"Thanks for the flowers", superzapp! It is indeed as you explained. In construction work, it is common to specify height measurements as negative and in mm because one always refers to the later ZERO line (with respect to the binding meter mark). -10mm thus means that with a standard thickness of the so-called smoothing compound of 2mm, 8mm remain for the top layer. No, it is indeed true that 20mm (i.e. 2cm) deviation no longer represents the limits of surface flatness. And smoothing (smoothing compound) is always necessary with elastic as well as textile floor coverings, but also with the click variants always. One reason for this is that the screed layer can only achieve the flatness of a screed according to line 3 of DIN 18202 (Table 3) with increased effort. It does not have to! It should not and cannot be relevant if complaints arise due to perhaps 200,--€ additional costs (for a smoothing compound) and, for cost savings, whatever the click method, it is simply applied directly to the screed. This will backfire. And very, very quickly. The saving possibly leads to "soft" zones in the usable plane when walked on and it crunches because the screed is breaking down (no fine smoothing compound was applied) and every step will be accompanied by a fine cracking noise. But a client can and may do that! Because he must live with the consequences he (consciously) caused - and, if necessary, also pay for the dismantling so that then someone who knows about floor technology carries out the covering installation. BUT: he (the client) will certainly not really save by not applying smoothing compound!! It has gotten late, good night to all! KlaRa