How deep should the laying course be beneath patio slabs?
A laying course beneath patio slabs is generally about 4 cm thick. Under DIN 18318, the permitted laying-course thickness is 3 to 5 cm, measured after compaction. The allowance required for bedding-in and compaction must be taken into account when screeding, and the precise allowance depends on both the laying material and the installation method.
The properly compacted sub-base beneath the laying course must not settle afterwards. At a thickness of less than 3 cm, the laying course can no longer reliably compensate for dimensional deviations in the slabs or minor irregularities. It should not be more than 5 cm thick: an excessively thick or unevenly laid course may compact further in use, causing the surface to sink in places.
The patio is supported by the compacted crushed-stone sub-base. Larger irregularities must therefore be removed before the laying course is screeded and must not be compensated for with a thicker layer of chippings.
Different aggregate gradings may be suitable as laying material, depending on the surfacing, loading and construction. DIN 18318 permits graded mixtures of crushed sand and chippings as well as certain crushed aggregates without fines. Suitability for the particular surfacing, particle shape, particle strength, water permeability and filter stability are decisive. Rounded aggregate is generally less favourable because it interlocks less effectively.
Because patio slabs made from rubber granules are elastic, their bearing surface must be especially even and stable. For these patio slabs, the thinner the chippings bed, the better: it serves only for fine height adjustment and should, depending on the slab type, be about 0.5 to a maximum of 2 cm thick. A laying-course thickness of 3 to 5 cm, as commonly used for rigid patio slabs, is unsuitable. Loose chippings can move under load, and elastic slabs follow depressions in the chippings bed.
No specific recommendation is made here as to which of the graded mixtures of crushed sand and chippings 0/4, 0/5 and 0/8, or crushed aggregates without fines 1/3 and 2/5, should be used.