How-to · 4 min read · Updated 6 November 2026
Gravel driveway sub-base: depths, Type 1 vs Type 3 and membranes
Why the sub-base matters more than the gravel
The gravel you see is a wearing course just a few centimetres thick. All the weight of a car, and the job of keeping the surface level, falls on the layer underneath. A thin or poorly compacted sub-base is the main reason gravel drives rut, sink in tyre tracks and turn muddy within a couple of years. Two drives with identical gravel can perform completely differently because of what lies beneath.
A typical build-up
| Layer | Typical depth | Purpose |
|---|---|---|
| Decorative gravel | 40 to 50 mm | Wearing surface, appearance |
| Gravel grid, if used | 30 to 40 mm | Stops stones moving and rutting |
| Crushed stone sub-base | 150 to 200 mm, more on soft ground | Carries the load, spreads weight |
| Permeable geotextile | Thin fabric | Separates sub-base from soil, lets water through |
| Formation | Firm subsoil | Base that everything sits on |
These are typical domestic figures for cars. Vans, motorhomes or heavy deliveries, and soft or clay ground, often need 250 mm or more of sub-base.
Type 1 or Type 3?
| Material | What it is | Best for |
|---|---|---|
| MOT Type 1 | Crushed stone with fines, compacts very tight | General use, good load-bearing on reasonably draining soil |
| Type 3 | Open-graded crushed stone, few fines | Permeable builds, water storage, poorer drainage sites |
| Clean crushed stone, 20 to 40 mm | Single-size stone, no fines | Drainage layers and some grid systems |
| Recycled Type 1 | Crushed concrete and aggregate | Lower cost alternative where quality is controlled |
Type 1 is strong and widely available, but its fines make it drain slowly once compacted. Type 3 has large voids that store water and let it soak away, but it needs care to compact and sometimes benefits from a thin blinding layer before a grid is laid. Recycled aggregates can be fine if they are clean and graded, but avoid loads containing plaster, wood, plastic or soil.
How it should be laid
- Strip all turf, topsoil and organic material down to firm subsoil
- Remove soft spots and replace with compacted stone
- Lay the geotextile with generous overlaps, typically around 300 mm
- Spread sub-base in layers of no more than about 100 mm
- Compact each layer with a plate compactor or roller, with several passes
- Set falls so water runs away from the house
- Fit edging before the final layers so the stone is restrained
Compacting in layers is important. One thick dump of stone compacted only at the top will settle over the following months, leaving tyre tracks and dips.
Membranes: what to use and what to avoid
Under the sub-base, a non-woven or heavy-duty geotextile separates the stone from the soil, stopping the two mixing when it rains and keeping the base firm. Some installers also lay a lighter membrane between sub-base and gravel to discourage weeds rooting into the base, though many gravel grids come with a fabric bonded to the underside that does the same job. Avoid black polythene sheeting: it is not permeable, so water sits on it and the gravel turns into a slurry.
Signs a sub-base was built badly
If you have inherited a gravel drive and are wondering whether it can simply be topped up, look for these warning signs. Any of them suggests that the problem is underneath, and that adding more stone will only hide it for a while.
- Deep tyre ruts that reappear soon after raking
- Mud or soil showing through the gravel
- Puddles that sit for hours after rain
- Edging that has leaned, sunk or broken away
- Sinking near manholes, drains or along old trench lines
- Membrane visible at the surface in worn areas
A trial hole in a worn spot, about 300 mm square, tells you a lot. If you find a good depth of firm, compacted crushed stone, a re-gravel may be all that is needed. If you hit soil or soft, muddy stone within a few centimetres, a proper rebuild is the long-term answer.
Questions to ask about the sub-base
- How deep will you excavate, and what if you find soft ground?
- Which sub-base material, and how thick once compacted?
- Will it be compacted in layers, and with what machine?
- Which membrane, and where will it go?
- Which way will the drive fall, and where will water go?
A written answer to each of these in the quote is a good sign. Vague promises of a solid base without numbers are worth questioning.
Frequently asked questions
How deep should I dig for a gravel driveway?
For a typical car driveway, total excavation is usually around 200 to 250 mm below finished level, allowing for 150 to 200 mm of sub-base and 40 to 50 mm of gravel. On soft ground, clay, or where heavier vehicles will park, digging deeper and adding more sub-base is common. Always dig down to firm subsoil rather than a fixed number.
Can you lay gravel straight onto soil?
You can, but it rarely works well for a driveway. Without a sub-base the gravel is pushed into the soil by car tyres, mud works up through the stones, and the surface ruts quickly. Laying gravel directly on soil is only really suitable for light-use paths or decorative areas, and even then a membrane underneath helps.
Can gravel be laid over an old concrete or tarmac drive?
Sometimes. If the old surface is sound and roughly level, a thin layer of gravel can be laid over it, often with a grid fixed down. The catch is drainage: concrete and tarmac are not permeable, so water may sit under the stones. In England this can also affect whether a front drive meets the permeable surface rules, so get advice first.
sub-base MOT Type 1 membranes construction
Related services
We cover 250 towns and cities, including Bath, Birmingham, Coventry, Nottingham, Mansfield, Manchester.
