The part of a patio or driveway you never see is what decides whether it stays flat. Most sunken slabs, rocking flags and rutted block paving come down to one thing: a sub-base that was too thin, or not compacted properly. This guide gives typical UK depths for each job, explains how deep to dig in total, and shows how much MOT Type 1 to order.
Already know your sizes? Work out tonnes and bulk bags of MOT Type 1, or plan the whole patio with the patio calculator.
Planning the whole job? Our New patio planner takes your measurements once and builds a single shopping list across every calculator you need.
1. What a sub-base does
A sub-base is a layer of compacted crushed stone between the ground and your paving. It does three jobs:
- Spreads the load from feet, furniture or cars over a wider area of soil, so the surface doesn't sink in one spot.
- Drains water away from the underside of the paving, so it isn't sitting on wet, soft ground.
- Gives a firm, level platform to lay onto, with the fall already built in.
In the UK the standard material is MOT Type 1: crushed stone or concrete graded from about 40mm down to dust, made to the Highways specification. The fines fill the gaps between the larger stones, so it compacts into a dense layer. You'll also see it sold as "Type 1", "DTp1" or "sub-base"; recycled Type 1 made from crushed concrete is usually cheaper and fine for domestic jobs.
2. Sub-base depths by job
These are typical compacted depths on firm ground. Add 50mm or more on soft clay, made-up ground or anywhere that stays wet.
| Job | MOT Type 1 (compacted) | Laying course | Surface | Total dig depth |
|---|---|---|---|---|
| Garden path (foot traffic) | 75–100mm | 30–40mm mortar bed | Slabs 20–40mm | ~140–180mm |
| Patio | 100mm (150mm on clay) | 30–40mm mortar bed | Slabs 20–40mm | ~160–180mm |
| Shed or garden room base (concrete) | 75–100mm | – | 100mm concrete | ~175–200mm |
| Gravel driveway | 100–150mm | Stabiliser grid (optional) | 40–50mm gravel | ~150–200mm |
| Block paving driveway | 150mm (200mm+ on soft ground) | 30–50mm sharp sand | 60mm blocks | ~250–300mm |
| Concrete driveway | 150mm | – | 150mm concrete | ~300mm |
3. Working out how deep to dig
Start from the finished surface level and work down. Add together the thickness of the paving, the laying course and the compacted sub-base. That total is how far below the finished level you need to dig.
Before you dig, set the finished level with pegs and a string line:
- Next to a house, the finished surface should be at least 150mm below the damp-proof course (DPC), so rain can't splash above it into the wall.
- Build in a fall of about 1:60 to 1:80 (roughly 15mm per metre) away from the house, so water runs off. Driveways should fall towards a drain, a gully or a planted area.
- Dig out all the topsoil and any roots, even if that takes you deeper than the table. Top up the extra depth with more sub-base, not soil.
Need to know how much soil you'll be getting rid of? The excavation and muck-away calculator works out the volume (allowing for the soil bulking up as it's dug) and the skips or grab lorry loads you'll need.
4. Soft ground, clay and drainage
Push your heel into the bottom of the dig. If it leaves a deep print, or the ground is sticky clay, the sub-base needs help:
- Add 50mm or more to the sub-base depth on clay, made-up ground or soft spots.
- Lay a woven geotextile membrane on the dug-out ground before the MOT Type 1 goes in. It keeps the stone from being pushed into the soil over the years.
- Dig out any very soft pockets and fill them with compacted sub-base, rather than bridging over them.
- Don't let water collect against the house or under the paving. On a large area, or where the ground slopes towards the building, add a channel drain or soakaway.
Front gardens in England: paving more than 5m² of a front garden with a non-permeable surface (such as slabs on a mortar bed or standard block paving) needs planning permission, unless rainwater runs off into a lawn, border or soakaway on your own land. Permeable block paving on a Type 3 sub-base, or gravel, avoids this. Check with your council if you're not sure.
5. Laying and compacting MOT Type 1
Depth only counts if it's compacted. Loose MOT Type 1 shrinks by roughly 20–25% under a plate compactor, so a sub-base that was raked out but never compacted will settle later, under your paving.
- Spread in layers of no more than 50–75mm (loose depth about 75–100mm). A 150mm driveway sub-base goes down in two or three layers.
- Dampen it lightly if it's dusty and dry. Damp (not wet) stone compacts much better.
- Compact each layer with 4–6 passes of a plate compactor in different directions, until the plate stops leaving marks. Use a hand tamper in corners.
- Check levels and fall against your string lines after each layer, and top up low spots before the next one.
- The finished sub-base should be firm enough to walk on without leaving footprints.
6. How much MOT Type 1 to order
Compacted MOT Type 1 weighs about 2.1 tonnes per m³. Most UK merchants sell it in bulk bags of about 850kg (check your supplier's figure; few "tonne bags" hold a full tonne).
| Compacted depth | Tonnes per m² | One 850kg bulk bag covers | Bulk bags for 20m² |
|---|---|---|---|
| 75mm | 0.16 t | ~5.4 m² | 4 |
| 100mm | 0.21 t | ~4.0 m² | 5 |
| 150mm | 0.32 t | ~2.7 m² | 8 |
| 200mm | 0.42 t | ~2.0 m² | 10 |
Add 5–10% for uneven ground and topping up. For larger jobs (roughly 6 tonnes and up), a loose tipper load from a local quarry or aggregates supplier is often cheaper than bulk bags, if you have somewhere to tip it. The MOT Type 1 calculator does the sums for your exact area and depth.
7. Common sub-base mistakes
- Leaving topsoil in place and laying sub-base on top. Topsoil is organic and keeps settling.
- Compacting it all in one thick layer. A plate compactor only compacts the top 75mm or so properly.
- Using soil, sand or rubble to make up depth. Use sub-base all the way up.
- No fall, so water ponds on the surface and soaks under the edges.
- Skimping on driveway depth. A patio build-up under a car will rut and crack quickly.
- Forgetting the edges. Block paving and gravel need a fixed edge restraint (kerbs or edging set in concrete) or the sub-base and surface spread outwards.
Ready to plan the rest? Use the patio calculator for slabs, bedding and jointing, the block paving calculator for driveways, or the concrete calculator for a shed base.