To create a basic gable roof, two walls must be specified as Full Gable Walls in the Wall Specification dialog.
To create a gable roof
Click on the floor plan view window to make it the active view.
Select the Roof Tools
parent button, then click Delete Roof Planes .
Click the Select Objects
tool, select the vertical wall on the left, hold down the Shift key, and select the vertical wall on the right. The two walls should be group-selected.
Click the Open Object
edit button and on the Roof tab of the Wall Specification dialog, check Full Gable Wall and click OK.
Select Build> Roof> Build Roof
to open the Build Roof dialog check Build Roof Planes, and click OK.