A crushed aggregate with a controlled spread of sizes
MOT Type 1 is crushed rock or crushed concrete supplied with a specified range of particle sizes, running from stones of a few centimetres all the way down to dust. It is used as a sub-base: the load-spreading layer that goes under a driveway, a patio, a path or a slab.
The name is a fossil. MOT is the Ministry of Transport, a department that stopped existing in 1970. The specification is now part of the Specification for Highway Works, where the material appears as a Type 1 unbound mixture, and the trade has carried on calling it MOT Type 1 for fifty years regardless.
The grading is the product, not the stone
This is the part that gets missed, and it is the reason substituting cheaper stone does not work.
A single-size material, pea shingle or 20mm gravel, is all the same size. The pieces touch each other and between every three or four of them there is a void with nothing in it. Load the layer and the stones have somewhere to move to, so they move. That never stops happening, at any depth, which is why a gravel sub-base keeps rutting no matter how much of it went in.
A well graded material has every size in it at once. The medium pieces fit between the large ones, the small pieces fit between those, and the dust fills what is left. Compact that and there is nowhere for anything to go. The mass locks together and starts behaving like a slab.
Unbound means the compaction is not optional
There is no cement in it and no bitumen. Every bit of its strength comes from friction and interlock between the particles, which means that until it has been compacted it is not really the material yet, it is a heap of stone that has the potential to become it.
That is a genuinely different mental model from concrete, which is a material when it arrives and then sets. Type 1 arrives as an ingredient. How it goes down, in what thickness and with what, decides whether it works at all, and the hardcore calculator covers that side including the layer thicknesses people routinely get wrong.
Type 1, Type 3, and the thing sold as Type 2
- Type 1. Dense graded, everything down to dust. Locks up hard, and once compacted it sheds water rather than passing it.
- Type 3. Deliberately open graded with most of the fines left out, so water drains through. The sub-base for permeable paving, at some cost in stability.
- Type 2. A grade from older specifications, not in the current one, and still sold under the name for material with more fines and often uncrushed content. Fine as fill, and a poor swap for Type 1 under anything a vehicle will use.
The distinction that actually matters on a domestic job is the first one against the second: if the finished surface has to drain through rather than off, dense graded Type 1 underneath it defeats the purpose.
Buying it, and what it is called elsewhere
It is sold loose by the tonne or in bulk bags, and it is bought loose while it is specified compacted, which is the arithmetic the hardcore calculator exists for. For the layer above it, the aggregate and bulk bag tool covers the sharp sand or grit bedding, and muck away handles the spoil coming out, which bulks up considerably once dug and catches people ordering skips.
Outside Britain, ask for it by description. In the United States it is a dense graded aggregate base, often called crusher run. In Ontario it is Granular A. In Australia it is road base. None of them will know what MOT means, and all of them sell the same thing.
Common questions
What does MOT stand for?
Ministry of Transport, which is a government department that has not existed since 1970. The material was defined in that ministry specification for road building, the name went into the trade, and it outlived the department by decades. The specification itself lives on in the Specification for Highway Works, where the material is described as a Type 1 unbound mixture, but nobody in a builders merchant is going to call it that.
Can I use gravel or pea shingle instead?
No, and the reason is worth understanding because it is the whole point of the material. Single-size gravel is all the same size, so the pieces touch but nothing fills the gaps between them, which means the mass can never lock up. Roll it, walk on it, drive on it, and the stones simply rearrange. MOT Type 1 contains everything from large pieces down to dust, so the small particles wedge into the voids between the big ones and the whole layer becomes something close to solid.
What does unbound mean?
That there is no cement, bitumen or any other glue in it. All the strength comes from mechanical interlock and friction between the particles, which is why compaction is not a finishing step but the thing that creates the material in the first place. Uncompacted Type 1 is a loose heap of stone. Properly compacted, the same stone carries traffic.
What is the difference between Type 1 and Type 3?
Fines. Type 1 is dense graded, with everything down to dust, which is what makes it lock solid and also what makes it nearly impermeable once compacted. Type 3 is deliberately open graded with much of the fine material left out, so it drains freely. That makes Type 3 the choice under permeable paving and in places where water has to get through the sub-base rather than run off it, at the cost of some of the stability.
Is Type 2 the same thing but cheaper?
Not exactly, and it is worth asking what you are being sold. Type 2 was a lower grade in older specifications and is not in the current one, but the name survives in merchants yards for a material with more fines and often uncrushed content, sometimes closer to what is sold as scalpings. It has its uses as fill and under light work. Buying it as a substitute for Type 1 under something that will carry vehicles is a false economy.
What is it called outside Britain?
The material is universal and the name is not. In the United States it is generally a dense graded aggregate base, often called crusher run or referred to by a state designation. In Ontario the equivalent is Granular A. In Australia it is usually just road base. If you are reading instructions from any of those places, well graded crushed rock is the description that carries across, and any of them will behave the same way for the same reason.