How deep should the laying course be beneath patio slabs?
The laying course should be 3 cm deep in its finished, compacted state. The acceptable range is 2.5 to 4 cm, also measured after compaction. When screeding with a screed board, allow about 1 cm extra because the layer settles beneath the laid patio slabs. Below 2.5 cm after compaction, the bedding can no longer accommodate thickness tolerances in the slabs.
The upper limit is 4 cm in the finished, compacted state. Above this depth, the layer may compact further in use and parts of the patio may sink. The patio is carried by the compacted sub-base below. Any unevenness in the sub-base should be corrected before screeding, rather than concealed by increasing the bedding depth.
These depths apply to the stable, compactable bedding aggregates commonly used in Britain, including 0/4 grit or sharp sand and 0/6 grano dust. They do not apply to rounded decorative gravel, which remains mobile within the laying course.
With rubber patio tiles, the levelness of the screeded surface matters more than the laying course depth. A rigid patio slab may bridge a small hollow, while an elastic rubber tile follows it.