The first time I ordered MOT Type 1 for a driveway project I got the maths badly wrong. I calculated the compacted volume I needed, ordered exactly that amount and ended up about 30% short. The tipper truck came, tipped the load, and even as I was spreading it across the excavated area I could see we were not going to make it. I had to get back on the phone and arrange a second delivery for the next day, which cost me both money and a day's work.
What I had not accounted for was the compaction factor. MOT Type 1 loses roughly 30% of its volume when properly compacted, which means you need to order significantly more loose material than your finished depth suggests. It is the single most common mistake people make when ordering sub-base material, and once you know about it you never forget it.
What is MOT Type 1?
MOT Type 1 (also written as Type 1 MOT or Granular Sub-Base Type 1) is a crushed stone aggregate with a specific grading specification originally set by the Ministry of Transport - hence the name. It is available almost everywhere in the UK from builders merchants, aggregate suppliers and landscape suppliers.
The reason it is so widely used is the grading: it contains a blend of sizes from fine dust up to 40mm pieces. When compacted, the smaller particles fill the gaps between the larger ones, creating a very dense, stable, self-locking layer. It is excellent at distributing load, resisting movement and draining freely.
Common applications include:
- Sub-base for block paving and concrete driveways
- Sub-base under patios and paved areas
- Foundation for shed and outbuilding bases
- Sub-base beneath gravel driveway surfaces
- General hard-standing base material
The compaction factor - the bit that catches everyone out
MOT Type 1 is a loose material when it comes off the tipper truck. When you spread it and compact it with a plate compactor, the particles lock together and the layer reduces in depth by around 20-30%.
The standard figure used in the industry is a compaction factor of 1.3. This means:
- For every 100mm of finished compacted depth, you need to order material equivalent to 130mm of loose depth
- For every 1.0 m3 of compacted sub-base volume, you need to order 1.3 m3 of loose material
Skip this step and you will be short. I have been short. Many people before me have been short. The calculator does this for you automatically, but it is worth understanding why.
The formula
Step 1 - Compacted volume (m3): Length x Width x Depth (all in metres)
Step 2 - Loose volume to order: Compacted volume x 1.3
Step 3 - Weight in tonnes: Loose volume x 2.1 (density of MOT Type 1, tonnes per m3)
Step 4 - Add wastage: Typically 10-15% for sub-base work
Worked example - driveway
Driveway 8m x 4m, 150mm compacted depth:
- Compacted volume = 8 x 4 x 0.15 = 4.8 m3
- With compaction factor = 4.8 x 1.3 = 6.24 m3 (loose)
- Weight = 6.24 x 2.1 = 13.1 tonnes
- Add 15% wastage = 15.1 tonnes to order
That is a large quantity for what feels like a modest driveway - and it is one of the reasons driveway projects are more expensive than many people expect. The stone alone, before any paving goes on top, can be a substantial cost.
How much depth do I need?
| Application | Compacted depth | Notes |
|---|---|---|
| Pedestrian path or patio | 75-100mm | Light foot traffic only |
| Standard driveway, cars | 100-150mm | Most common residential spec |
| Driveway on clay or soft ground | 150-200mm | Extra depth for poor ground |
| Commercial or heavy vehicle use | 200mm+ | Seek specialist advice |
| Under concrete slab | 100mm | Provides drainage and stability |
For most residential driveways in ordinary garden soil, 150mm compacted depth is the right target. On clay, I would always go to 150mm at minimum - the clay expands in winter and contracts in summer and a thicker sub-base gives you more insurance against that movement appearing at the surface.
Tonnes needed by area - quick reference
All figures below are at 150mm compacted depth, including 1.3x compaction factor and 15% wastage:
| Area | Tonnes to order |
|---|---|
| 5m2 | 3.0t |
| 10m2 | 6.0t |
| 15m2 | 9.0t |
| 20m2 | 12.0t |
| 30m2 | 18.0t |
| 50m2 | 30.0t |
At 100mm compacted depth (for patios and lighter-duty areas):
| Area | Tonnes to order |
|---|---|
| 5m2 | 2.0t |
| 10m2 | 4.0t |
| 20m2 | 8.0t |
| 30m2 | 12.0t |
Bulk bag or loose load?
MOT Type 1 is available in bulk bags (approximately 0.85 tonnes each) or as loose loads delivered by tipper truck.
Bulk bags are better if:
- You need less than 3-4 tonnes
- Access is tight and a tipper truck cannot get close
- You want to manage the material over multiple days
Loose load is better if:
- You need more than 4-5 tonnes
- You have good truck access to the site
- You want to save money per tonne (loose load is typically 10-20% cheaper)
For most residential driveways you will be into loose load territory. For patios, paths and smaller areas, bulk bags are usually the right call.
My tips for ordering and laying MOT Type 1
I have done enough sub-base work to have learned some things the hard way. Here is what I would tell anyone starting out:
Always order a little extra. Even with the calculator, add a touch more than the calculated figure. You cannot return stone easily, but you also cannot leave the last corner of a sub-base 30mm too thin just because you ran out. I keep any leftover MOT Type 1 in bulk bags for future small projects.
Hire a plate compactor - do not skip this. A hand tamper does not get close to the compaction you need. A plate compactor is typically £30-50 per day from a hire centre and makes an enormous difference to the stability and longevity of the finished job. Compact in layers of no more than 100mm.
Compact the edges properly. It is easy to under-compact corners and edges. Run the plate compactor parallel to each edge at the end, making a final pass around the perimeter.
Check your falls as you go. MOT Type 1 is your best opportunity to set drainage falls correctly - aim for at least 1:60 away from buildings. Do this at sub-base stage rather than trying to correct it with the surface later.
Use a geotextile membrane underneath. Laying a non-woven geotextile membrane between the natural soil and the MOT Type 1 prevents the stone from gradually migrating into softer ground over the years. It costs very little and adds years to the life of the whole structure.
Use the MOT Type 1 Calculator to get accurate quantities before you order.