How deep should a patio sub-base be?
Last reviewed 1 July 2026
Short answer
100 mm of compacted sub-base for a pedestrian patio on firm ground, 150 mm over soft or clay subsoil, and 150–200 mm where vehicles will cross. Order about 30% more loose material than the compacted depth — a 100 mm compacted layer over 20 m² needs roughly 2.6 m³ of loose Type 1, not 2 m³.
Work it out for your project with the patio and paving calculator →
Depth by use and ground
| Use | Firm subsoil | Soft or clay subsoil |
|---|---|---|
| Footpath | 75–100 mm | 100–150 mm |
| Patio, pedestrian only | 100 mm | 150 mm |
| Patio with occasional light vehicle | 150 mm | 200 mm |
| Car driveway | 150 mm | 200 mm |
| Driveway with delivery vehicles | 200 mm | 250 mm or designed |
Depths are of compacted material, measured after whacking down.
The compaction allowance
MOT Type 1 and equivalent dense graded bases lose roughly a quarter to a third of their loose volume when compacted. The standard working figure is a factor of 1.3.
So a 100 mm compacted layer needs about 130 mm of loose material spread before compaction. Across 20 m² that is 2.6 m³ loose, which at 2,000 kg/m³ is about 5.2 tonnes — six UK bulk bags rather than the four you would order from the compacted volume.
This is the single most common reason a sub-base order comes up short.
loose volume = area × compacted depth × 1.3
Laying it properly
- Excavate to a firm, undisturbed formation. Remove all topsoil and organic material — it will rot and settle.
- If the formation is soft or wet, consider a geotextile separator so the sub-base is not pumped into the subgrade.
- Spread in layers of no more than 100 mm. A plate compactor cannot compact more than that in one pass.
- Compact each layer until the surface stops moving under the plate — typically three or four passes in different directions.
- Check levels and falls as you go. A patio should fall about 1:80 away from the building.
- Blind the surface with a thin layer of grit or sharp sand if it is too coarse to screed the bedding onto.
Important: The sub-base is where a patio is won or lost
Working out the excavation
Total dig depth is the sub-base depth plus the bedding depth plus the slab thickness. For a typical patio that is 100 + 40 + 35 = 175 mm below the finished paving level.
Remember the finished level must be at least 150 mm below the damp-proof course of any adjoining building.
The spoil has to go somewhere. Excavating 175 mm over 20 m² produces about 3.5 m³ of soil, which is roughly 4.5 tonnes and a full builders' skip.
Safety notice: Material planning only