MOT Type 1 vs Type 3 - where crusher run and 6F2 fit in

Pick the wrong sub-base and you either build something that will not drain, or something that will not carry the load. This guide goes to the actual specification — the Specification for Highway Works — rather than repeating what the trade says. Four of the claims you will read elsewhere turn out to be wrong.

What "MOT" actually means

MOT stands for Ministry of Transport. That department no longer exists under that name, but the label stuck. What it signals today is that a material is made to a national specification rather than to whatever a quarry happens to produce.

That specification is the Specification for Highway Works, part of the Manual of Contract Documents for Highway Works. Sub-base materials live in Series 800. Since 2004 they have also been tied to a European standard, BS EN 13285, which covers unbound mixtures.

The clause numbers matter, because they are what a specifier will quote:

One thing worth knowing: these clause numbers have changed. In older editions of the specification, Clause 805 covered slag bound material and Type 4 sat at 806. If you are reading an old document or an old web page, check which edition it refers to.

MOT Type 1 explained

Type 1 is the default sub-base across the UK. If someone says "sub-base" without qualifying it, this is what they mean.

What it is made of

Crushed rock, crushed slag, crushed concrete, recycled aggregate, or well-burnt non-plastic shale. Up to 10% of it can be natural sand passing the 4mm sieve. Crushed gravel is only allowed where the contract specifically permits it.

The grading

Type 1 is a 0/31.5 material, category GP, with fines capped at 9% — the specification calls this UF9.

In plain terms: everything from around 31.5mm down to dust, with a continuous spread of sizes in between.

Why it carries load

The continuous spread of sizes is the whole point. Large stones form the skeleton. Medium stones fill the gaps between them. Fines fill what is left. Compact that, and the particles lock together into a dense mass with very little air space.

The specification is strict about this continuity. Under BS EN 13285, the jump in percentage passing between one sieve and the next must be neither too small nor too large. A mix that skips sizes will not lock together properly.

The consequence people miss: filling every void means water cannot get through. Compacted Type 1 is effectively impermeable. That is not a flaw — it is the direct result of the density that makes it strong. But it means Type 1 is the wrong material if you need water to drain downwards.

One more requirement: the fraction passing 0.425mm must be non-plastic. Plastic fines swell and soften when wet, which is exactly what you do not want under a driveway.

View MOT Type 1 Sub Base

MOT Type 3 explained

Type 3 is the drainage option. It is made to the same family of standards, from the same kinds of rock, but graded differently on purpose.

The grading — and the claim everyone gets wrong

Type 3 is a 0/40 material, category GO for open graded, with fines capped at 5% — category UF5.

You will read in many places that Type 3 has "no fines", or that it starts at 4mm, or that the fines are "removed entirely". That is not what the specification says.

This matters practically. If you are expecting a clean, open stone that water pours through like a soakaway, you will be surprised when your Type 3 arrives with visible dust in it. That is correct material, not a bad batch.

Why it drains

With most of the fines gone, the voids between stones stay connected. Water can move down through the layer instead of running off the top.

For permeable block paving, the relevant standard (BS 7533-13) requires the sub-base to keep a voids ratio of at least 30%. That is the space the water travels through and is stored in during a downpour.

Is Type 3 weaker?

Less dense, yes. Weak, no.

A well-graded material always compacts to a higher density than an open-graded one from the same rock, so Type 1 gives the stiffer platform. But Type 3 is a proper structural sub-base, used routinely under trafficked permeable car parks and driveways.

The specification actually compensates for the reduced density by demanding tougher stone. Type 3 must meet a Los Angeles abrasion category of LA30, against LA50 for Type 1. In other words, Type 3 requires harder-wearing aggregate than Type 1 does.

View Type 3 Sub Base

Type 1 vs Type 3 head to head

Note the last row. Type 3 is the coarser material, with a 40mm top size against 31.5mm. Sources claiming Type 3 gives a "smoother finish" better suited to pathways have it backwards.

When Type 3 is required, not just preferred

Most of the time the choice is yours. In three situations it is not.

Front gardens over five square metres

Since 1 October 2008, hard surfacing more than five square metres of a front garden in England is permitted development only if the surface is permeable, or the rainwater is directed to a permeable area such as a lawn, border or soakaway.

An impermeable build-up over Type 1, draining straight to the road, needs planning permission. A permeable build-up over Type 3 generally does not.

Under permeable block paving

BS 7533-13 sets out what goes beneath permeable paving: a crushed aggregate graded 4mm to 20mm, with at least 30% voids, at a minimum structural thickness of 250mm for domestic use with no HGV traffic. Note this is a tighter specification than generic Type 3 — on designed schemes, follow the standard rather than substituting.

Under permeable resin-bound surfaces

A resin-bound surface is only permeable if everything under it is too. That means a porous base over an open-graded sub-base. Laying permeable resin over Type 1 gives you an expensive surface that holds water.

What about SuDS law?

Here the picture is often overstated. Schedule 3 of the Flood and Water Management Act 2010, which would make sustainable drainage approval mandatory, has been in force in Wales since January 2019 but has never been commenced in England.

In England, sustainable drainage is delivered through planning policy rather than statute. So "Type 3 is legally required for SuDS" is not accurate for England. The front-garden rule and the paving standards above are the requirements that actually bite on domestic work.

The position may change. Government has signalled a preference for continuing with the planning-policy route rather than commencing Schedule 3, but a final decision was still pending. Check the current position on larger schemes.

Crusher run, scalpings and hoggin fill

These are the budget options. They can be perfectly sensible — but you need to know what you are buying.

What they are

Crusher run is rock that has been through a crusher and screened only at the top end. There is no controlled lower grading, and fines content is typically higher than a specified material.

Scalpings are the off-cuts and initial screenings from quarry processing. Angular, variable, no fixed recipe.

Hoggin fill or "as dug" is naturally occurring gravel, sand and clay sold as fill.

The key point

When they are good enough

• Bulk filling and levelling before a proper sub-base goes on
• Shed and greenhouse bases
• Farm tracks and field access
• Temporary hardstanding and site roads
• Firming up ground that holds water

When they are not

• Anywhere the job must meet a highways or council specification
• As the final sub-base layer under paving, where irregular sizing makes a flat, stable finish hard to achieve
• Where frost performance matters — fines content and parent material are uncontrolled, so frost susceptibility is unpredictable
• Under anything you would be unhappy to dig up again

A common and sensible approach is to use crusher run as a lower bulk layer and finish with 100mm of Type 1 on top. You get most of the saving without compromising the layer that actually matters.

6F2 — and the mistake almost everyone makes

6F2 is widely sold as a cheap sub-base. It is not a sub-base.

The pavement layers, in order

6F1, 6F2, 6F4, 6F5 — what the numbers mean

6F1 is fine-graded capping. 6F2 is coarse-graded capping. Both are now reserved for material won from within the site itself. The imported equivalents are 6F4 (fine) and 6F5 (coarse).

That last point is worth noting, because 6F2 and 6F5 are often sold interchangeably. Technically they are not the same class — the difference is whether the material came from the site or was brought in.

What 6F2 may contain

Recycled and secondary aggregates, blast furnace slag, crushed concrete and masonry. Up to 50% recycled asphalt is permitted. Up to 1% foreign material — wood, plastic, metal — is allowed. Aggregate toughness need only meet LA50.

The risk nobody mentions: sulfates

This is the part that matters if you are laying a concrete slab.

Recycled and crushed-concrete fills can carry sulfates and sulfides. The specification recognises this: unbound material placed within 500mm of concrete must meet limits on water-soluble sulfate and total sulfur. Within 500mm of metal, the sulfate limit is much tighter still.

Published research has warned specifically that guidance written for intact concrete in aggressive ground should not simply be applied to crushed concrete, and that recycled crushed concrete should not be used in conditions equivalent to the more aggressive ground classes.

In practice: putting unspecified 6F2 straight under a concrete slab or foundation is a real durability risk, not a theoretical one. If you are going to do it, ask your supplier for sulfate test results.

So is 6F2 useless?

Not at all. It is a good, economical material for what it is designed to do — capping over weak ground, bulk fill, yards, tracks and hardstanding. It compacts well because the crushed brick and concrete are angular and interlock.

The problem is only the labelling. Buy it as capping and fill, not as a Type 1 substitute.

View 6F2 / 6F5 coarse recycled aggregate → · Read our guide to recycled aggregates →

Which one for your job

Building a self binding gravel path? The sub-base choice is covered in more detail in our self binding gravel guide.

Depths, compaction and frost

How deep

On clay or soft ground, go deeper and consider a geotextile membrane between subgrade and sub-base.

How to compact it

Material up to 225mm compacted thickness can go down in one layer. Thicker than that, use two or more layers, each at least 110mm, with the lowest layer the thickest.

The specification sets compaction by number of passes rather than by a density test. As a guide, at 110mm layer thickness a vibrating plate over 2100 kg/m² needs around 3 passes, while a vibratory roller in the 1300–1800 kg/m² range needs around 16. Compact at or near optimum moisture content — dry material will not lock together, and saturated material pumps.

The layer is finished when the surface is closed and shows no movement under the plant.

Frost

The specification requires that material must not be frost susceptible if it is used within 450mm of the finished surface — or within 350mm where the site's annual frost index is below 50. Material counts as non-frost-susceptible if mean heave in the standard test is 15mm or less.

Properly made Type 1, Type 2 and Type 3 from crushed rock or concrete are specified to meet this. Crusher run, scalpings and 6F2 are not controlled to the same extent, so their frost behaviour is less predictable.

Geotextile separator

Not required by the specification for a standard build, but strongly advised over clay or soft, cohesive subgrades. Without one, sub-base sinks into the ground and clay works its way upwards.

This matters more with Type 3 than Type 1. Type 3's open voids are exactly what clay fines will migrate into, and a clogged permeable sub-base is no longer permeable. The specification even provides for a granular filter layer beneath open-graded sub-bases for this reason.

Four things people get wrong

A fifth, more subtle one: "Type 3 is cheaper because it has less material in it." Screening fines out is an extra process, and Type 3 requires tougher stone (LA30 against LA50). There is no technical reason to expect it to cost less.

The documents behind this article

Order your sub-base

The decision comes down to one question: does water need to get through the sub-base, or not? If yes, Type 3. If no, Type 1. Everything else — crusher run, 6F2 — is fill and capping, useful in its place but not a substitute for either.

Browse all sub base materials

This article is general guidance. Specification figures are drawn from the current Specification for Highway Works and associated standards; the standards themselves are the definitive source. For designed schemes, particularly those with drainage, structural or planning requirements, seek professional advice and confirm the current position with your local authority.