"Lean-To" roof

[phoebes13]

Hi! I'm using Interior Designer 7.0 and trying to build a single room (using 4 exterior walls) to create the lean-to style addition on my house. Where it's attached to the rest of the house the ceiling height is 8.5', dropping back opposite the house to a knee-wall that is 67" high.

Can't get it to generate a roof no matter what I've tried. Any suggestions?