How the block paving calculator works
Block paving is laid on a loose (screeded) bed of sharp sand over a compacted sub-base, with a concrete-haunched edge course to stop the blocks spreading. The calculator works out each layer:
Recommended build-up
| Use | Blocks | Sharp sand bed | Sub-base |
|---|---|---|---|
| Paths & patios | 50mm | 30–40mm | 100mm |
| Domestic driveway | 60mm | 30–40mm | 150mm |
| Driveway on clay / soft ground | 60–80mm | 30–40mm | 200mm+ with geotextile |
Tips
- Check drainage first. In England, new driveways over 5m² draining onto a road usually need permeable paving or a soakaway to avoid needing planning permission.
- Lay the edge course on a concrete bed with haunching before screeding the sand.
- Screed the sharp sand once, don't walk on it, and lay blocks from the edge working forward over the laid blocks.
- Brush in kiln-dried sand, compact with a rubber-mat plate compactor, then brush in more sand.
Worked example: an 8m × 5m driveway
40m² of 200 × 100mm blocks in herringbone: 40 × 50 = 2,000 blocks, plus 10% for the angled cuts at the edges = 2,200 blocks. With all 26m of the edge restrained, you need about 137 edging blocks and roughly 0.4m³ of concrete to bed and haunch them.
Underneath: 150mm of MOT Type 1 is about 13.2 tonnes (16 bulk bags), and a 40mm loose sharp sand bed is about 2.9 tonnes (4 bulk bags). Add about 180kg of kiln-dried jointing sand, or 8 × 25kg bags. At this size, a loose tipper load of sub-base is often cheaper than 16 bulk bags. Ask your merchant for both prices.
How deep to dig
Add the layers together: 60mm blocks + about 30mm of sand once compacted + 150mm sub-base = roughly 240mm below the finished surface. Finish at least 150mm below the house's damp-proof course, with a fall of about 1 in 60 to 1 in 80 away from the building. For this driveway that's nearly 10m³ of soil in the ground, closer to 12m³ once dug. That's too much for one skip, so consider a grab lorry. The excavation calculator and skip size calculator work this out.
Common block paving mistakes
- Thin or poorly compacted sub-base. Ruts and dips in a few years nearly always start here. Compact MOT Type 1 in layers of no more than about 75–100mm.
- Using building sand for the bed. The laying course must be sharp sand, which drains and locks. Soft sand stays mobile and the blocks sink.
- Weak edges. Without a concrete-bedded, haunched edge course, the blocks creep outwards and the joints open.
- Ignoring drainage rules. A non-permeable front drive over 5m² that drains onto the road can need planning permission in England. Permeable block paving or drainage into the garden avoids it.
- Skipping the second sanding. Kiln-dried sand settles into the joints after compacting. Brush in more a few weeks later, too.
Our sub-base depth guide explains depths for different ground. For a slab patio instead of blocks, use the patio calculator, and for the sub-base alone, the MOT Type 1 calculator.