Sub-base is the unsexy part of any paving or driveway project. Nobody photographs their sub-base. It goes underground, nobody sees it, and the temptation to cut corners or skip it entirely is real - especially when the budget is tight and you just want to get to the interesting bit.
I relaid a patio at our house a few years after we moved in because the previous owners had put slabs down directly on bare soil with barely 20mm of sharp sand beneath them. Within three winters, two slabs had sunk noticeably at one corner, water pooled there every time it rained, and the whole thing was an uneven hazard to anyone who did not know where to walk. The cost of digging it all up and doing it properly was far more than getting it right first time would have been.
The sub-base is not optional. Here is how to do the calculation properly.
What is a sub-base?
A sub-base is a compacted layer of granular material laid between the natural ground and whatever surface goes on top - whether that is paving slabs, block paving, gravel, or a concrete slab. Its job is to:
- Distribute load evenly across the ground below
- Provide free drainage to prevent water sitting under the surface
- Create a stable, consistent platform that resists movement
- Protect against frost heave in winter
Without it, paving is only as stable as the soil underneath, and soil is rarely as stable as you need it to be.
Sub-base material options
| Material | Best for | Pros | Cons |
|---|---|---|---|
| MOT Type 1 | Driveways, patios, anything load-bearing | Consistent spec, best compaction, widely available | Slightly more expensive |
| Hardcore (mixed) | Light garden areas, rough fill | Very cheap | Variable quality, unpredictable compaction |
| Crushed concrete | Patios, paths | Good recycled option, often competitively priced | Quality varies |
| Crushed stone | Most applications | Good performance | Price varies by region |
MOT Type 1 is the default recommendation for any application where stability and longevity matter - driveways, patios, paths around buildings, shed and garage bases.
The compaction factor - why you order more than you think
This is the most misunderstood part of ordering sub-base. When MOT Type 1 arrives on your site, it is in a loose state. When you spread it and compact it with a plate compactor, it reduces in depth by around 20-30%.
The standard compaction factor is 1.3. This means for every 1.0 m3 of compacted sub-base you need, you must order 1.3 m3 of loose material.
Skip this and you will be short - sometimes significantly short. Our Sub Base Calculator and MOT Type 1 Calculator apply this factor automatically.
How to calculate sub-base quantities
- Measure your area (Length x Width in metres)
- Decide on your compacted depth
- Calculate compacted volume (L x W x depth in metres)
- Multiply by 1.3 (compaction factor)
- Multiply by 2.1 t/m3 (MOT Type 1 density) to get tonnes
- Add 15% wastage
Worked example - patio
Patio 6m x 4m, 100mm compacted depth, MOT Type 1:
- Compacted volume = 6 x 4 x 0.1 = 2.4 m3
- With compaction factor = 2.4 x 1.3 = 3.12 m3 (loose)
- Weight = 3.12 x 2.1 = 6.55 tonnes
- Add 15% wastage = 7.53 tonnes to order
Without the compaction factor, the same calculation gives only 5.04 tonnes - that is 2.5 tonnes short on a single patio project.
How deep does the sub-base need to be?
| Application | Compacted depth |
|---|---|
| Garden path, foot traffic only | 75mm |
| Patio with slabs on mortar | 75-100mm |
| Block paving path | 100mm |
| Residential driveway, average soil | 100-150mm |
| Driveway on clay | 150-200mm |
| Driveway with occasional vans | 150-200mm |
| Under a concrete slab | 100mm |
When in doubt, go deeper. The extra material is not expensive compared to the labour cost of having to lift and re-lay a surface that moves.
Tonnes by area - quick reference
At 100mm compacted depth, MOT Type 1, including 1.3x compaction factor and 15% wastage:
| Area | Tonnes to order |
|---|---|
| 5m2 | 1.6t |
| 10m2 | 3.2t |
| 15m2 | 4.8t |
| 20m2 | 6.4t |
| 30m2 | 9.6t |
| 50m2 | 15.9t |
At 150mm compacted depth:
| Area | Tonnes to order |
|---|---|
| 5m2 | 2.4t |
| 10m2 | 4.8t |
| 20m2 | 9.6t |
| 30m2 | 14.4t |
| 50m2 | 24.0t |
My tips for sub-base work
After some expensive mistakes and a lot of projects, here is the honest version of what you should know:
Hire the plate compactor. I cannot stress this enough. A hand tamper does not compact sub-base material properly. A rented plate compactor from a hire centre is typically £30-50 a day and makes an enormous practical difference to the finished stability. Compact in 100mm layers - do not try to compact 200mm in one pass.
Compact the edges properly. It is easy to under-compact the edges and corners of a sub-base. Run a final pass along each edge parallel to the boundary. Edge settlement is one of the most common causes of slabs rocking near the perimeter.
Set your falls at sub-base stage. The sub-base is the right place to establish your drainage falls - typically at least 1:60 away from buildings. Do it here rather than trying to correct it at surface level.
Order a bit more than the calculator says. Even with the calculator, I always add an extra 0.25-0.5 tonnes on top of the calculated figure. Unexpected soft spots, a slightly lower-than-expected subgrade level, or the spreading process itself all eat into the material slightly. Running out with two square metres to go is one of the most frustrating moments in any project.
Use a geotextile membrane. Laying a non-woven geotextile (not a lightweight weed membrane - a proper heavy-duty geotextile) between the natural ground and the sub-base prevents the stone from being pushed down into soft soil over time. It adds years to the life of the whole structure and costs almost nothing.
Use the Sub Base Calculator to get your quantities before ordering.