How thick can the gravel be under the paving slabs?
Firstly, it must be noted that only specific terrace slabs can be laid on a crushed stone bed. The essential requirement is that the terrace slab does not deform or break under load. Typical terrace slabs for installation on a crushed stone bed are concrete or natural stone slabs, each with a minimum thickness of 40 mm.
A layer of crushed stone is not a load-bearing base! Crushed stone cannot be installed in a stable layer nor compacted. Therefore, if the layer of crushed stone (or gravel or sand) is too thick beneath the terrace slabs, it may shift. Crushed stone is only permitted as a leveling layer on top of a compacted gravel base!
On the most even and properly sloped gravel base available, a layer of crushed stone is applied as a leveling layer and then smoothed over. The thickness of the crushed stone layer should be kept to a minimum to ensure a permanently stable positioning of the terrace slabs.