This video demonstrates the following.
Revit roof dormer.
Create the dormer roof and join it to the main roof.
Move the walls to the dormer.
Create the walls and roof elements that make up the dormer.
To use this do the following.
Use the modify sub elements button to select your newly placed points.
Do not use the join geometry roof tool for this task or you may encounter an error when creating the dormer opening.
Use the join roof tool to join the dormer roof to the primary roof.
After adding a dormer cut an opening for it through the roof.
Create the roof opening and finish the dormer.
Select the wall type and constraints for a dormer that will be added to a roof.