This video demonstrates the following.
Revit roof dormer.
Move the walls to the dormer.
Use the modify sub elements button to select your newly placed points.
Create the roof opening and finish the dormer.
Select the wall type and constraints for a dormer that will be added to a roof.
Create the walls and roof elements that make up the dormer.
Add point or split line if better suited creates points at intersections place your new points as required.
Do not use the join geometry roof tool for this task or you may encounter an error when creating the dormer opening.
Select your flat roof press the modify sub elements button that appears.
Create the dormer roof and join it to the main roof.
After adding a dormer cut an opening for it through the roof.