Thank you for the reply. Both of you.
I am working with cm (centimeters).
My problem was, that I needed to set 1 Decimal in the Working Units (both for Model and Layout units)
Now I can enter .5 as a layer thickness into the Composite structures. Andro this is very strange, maybe we are using different types of Archicad 14. At my place I can enter numbers with .5 but not with ,5
Now I am interested in new problem: Why Dimension tool does not show the exact value of mortar thickness? The thickness is 1.5 cm, but when I add a dimension marks on my wall, the value varies to 1.5 +- 0.10. Why is that?
Take a look at the video, maybe it will be more clear:
http://www.youtube.com/watch?v=w6M3ZfSuaFI