On Land

Environment Information
At Rill Architects we run ArchiCAD on Mac OS X. If you work at Rill, this is your stuff. If you don't, but you work in ArchiCAD, you may find something interesting. Anybody else, I don't know.
RSS

As you know, you don't cut the grade mesh using the building elements. You place slabs in the shape of the holes you need to fit the building.

You might find cases where it's more appropriate to use a roof:

Roof Cuts Site

Create the roof with the pivot line at one edge. Go to a section and fix the slope graphically, using the slope button.

Roof slope edit