I draw your attention to the US Version Reference Guide accessed through Archicad's Help Menu.
Specifically pages 201 - 210. Of particular interest to users needing irregular roofs, the POLYROOF described on page 202 is probably most helpful.
Archicad Training Guides can be found through the HELP Menu at:
http://www.graphisoft.com/products/archicad/ac11/ITG/
I suggest the Basic Interactive Guide and scrolling down further to "Modeling".
If you post an image of what you are attempting or even the walls shaping the base, help will be quickly forthcoming.
Dwight Atkinson