Sometimes a parapet is created just by virtue of installing a roof lower than the top of the wall, as is not uncommon with tilt-up construction.
If the walls are structural, then the supports for the roof structure are built in ahead of time.
If there is a separate structure, then the walls just attach to it and stick up higher.
The roof materials will need to be properly flashed into the parapet and the parapet will perhaps get a cap.
I can't tell if I'm giving you information you want, or if I'm totally off base. The right answer might be, "Use Revit's Wall tool before you use Revit's Roof tool."
