Clipped Ceiling

[grenouillaise]

I am using AHD8 Build 15 and wish to create a second-storey room with a gambrel roof, BUT ---
I want the ceiling to be at the line of the pitch change, not at the top of the side walls, thus creating a room with gables each end and 'clipped' walls on the long sides. I have managed to do this using invisible knee-walls on floor 2, but these appear in the glass-house view and also affect the room definition. Any suggestions?
Peter
Melbourne Australia