Sub-Base Explained: How Much MOT Type 1 Do You Need?
Quick answer: MOT Type 1 is a crushed-stone sub-base laid compacted beneath driveways, paths, and patios. For a driveway, lay it 100–150mm deep; for a patio or path, 75–100mm is usual. One tonne of MOT Type 1 covers roughly 6–7 m² at 100mm deep once compacted. It’s sold by weight, so you convert area × depth to tonnes using its density — around 2.0–2.1 tonnes per cubic metre.
MOT Type 1 is the layer nobody sees and everybody underestimates — it’s what stops a driveway or patio from sinking and rutting, yet people routinely order too little because it’s denser than they expect and compacts down. Order short and the surface fails; order too much and you’ve paid for a wasted delivery. This guide explains what MOT Type 1 is, how deep to lay it, how to work out the tonnage, and why the compaction allowance matters. Use the calculator above to calculate your sub-base.
What is MOT Type 1?
MOT Type 1 is a graded crushed stone — a mix of stone sizes down to fine dust, made to a specification originally set by the Ministry of Transport (hence “MOT”). The range of sizes is deliberate: the fines fill the gaps between the larger stones so that, when compacted, it locks into a dense, stable, load-bearing layer.
That compaction is the whole point. Loose, it’s just a pile of stone; compacted, it becomes a solid base that spreads load and resists rutting. This is also why it’s the wrong material for drainage — the fines that make it compact so well block water flow, so for French drains and soakaways you use clean stone with no fines instead.
How deep to lay it
Depth depends on what’s going on top and how much weight it carries:
| Application | Compacted MOT Type 1 depth |
|---|---|
| Patio / light path | 75–100mm |
| Garden path (used) | 75mm |
| Driveway (cars) | 100–150mm |
| Heavy vehicles | 150mm+ |
These are compacted depths. MOT Type 1 reduces in volume when compacted, so you lay it slightly thicker than the finished depth and compact it down. Allow roughly 20–25% extra loose material to reach the compacted figure — this is the step most people forget, and it’s why sub-base orders so often come up short.
Coverage and tonnage
At a compacted depth of 100mm, one tonne of MOT Type 1 covers roughly 6–7 m². Some examples:
| Area | Depth (compacted) | Approx. MOT Type 1 |
|---|---|---|
| 20 m² driveway | 100mm | ~4 tonnes |
| 20 m² driveway | 150mm | ~6 tonnes |
| 15 m² patio | 100mm | ~3 tonnes |
| 10 m² path | 75mm | ~1.5 tonnes |
Remember to allow extra for compaction — the loose delivered volume is more than the compacted volume you measure, so order a little above the calculated figure.
Why the density step matters
MOT Type 1 is denser than decorative gravel — it weighs more per cubic metre because of the fines packed into it, around 2.0–2.1 tonnes per cubic metre against roughly 1.5 for loose gravel. So a sub-base and a gravel top layer covering the same area at the same depth come out as different tonnages. Using the correct density for MOT Type 1, rather than a generic gravel figure, is what makes the order accurate — and it’s exactly the step a one-size calculator gets wrong. The calculator here applies the right density when you select MOT Type 1.
Laying it right
The sub-base only works if it’s compacted properly. Lay it in layers — no more than about 75mm at a time — and compact each layer before adding the next. Use a plate compactor (a “wacker plate”), which you can hire; treading it down by foot isn’t enough for a driveway. Aim for a firm, even surface with no soft spots or movement before you lay gravel, slabs, or paving on top. A well-compacted base is the foundation of everything above it, so it’s worth taking the time to get it solid and level.
Common mistakes to avoid
The most common mistake is forgetting the compaction allowance and ordering only the compacted volume, which leaves you short. Others include compacting the whole depth in one go rather than in layers (so the lower part stays loose), using MOT Type 1 for drainage where its fines cause blockages, and calculating with a generic gravel density that under-estimates the heavier sub-base tonnage. Not compacting at all — just raking it level — is the mistake that leads to a driveway rutting within months.
Frequently asked questions
How much MOT Type 1 do I need for a driveway?
For a car driveway, lay 100–150mm compacted. That’s roughly 4 tonnes per 20 m² at 100mm, or about 6 tonnes at 150mm, plus a compaction allowance of around 20–25% extra loose material.
How deep should MOT Type 1 be?
75–100mm for patios and paths, 100–150mm for driveways, and 150mm or more for heavy vehicles — all compacted depths. Increase the depth on soft or wet ground, and always compact in layers rather than all at once.
How much does a tonne of MOT Type 1 cover?
About 6–7 m² at 100mm compacted depth. Deeper layers cover less area per tonne — roughly 4–5 m² at 150mm. Remember MOT Type 1 is dense, so a tonne doesn’t go as far as the same weight of loose decorative gravel.
Do I need to compact MOT Type 1?
Yes — compaction is what turns it into a load-bearing base. Lay it in layers of no more than 75mm and compact each with a plate compactor, allowing extra loose material for the reduction in volume. Uncompacted, it won’t bear weight and the surface above will rut.
Can I use MOT Type 1 for drainage?
No — its fine content blocks water flow, which is exactly what makes it good as a compacted base but bad for drainage. For French drains, soakaways, and pipe bedding, use clean single-size stone with no fines instead.
How much extra MOT Type 1 should I order for compaction?
Around 20–25% more than the compacted volume, because the material reduces in bulk when compacted. So if your compacted volume works out at 4 tonnes, order closer to 5 tonnes of loose material to reach that finished depth.