Like Martin, I'd never noticed that command before ...
From the HELP files - for anyone who's interested:
Using Clip Roof to Walls
Use the Clip Roof to Walls command to cut complex shaped holes into the roof plan. For example, the command can be used when the upper floor of a house is nested in the roof of the main floor.
To Use Clip Roof to Walls
1. With the roof plan displayed, select Draw > Hole > Clip Roof To Walls.
2. Click on the roof section you want to cut a hole in. The selected roof becomes highlighted.
3. Click on each wall around the perimeter of the section you would like to cut the hole out of. The walls will highlight.
4. Right-click and the hole will be cut.