I need to create a Saltbox style roof, which is similar to an Offset Gable, in my plan. How can I create this?
A saltbox roof can be created automatically by generating an additional floor level and utilizing the Knee Wall setting located on the Roof panel of the Wall Specification dialog.
, then click and drag out 4 walls to create a basic 30' x 40' structure. 
edit tool.
, and in the New Floor dialog that opens, choose to Derive new 2nd floor plan from the 1st floor plan, then click OK.

edit tool.
A knee wall's height is defined by the roof above, not the ceiling height.

from the menu, check the Build Roof Planes box, then click OK to see the changes reflect in your plan.