Conventional depths, reported not decided
Sub-base depth is a function of the ground underneath and the load going on top. Choosing one is a design decision, and this site does not make design decisions.
What is publishable is what the trade conventionally uses for each kind of job, said plainly as convention. That is reporting, not specifying, and it is genuinely what most people are looking for:
- Footpath, patio, shed base: 100mm compacted
- Driveway, cars: 150mm compacted
- Drive taking a lorry or skip wagon: 225mm compacted
- Under a concrete slab: 150mm compacted
- Artificial grass: 75mm, since it carries almost nothing
Pick the job and the depth fills in. Change it and the tool tells you you are under the usual figure without telling you not to be, because your ground may well justify either.
The one that catches people
A drive built for cars, with a skip wagon on it. A loaded skip wagon is several times the axle load of a car, and 150mm of Type 1 under block paving does not carry it. That is how a two-year-old drive ends up with two ruts down the middle where the wagon parked.
If a lorry is ever coming on, build for the lorry.
Layers, which nobody asks about and everybody gets wrong
A whacker plate compacts about 100mm down. Not 200.
Tip the lot in one go and the bottom half stays loose no matter how long you run over the top of it. It feels solid, it passes any test you can do by eye, and then it settles into itself over the next twelve months and takes the surface with it.
So 150mm goes in as two layers and 225mm as three, each one whacked before the next goes on. The tool works out how many and says so: a correctly-specified drive most often fails for exactly this reason.
The membrane is the cheapest thing on the job
A geotextile under the sub-base does two things at once: stops the stone migrating down into soft ground, and stops the soft ground pumping up into the stone.
Without one, on clay, you lose the bottom of the sub-base you paid for. It happens slowly and invisibly and there is no way to fix it afterwards without digging it all out.
It is on by default here and the tool works out the area with a bit of overlap.
You buy it loose and you specify it finished
Type 1 loses roughly a fifth of its volume being compacted, so the loose volume you order is always more than the finished volume you measured. Order by the finished figure and you come up short.
Both numbers are shown, and the bag count is worked off a real 850kg bulk bagrather than a notional tonne. More on both of those on the aggregate calculator.
Whether your ground is any good
Whether your ground is any good. That is the thing that decides it, and the commonest cause of a failed drive is a perfectly correct depth laid over the wrong ground.
Soft ground, made ground, recent fill, an old pond, water sitting on it after rain: any of those and the conventional figure is not the right figure, and no amount of Type 1 fixes a subgrade problem. That is a question for somebody who can put a spade in it and look.
Next to it: aggregate quantities for other materials, concrete for the slab on top, and muck away for getting rid of what you dug out.
Common questions
How deep should a sub-base be under a driveway?
For cars, 150mm of compacted Type 1 is what the trade conventionally uses. For a drive that takes a lorry or a loaded skip wagon, 225mm. That is convention rather than a specification: the ground underneath changes it, and soft or made ground can change it completely.
How deep for a patio or a path?
Conventionally 100mm of compacted Type 1 for both, and for a shed base. Artificial grass gets away with 75mm because it carries almost nothing: it just has to drain and stay flat.
Why does it want the depth in layers?
Because a whacker plate only compacts so far down. Tip 150mm in one go and the bottom half never properly compacts, so it settles into itself over the following year and the surface goes with it. Two layers of 75 to 100mm, whacked separately, is the difference.
Do I need a membrane under it?
On clay or made ground, yes. Without a geotextile the sub-base slowly migrates down into soft ground and the soft ground comes up into it, and you lose the sub-base you paid for. It is the cheapest thing on the whole job and the one people leave out.
Why is the amount to order more than the volume I worked out?
Because you specify a sub-base by its finished depth and buy it loose. Type 1 loses about a fifth of its volume being compacted, so the loose volume you order is always bigger than the finished volume you measure. This tool shows both.
Will this tell me if my ground is good enough?
No, and nothing on a screen can. The commonest cause of a failed drive is a perfectly correct depth laid over the wrong ground. If it is soft, made up, recently filled, or the water sits on it, that is a question for somebody who can look at it.