Screed thickness for underfloor heating - is 6 cm sufficient?

According to our construction service description, 6 cm of floating screed (on 12 or 6 cm insulation) is planned - the heating engineer now said that ideally he would need 0.5 cm more. The underfloor heating pipes have a diameter of 17 mm. The builder says he would always do it that way and there have been no problems so far. Did you have more than 6 cm of screed? I’m a bit confused.

Thank you and best regards.

Much more important, what kind of screed is that? With [Fließestrich], the coverage of the pipes does not need to be as thick. With [Zementestrich], the coverage must be thicker.

floating screed. We checked once, a standard states 4.5 cm coverage.