Georgios,
Can we agree that what you want to make is what is commonly known
as a barrel vault roof with 180° of included angle ?
If so, there are several ways to accomplish this.
I will only describe two ways hear because they are the easiest
to describe and the easiest to do.
One- use the "Shell 10" object in the Basic shapes folder in the
Archicad library.
Two- Use TrussMaker. Draw an 180° arc in a section/elevation window
Make the radius of this arc the radius of the under side of vault
plus one half the thickness of the vault shell. Select the arc,
go to design extras->TrussMaker, select the cross section shape
solid beam, enter the vertical dimension as the thickness of shell
make the width three or four meters, and hit OK.
In plan you will see the 2D symbol of the barrel vault with
center line lying along the section tool line. Stretch the
2D symbol to the length of the vault you want and check
in the 3D window and see if the vault is what you want.
Peter Devlin