.obj file works well. Cool house, but I am glad I'm not your builder...
Either you can use 3d modelling software, or you can look at hansmex' roof-guide: http://www.sweethome3d.com/slopingCeilingGuide.pdf
In short, it is probably doable by using separate SH3D drawings of slanted walls in the correct angle which you later export to .obj files and re-import and rotate. But it is not all that straightforward.
For this kind of roof, I would prefer 3d modelling software that can work with accurate dimensions. But learning how to use such software will require a bit of patience....
ok