Stop your planar thinking.
These shapes are made with curved wall with a complex profile.
Make a swoopy section and then trim it with a Solid Element Operation [SEO]
Once you establish the ceiling geometry [in my illustration, the wire frame as a complex profile extruded as a wall], then you make a slab to match the irregular planar size and "Intersect" the SEO.
These section shapes can be stair or ramp-like, tilted and doubly-curved....
See:
http://www.archicadwiki.com/Creative%20Uses%20of%20Complex%20Profiles
and
http://www.archicadwiki.com/Solid%20Element%20Operations
And all words come from the Greek.
Dwight Atkinson