Normally when you are modelling a roof in revit you will have a level set up to control it s height this may by a level that is also controlling the top of your external walls.
Revit roof plam.
Make sure the floor plan view you choose to work in is associated with the level at which you want to create your roof at.
In the view properties expand the underlay drop down and select the level on which the roof was created to display as an underlay.
Open the plan view of the level in which you want to see the roof line.
Display a floor plan view or a reflected ceiling plan view.
Some may call this wysiwyg or what you see is what you get.
If you try to add a roof on the lowest level a dialog prompts you to move it to a higher level.
If you choose not to move the roof to a different level revit notifies you later if the roof is too low.
Out of the box revit doesn t do this because revit shows it as it s built.
In residential architecture as well as other types it is often a required part of the working drawing set to be able to show the roof overhang as a hidden line on the floor plan views.