Do not use the join geometry roof tool for this task or you may encounter an error when creating the dormer opening.
Revit roof dormer.
Move the walls to the dormer.
Create the walls and roof elements that make up the dormer.
Revit 2021 inclined rafter joining roof rafter to dormer rafter paul building and architectural design autodesk beam inclined micrographics revit 2021 roof rafter when a dormer roof joins the main roof the rafters are not in line.
After adding a dormer cut an opening for it through the roof.
To use this do the following.
Create the roof opening and finish the dormer.
This video demonstrates the following.
Select your flat roof press the modify sub elements button that appears.
Use the modify sub elements button to select your newly placed points.
Use the join roof tool to join the dormer roof to the primary roof.