We have a neighbour who has been topping up their gravel driveway every spring for as long as I can remember. Every year, a couple of bulk bags of fresh gravel go down, it looks good for a few months, and by the following spring it is thin and patchy again. I asked them about it once and they told me the driveway was "always like this."
Their driveway has no sub-base and no membrane. The gravel is going straight down into the clay beneath it, season after season. They have probably spent more on topping up gravel over eight years than a properly built driveway would have cost in the first place.
Getting the depth and structure right is what makes the difference.
The complete build-up
A gravel driveway is not just gravel. It is a layered system where every layer has a purpose.
| Layer | Depth | Material |
|---|---|---|
| Surface gravel | 50mm | 10-20mm shingle or gravel |
| (Optional upper membrane) | - | Geotextile |
| Compacted sub-base | 100-150mm | MOT Type 1 |
| Lower membrane | - | Geotextile (non-woven) |
| Compacted subgrade | - | Natural ground |
Total excavation depth from finished surface: approximately 150-200mm.
The two membranes, upper and lower, are optional in principle but strongly recommended in practice. The lower one stops the sub-base sinking into soft ground. The upper one stops the gravel mixing with the sub-base and helps maintain a clean surface. Together they add less than £1 per m2 to the project cost and add years to the life of the driveway.
Surface gravel depth
For the gravel layer itself, 50mm is the standard:
- Enough for tyres to compress slightly and gain grip
- Gives a full, generous appearance without being wasteful
- Sufficient depth to last 2-3 years between occasional top-ups
Going shallower than 40mm looks thin and will start showing sub-base through in trafficked areas within a season. Going deeper than 75mm makes the surface feel unstable and is harder to walk on - you do not want to be sinking into gravel every time you walk to your car.
Sub-base depth by ground type
| Ground type | Minimum sub-base depth |
|---|---|
| Firm sandy or chalky ground | 100mm |
| Average garden soil | 100-125mm |
| Clay soil | 150mm |
| Made ground or soft areas | 150-200mm |
Clay is the challenging one. In a wet UK winter, clay absorbs water and swells. In a dry summer, it shrinks back. This seasonal movement, repeated year after year, will gradually push a thin sub-base to the surface and create ruts and dips. On clay, 150mm is not optional - it is the minimum for a driveway that will stay level.
What happens without a sub-base?
If you are determined to skip the sub-base, the surface gravel needs to be deeper to compensate - typically 75-100mm. Even then:
- Vehicle weight will push gravel into soft ground
- Ruts will develop in trafficked areas
- The driveway will need more frequent topping up
- Long-term, the surface will continue to degrade
For anything other than a very lightly used path on firm ground, the sub-base is not optional if you want a driveway that lasts.
Geotextile membrane: the overlooked hero
The membrane is cheap, easy to lay and one of the most impactful decisions you can make on a gravel project. Here is what it does:
Lower membrane (between soil and sub-base): Prevents the sub-base stone from being pushed down into softer soil by vehicle loads. Without it, the sub-base gradually disappears into the ground over years, causing dips and ruts.
Upper membrane (between sub-base and surface gravel): Keeps the gravel separated from the sub-base. Without it, the surface gravel mixes with the sub-base over time, reducing the depth of usable surface material.
Both membranes should be a proper non-woven geotextile - not a lightweight garden weed membrane. The weed membrane will tear under the weight of vehicles and stone.
How much gravel do I need?
Once you know your surface area and 50mm depth:
Volume (m3) = Area (m2) x 0.05
Weight (tonnes) = Volume x 1.7 (for standard shingle)
Order weight = Weight x 1.1 (10% wastage)
| Driveway area | Tonnes of surface gravel (50mm, inc. 10% wastage) |
|---|---|
| 10m2 | 0.9t |
| 20m2 | 1.9t |
| 30m2 | 2.8t |
| 50m2 | 4.7t |
| 75m2 | 7.1t |
| 100m2 | 9.4t |
For your exact figure, use the Gravel Calculator.
My tips for building a gravel driveway that lasts
Here is what I have learned from our own driveway project and from watching other people's succeed and fail:
Mark out and excavate accurately. It is tempting to roughly dig out the area and estimate the depth. Take the time to mark accurate depths with pegs and string. Under-excavated areas mean insufficient sub-base depth in places, which are exactly where you will get the first problems.
Compact the subgrade before putting anything down. Before the membrane goes on, compact the natural ground with a plate compactor. Loose soil under a membrane compresses unevenly under vehicle weight.
Get the sub-base level and at the right falls. This is your best opportunity to set drainage falls correctly - aim for 1:60 minimum away from any buildings. A level laser makes this much quicker. Setting falls at sub-base stage is far easier than trying to adjust at surface level.
Contain the edges with quality restraint. Gravel will migrate outward without edge restraint. Steel edging, treated timber boards or block kerbs all work. Cheap plastic edging tends to flex and pop out under ground movement - it is false economy. The edge restraint is what keeps the driveway looking neat for years.
Do not use self-seeding plants near the borders. Gravel and weeds are natural enemies. A good membrane beneath the gravel controls most weeds, but if you have seeding plants nearby, the seeds will establish in the gravel layer regardless of what is underneath. Worth thinking about at the design stage.
Use the Gravel Calculator and Sub Base Calculator together to plan the full project quantities.