The most expensive block paving job I have ever seen was a driveway that had been relaid three times in twelve years. The original job was lovely - charcoal and buff blocks in a herringbone pattern, looked fantastic. But within two years, ruts had appeared where the car wheels always tracked, and the area near the dropped kerb had sunk noticeably.
The second relaying addressed the most visible problems but did not touch the sub-base. Two years later it was rucked again. The third time, a reputable contractor dug everything out to 250mm, put in 150mm of properly compacted MOT Type 1, and relaid it correctly. That was six years ago and it still looks perfect.
Three times the labour and materials cost. All of it preventable with the right sub-base first time.
The full block paving build-up
Block paving is a flexible system that sits on a bed of sand over a compacted sub-base. Understanding the complete build-up explains why each layer matters.
From bottom to top:
| Layer | Depth | Material |
|---|---|---|
| Compacted subgrade | - | Natural ground, well compacted |
| Geotextile membrane | - | Non-woven geotextile |
| Compacted sub-base | 100-150mm | MOT Type 1 |
| Sharp sand bed | 25-40mm | Coarse sharp sand |
| Block paving | 60-80mm | Concrete or clay blocks |
| Kiln-dried sand | - | Joint filling after laying |
Total excavation depth for a driveway: approximately 220-270mm below finished surface level.
Each layer has a specific function. The MOT Type 1 distributes load across the ground. The sand bed allows fine adjustment of levels and gives the blocks a consistent bedding depth. The blocks themselves flex slightly as a unit, which is why block paving handles ground movement better than rigid concrete.
Recommended sub-base depth
| Application | Ground type | Sub-base depth |
|---|---|---|
| Pedestrian path | Any | 75-100mm |
| Patio (light use) | Firm | 75-100mm |
| Residential driveway (cars) | Firm sandy/chalk | 100-125mm |
| Residential driveway (cars) | Average soil | 100-150mm |
| Residential driveway (cars) | Clay | 150-200mm |
| Driveway (occasional vans) | Any | 150-200mm |
| Commercial driveway | Any | 200mm+ |
For most residential driveways, 150mm is the safe standard. It costs slightly more than 100mm in material but provides meaningfully better resistance to the combination of vehicle loading and UK clay ground conditions.
How much MOT Type 1 do I need?
MOT Type 1 requires a 1.3x compaction factor - because it compacts by approximately 20-30% when properly laid. The quantity to order is:
Tonnes = Area x Compacted depth x 1.3 x 2.1 (density) x 1.15 (15% wastage)
Quick reference at 150mm compacted depth
| Driveway area | Tonnes to order |
|---|---|
| 15m2 | 7.1t |
| 20m2 | 9.5t |
| 25m2 | 11.9t |
| 30m2 | 14.3t |
| 40m2 | 19.1t |
| 50m2 | 23.8t |
For accurate quantities including the compaction factor, use the MOT Type 1 Calculator or Sub Base Calculator.
The sand bed: thickness and material
The sand bed on top of the sub-base should be:
- 25-40mm deep, typically 30mm is the target
- Coarse sharp sand - not soft building sand
- Screeded level and at the right falls before blocks go down
- Not compacted before laying blocks - it needs to remain loose enough to allow blocks to bed in
A common mistake is making the sand bed too thick to compensate for an uneven sub-base. The sand bed should be consistent at 25-40mm across the whole area. If the sub-base is uneven, correct the sub-base - do not try to use the sand bed as a levelling layer.
Edge restraints: essential for block paving
Unlike a concrete slab, block paving has no structural connection between individual blocks. The whole surface is held in place by edge restraints around the perimeter. Without them, blocks migrate outward over time, gaps open up and the surface gradually spreads and loosens.
Edge restraints for block paving include:
- Concrete kerb channels (most permanent)
- Haunched concrete edging
- Pre-cast edging blocks on concrete haunching
- Solid boundary features (walls, fences on concrete footings)
Edge restraints need to go in and be haunched in concrete before any blocks are laid.
My tips for block paving sub-base
Having seen block paving done well and very badly, here is what makes the difference:
Check the sub-base level very carefully. Block paving reveals every imperfection in the sub-base because you can see where individual blocks are slightly proud or slightly low. Run a long straight edge across the screeded sand bed before laying any blocks. Fix any hollows in the sub-base before you get to the sand stage.
Hire the plate compactor for two days. You need it for the sub-base compaction and again after the blocks are laid (with a rubber plate attachment or rubber pad to avoid marking). One day is rarely enough.
Order more blocks than you think. The standard 10% wastage is a minimum for a rectangular driveway. If there are curves, diagonal cuts, multiple colours or complex patterns, go to 12-15%. Running short when the blocks are in full production is a painful experience.
Check the colour batches match. Block paving from different production batches can be noticeably different in colour, even of the same nominal colour. Buy all your blocks from the same batch code where possible, or mix blocks from different packs as you lay to average out any variation.
Use the Sub Base Calculator to get your MOT Type 1 quantities before starting.