You can use phase filters to control the flow of building model information into views and schedules.
How to change a roof in new renovation revit.
This should keep you existing roof in tact and show a cut in the new construction phase.
See hd version of video tutorial here.
A plan view representation of the roof displays.
Https youtu be pl5jqj8btfc wall layers in revit tutorial.
Create a void extrusion and make to required cut in the new construction phase apply the cut and done.
Roof construction in revit.
If the go to view dialog displays select the appropriate plan view in which to edit the profile.
In either case you ll probably need to consult with a structural engineer to make sure the home itself can handle the change.
Select the extruded roof and click modify roofs tab opening panel vertical.
Hope this is of assistance.
If anyone is still looking for a solution you can make a roof cut into an existing roof using component model in place.
Many projects such as renovations proceed in phases each representing a distinct time period in the life of the project.
Cut openings in an extruded roof.