Every failed driveway I have looked at starts with the same question: what is underneath? The answer is almost always either nothing (the tarmac or block paving was laid straight onto soil) or Type 1 that was not properly compacted.
MOT Type 1 is straightforward material. The rules around it are not complicated. The mistakes are almost always in depth and compaction.
What is MOT Type 1?
MOT Type 1 (also written as DoT Type 1 or just Type 1) is a crushed aggregate sub-base material specification defined by the UK Department of Transport. It consists of crushed stone (or recycled concrete/demolition material in some products) graded to a specific range of particle sizes.
The key characteristic is the grading - Type 1 includes particles ranging from large (37.5mm maximum) down to fines (dust). This wide grading means that when the material is compacted, the fines fill the gaps between the larger stones, creating a dense, interlocked mass that is both stable and resistant to water infiltration.
Typical composition:
- Crushed limestone (most common in UK)
- Crushed granite
- Crushed basalt
- Recycled Type 1 (crushed concrete and hardcore - check suitability before specifying)
Why Type 1 and not just compacted soil?
Soil is unstable under load and in wet conditions. The bearing capacity of natural clay or sandy soil is inadequate for driveways and paved surfaces. Over time:
- Soil compresses under load, causing surface settlement
- Clay expands when wet and contracts when dry, causing surface movement
- Frost heave lifts and disrupts surfaces
A properly compacted Type 1 sub-base bridges this gap. Its high bearing capacity distributes loads, and its free-draining characteristics avoid the freeze-thaw and water saturation problems that affect soil directly under paving.
Compaction: the critical step
Type 1 does not achieve its design performance without compaction. You must use a vibrating plate compactor (wacker plate).
Compaction guidelines:
- Lay and compact in layers of no more than 150mm loose depth
- Make at least 2-3 passes with the plate compactor across each layer
- Check by walking across the compacted surface - if it moves or sinks under foot, compact more
- Add the next layer only after the previous is fully compacted
After compaction, Type 1 reduces in depth by approximately 20-25%. A 200mm loose layer will compact to approximately 150-160mm. This is the compaction factor you must account for when working out how much to order.
How much Type 1 do you need?
Use the MOT Type 1 Calculator for a precise quantity. As a rough guide:
| Compacted depth | Tonnes per m2 (approx) |
|---|---|
| 100mm | 0.18-0.20 t |
| 150mm | 0.27-0.30 t |
| 200mm | 0.36-0.40 t |
Type 1 typically has a loose bulk density of approximately 1.6-1.8 t/m3 and compacts to approximately 1.9-2.0 t/m3. For ordering purposes, assume approximately 1.8t per m3 of loose material to allow for compaction loss.
Recommended depths for different applications
| Application | Compacted Type 1 depth |
|---|---|
| Footpath | 75-100mm |
| Patio (pedestrian) | 100mm |
| Residential driveway (light vehicles) | 150mm |
| Driveway with regular van access | 200mm |
| Heavy vehicle access (commercial) | 200-300mm |
Always check against the specific surface layer requirements - block paving, tarmac and concrete each have their own depth recommendations that may specify different sub-base thicknesses.
Type 1 vs other sub-base options
| Material | Use | Comparison |
|---|---|---|
| MOT Type 1 | Driveways, paths, patios | Standard, widely available |
| MOT Type 3 | Permeable paving | Open-graded, allows water through |
| Recycled Type 1 | Similar to primary but lower cost | Confirm suitability, variable quality |
| Hardcore (unprocessed) | Not recommended | Variable grading, unreliable compaction |
| Sand | Bedding layer above Type 1 | Not a sub-base material |
Do not use unprocessed hardcore as a sub-base. It may appear similar to Type 1 but has unpredictable grading and compaction performance.
My tips on MOT Type 1
Order slightly more than calculated. Type 1 is cheap per tonne and the consequences of running short (having to pause the job and wait for a second delivery) are disproportionate. Add 10% to your calculation.
Keep it dry when possible. Saturated Type 1 is harder to compact properly and takes longer to achieve stable bearing. If rain is forecast, cover the stockpile and do not compact when the material is sodden.
Compact before your final level check. If you are hitting a target level, compact first, then check. Compaction reduces the height by 15-25% - if you try to set the level before compacting, you will be 20-30mm too high after.
Use the MOT Type 1 Calculator to get accurate tonnage for your project.