flexible strip (such as bitumen-impregnated fibreboard) where the slab meets the house, a wall or a garage, so the drive can move without pushing on the building.

A drive with no joints will crack anyway. It just does it in random places across the pattern.

Does imprinted concrete need to be thicker?

No. The structural slab is the same as plain concrete; the colour hardener and stamping only affect the top few millimetres. What imprinting does add is cutting joints on a pattern line so they're less visible, and the sealer on top.

Concrete thickness questions, answered

Is 75mm of concrete enough for a driveway?

Not for cars. 75mm is the LABC minimum for paths. Under a car it's likely to crack, especially at the edges and where you turn the wheel.

Do I need 150mm for a van or motorhome?

At least. A motorhome or a loaded van is a heavier-load case than the "typical family car" the 100mm minimum is written for, and the slab and sub-base should be designed for it.

How does this compare with tarmac?

Tarmac is a flexible surface and is built differently, in a binder course and a surface course over the sub-base. If you're weighing the two, our tarmac driveways page explains why the dig-out and sub-base matter just as much there. For how concrete stacks up against blocks on cracking and sinking, see imprinted concrete vs block paving, and for what you can put on top of a well-built slab, browse our pattern imprint driveway ideas.

When Pete measures up for a concrete drive anywhere around Wolverhampton, the quote states the slab thickness, sub-base depth and reinforcement in writing, so you can hold any other quote against the same build.