However this is not always straight forward with Revit. What I’ll do is go through many of the little items you should know about roofs, which will help you in your understanding.
It should be worth knowing that if you place a roof by footprint using walls the level of the roof will be different from using lines. By lines the bottom level of the roof will be off the bottom of the facia. By using walls, the level of the room will be determined by where the roof meets the walls.
Roof placed by wall.
Roof placed by line.
Truss or Rafter:
Setting the roof to Truss or Rafter affects how a “By Wall” roof sits in the wall.
By rafter sits the roof on the inside of the wall. By truss sits it on the outside.