When a levelling screed is to be used, the designer should consider both the surface regularity and departure from datum of the finished surface of the base slab, not only in relation to the type and thickness of levelling screed intended to be specified, but also the surface regularity and departure from datum of the finished surface of the screed.
The limits within which it is permissible to apply that type of levelling screed (e.g. bonded, unbonded or floating) should be considered. A nominal screed thickness close to the minimum value (or indeed maximum value) for that type of screed should not be specified unless the base slab can be laid to sufficiently close limits without prejudicing the requirements for level and surface regularity of the surface of the finished screed. It might not be possible to accommodate the levels of base slabs that are outside the specified tolerance limits using levelling screeds at the thicknesses recommended in this standard.