thank you all for your time!
i managed to generate a cycloid with CCycloid 3.4.3 (
http://www.cgl.uwaterloo.ca/~smann/ccycloid/
) and then imported the *.dxf file into archicad; still had to use the spline though.
jean-luc regnier, thank you for the code!
Beart, i couldn't design a barrel vaulted roof with the magic wand; i'll check it out on the manual... thank you!
kliment, thank you too but can you try to mirror your half circle... and notice the difference?