At least two factors have an impact on whether or not the located terrace slabs are just.
First the solid ground is not always with a flatness within (one or two) millimetres.Second., also have the located terrace slabs their thickness tolerances.
In order to compensate for these influences, together with the paving support pads, the levelling shims offered. The levelling shims have (depending on the manufacturer) a thickness of 1.5 to 3 mm. Also include the levelling shims cutouts for the joint crosseson the paving support pads.
The levelling shims are placed on the paving support pads und then the terrace slabs laid on.