Sub-base for block paving: layer depths

The sub-base under block paving is built up from the bottom: a geotextile membrane, a MOT Type 1 sub-base, and a 4-8mm laying course. Total depth is around 300mm for a path, 400-450mm for a driveway. The key is compacting the subgrade and every layer above it — on a properly compacted sub-base, paving doesn't sink.

Sub-base thickness depends on what will walk or drive over it. The difference between a path and a driveway isn't cosmetic: under a car, a thin sub-base rides and the paving drops in the wheel tracks.

Under heavier loads, add thickness and go to a coarser stone (MOT Type 3). If the subgrade is clay and holds water, allow for a thicker sub-base than the table shows, or drainage. Clay loses bearing capacity once saturated and moves in winter.

The area needs a fall of 1:100 to 1:50 away from the house — 10 to 20mm per metre. The fall is built into the sub-base, not made up in the paving. If you level out a difference in the laying course instead, it'll be thicker in places, and that's exactly where the area settles over time.

Block paving is only partly permeable, but water still needs somewhere to go. On larger areas, and on a driveway boxed in by a wall, a channel drain or a soakaway strip at the edge is worth having. Water sitting in the sub-base that freezes in winter heaves the paving up.

1. Compact the original ground at the bottom of the dig-out (skip this and everything above settles).
2. Roll out the geotextile membrane.
3. Lay and compact the MOT Type 1 in layers, not the full thickness in one go.
4. Screed the laying course level — don't compact it.
5. Once the paving is laid, vibrate the whole area with sand in the joints.

• Uncompacted dig-out. The most expensive mistake, because it doesn't show up for a year, and fixing it means lifting the whole area.
• MOT Type 1 tipped in at full depth. The plate compactor only compacts the top, and the bottom stays loose. Work in 100-150mm layers.
• Geotextile membrane skipped. The subgrade mixes with the stone over time, the sub-base loses thickness and the area settles.
• A thick laying course instead of a level sub-base. Sand isn't a levelling material. Above 50mm it compresses under load.
• Laying course compacted before laying. It should only be screeded level. Compaction happens once, on the finished paving with sand in the joints.
• Kerbs left out. Without edge restraint the outer rows spread and the pattern opens up.

Convert volume to tonnes using the density of the stone. The calculator on this site uses 1.7 t/m³ for MOT Type 1 and 1.6 t/m³ for sand, standard figures for dry crushed aggregate. Wet material weighs more, so a supplier may deliver a slightly smaller volume than you expect for the same tonnage.

Example: a 40 m² patio with a 200mm sub-base needs roughly 13.6 tonnes of MOT Type 1 and 2.6 tonnes of sand. Order with a small margin — volume drops during compaction, and the last load is always short.

Work out the tonnes of MOT Type 1 and sand for your area and layer thicknesses in the block paving calculator. Related: the step-by-step laying guide.