Brett Brown,
Thanks a lot! But still, I'm not able to create the roof as it needs to be.
I also never used the Shell Tool in ArchiCAD before.
I guess if it's that complex to create that kind of roof, it just won't work and I need to take peace with the roof ArchiCAD generates by using the cmd-button.
On the picture you can see the building I'm drawing.
It's bingmaps, so it's not that clear.
Maybe there's still someone reading this that hopefully knows a simple solution.
Thanks again, Brett!
Kilian