Corner cut out of roof

[Jason42]

is there a way to automatically create this roof?  The second image is what it auto rebuild does to it.  

[Jo_Ann]

Easiest solution using auto build roof?
Create  (build) a new floor; Choose 'make new blank plan'.
With the floor reference turned on, draw 4 new walls as shown.  Obviously set the front wall to be a gable.
Open the room, and start raising or lowering the ceiling height until the roof perfectly blends with the existing roof.