A gambrel or barn style roof has two pitches on each side of the ridge. The first (lower) pitch on either side is steeper than the pitch near the ridge.
To convert the saltbox to a gambrel
Click on the floor plan view window to make it the active view.
Click the Roof Tools
parent button, then Delete Roof Planes .
Group select both horizontal walls and open them for specification. On the Roof tab:
Click OK to close the Wall Specification dialog.
Open the two vertical walls for specification, they should remain Full Gable Walls.
Open the Build Roof dialog, check Build Roof Planes, and click OK.