Set the height of the pivot line at 'b' in screenshot. Then when you draw the actual shape of the roof, start it at this same spot/line.
In a typical wood frame, the pivot line is the plate line along the inside face of stud. Other construction types can be different. WHen you draw your roof you can start it anywhere, at the eave edge if you want, where the eave edge is vertically will be set by the pivot plane height and the roof slope.
For domed roofs, you are not drawing the pivot line. With this roof type the center of the dome is the start height of this roof type. It too is set in 'b'.
HTH
Erika
Architect, Consultant
MacBook Pro Retina, 15-inch Yosemite 2.8 GHz Intel Core i7 16 GB 1600 MHz DDR3
Mac OSX 10.11.1
AC5-18
Onuma System
"Implementing Successful Building Information Modeling"