MOT Type 1: The Sub-Base That Decides How Long Your Drive Lasts
MOT Type 1 is a crushed stone sub-base, graded from about 31.5mm down to dust, that is spread and compacted under a driveway to carry the weight of cars. For a typical domestic drive it goes down at around 100mm compacted, or 150mm on soft ground or where vans park. Get this layer wrong and no surface on top of it will last, however good it looks on day one. It is the part of the job we spend most time on when laying driveways in Wolverhampton and the Black Country, because it is the part you never see again.
What is MOT Type 1?
Type 1 is an unbound granular sub-base defined in the Specification for Highway Works, the national highways spec, under Clause 803. The "MOT" part is a leftover from the old Ministry of Transport, which wrote the original spec. Builders also call it Type 1, DTp1 or "40 to dust", though the last name is not accurate.
Under the current wording, Type 1 is a 0/31.5 mixture: most of it (75-99%) passes a 31.5mm sieve, a little oversize stone up to 63mm is allowed, and everything below that is present in a continuous spread, right down to fine dust. The fines (the material passing a 0.063mm sieve) are capped at 9%. That spread is the whole point. When it is compacted, the small particles lock into the gaps between the large ones and the layer becomes dense and stiff, spreading the load of a wheel over a much wider area of the soil below.
It can be crushed limestone, granite or other hard rock, and recycled versions made from crushed concrete are common. What matters on a driveway is that it is properly graded, laid at the right moisture and compacted in thin layers.
Type 1 vs Type 3: which one goes under a driveway?
The difference is drainage. Type 1 is close graded, so its fines block most water from passing through. Type 3 (Clause 805) is a 0/40 open graded mixture with fines capped at 5%, which leaves voids between the stones for water to pass through and be stored.
Type 1: the default under tarmac and pattern imprinted concrete, where the surface itself is sealed and the water is dealt with by falls and drains.
Type 3 (or 4/20 clean stone): used under permeable and porous surfaces, where rain is meant to soak down through the drive.
This matters for planning. The government guidance on paving front gardens explains that conventional drives use Type 1, which has "a lot of fine material" that stops water passing through, while permeable drives need open graded materials such as Type 3 or 4/20. A permeable surface laid on dense Type 1 will not drain the way the front garden rules expect it to.
How deep should a driveway sub-base be?
For a domestic drive used by cars, 100mm of compacted Type 1 is the common light to medium duty figure. Go to 150mm where the ground is soft clay or where vans or a motorhome will park, and get a site-specific design for anything heavier. The government front garden guidance describes "about 150mm" of compacted sub-base over a geotextile for a typical permeable drive.
The total dig is the sub-base plus everything above it, measured down from the finished level:
Tarmac: a typical build is 100mm sub-base, a 50mm binder course and a 25mm surface course, so roughly 175mm below the finished level before allowing for any extra on poor ground.
Pattern imprinted concrete: the slab itself should be at least 100mm thick on a driveway, laid over a levelled, compacted base.
Resin bound: the resin layer is thin (around 18mm on a driveway, about 15mm on a path), so it needs a solid base underneath. On a driveway a common base is at least 50mm of compacted asphalt over a prepared sub-base, or a porous asphalt over an open graded sub-base where the drive must be permeable.
Artificial grass: no cars, so a lighter compacted stone layer and a fine levelling course are enough. The base still has to drain.
Those are typical builds, not legal minimums. There is no single legal thickness for a private driveway. The right depth is decided on site, after looking at the soil, the levels and what will park on it.
How is Type 1 compacted properly?
Compaction is where cheap jobs cut corners, and it is invisible once the surface goes on. Done properly, it looks like this:
Dig out to the formation level and remove soft spots, roots and topsoil.
Lay a geotextile membrane where the ground is soft, to stop the stone working down into the soil.
Spread the Type 1 in layers no thicker than 150mm, which is about the most domestic compaction plant can compact properly. A deeper sub-base is built up in two or more layers, each compacted before the next goes on.
Compact each layer with a vibrating plate or roller, with repeated passes in both directions where the space allows.
Check the levels. The finished sub-base should be within about 10mm of its design level and already follow the falls of the finished drive.
Loose Type 1 shrinks as it compacts. As a very rough guide, around 130mm of loose material compacts to about 100mm, but the figure varies with the rock type and the moisture. Too dry and it will not lock up; too wet and it pumps and moves. A good sub-base does not shift when you walk on it and has a tight, closed surface.
Why the sub-base decides how long a drive lasts
Most driveway failures are base failures, not surface failures. Tarmac that has sunk in two wheel tracks, imprinted concrete that has cracked across the middle, resin that has split along a line: dig into any of them and you usually find a thin layer of stone, no compaction, or old soil left in place.
The surface only spreads load a short distance. It is the sub-base that takes the weight of a car parked in the same spot every night for fifteen years and passes it to the ground evenly. That is why a properly dug and compacted base is the most valuable part of any drive, and why it is the first thing to ask about when comparing quotes. Our full comparison of driveway materials covers how each surface behaves once that base is right.
On our own jobs the excavation and sub-base are done by our team, not subcontracted. On one tarmac drive in Finchfield, for example, the job meant removing five large tree roots and laying 16 tonnes of Type 1 before any tarmac went down. Every drive we lay starts with that groundwork, and it is the reason we can put a 10-year guarantee on materials, workmanship and structure.
MOT Type 1 questions, answered
Can I lay a new drive over the old one instead of digging out?
Sometimes. If the existing base is sound, well drained and the extra height will not reach the damp proof course or block airbricks, an overlay can work. If the old drive is cracked, rutted or sinking, the problem is underneath, and overlaying it just hides that for a year or two.
Is recycled Type 1 as good as quarried?
It can be, provided it is properly processed and graded to the Clause 803 envelope. Badly crushed recycled material with brick, plaster or timber mixed in is not Type 1, whatever the delivery note calls it.
Does a resin drive need Type 1 or Type 3?
It depends on how the water is handled. If the resin is meant to be permeable and meet the front garden rules on its own, the layers underneath must drain too, so an open graded sub-base and porous asphalt are used. Resin laid over dense Type 1 and solid asphalt will not drain through. We explain the options on our resin bound driveways page.
How much Type 1 do I need?
Work out the area in square metres, multiply by the compacted depth in metres, then allow extra for compaction. For an accurate tonnage, ask your installer: it depends on the stone and the ground.
If you are planning a new drive and want to see what sits under it before you choose a surface, our post on driveway ideas is a good place to start, and we are happy to talk through the groundwork when we visit to quote.
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