Do not use the join geometry roof tool for this task or you may encounter an error when creating the dormer opening.
How to cut into a roof on revit.
In the family category and parameters dialog select cut with voids when loaded and click ok.
Check out the full thing.
An option would be to cut hole in the roof is to use roof voids the following way.
Create an in place family roof at choosing void extrusion.
Structure tab opening panel.
Click modify tab geometry panel cut drop down cut geometry.
Ideally in a section view draw the contourline of the void using the pick line option on the base of the inside edge of the roof.
After adding a dormer cut an opening for it through the roof.
Create the walls and roof elements that make up the dormer.
Use one of the opening tools to cut a vertical or perpendicular opening in a roof floor or ceiling for example to accommodate a chimney.
Select the object to be cut.
Click by face or vertical.
Load the family into the project and place it.
Autodesk revit structure 2014 hope you enjoyed this lesson on creating roof openings.
Architecture tab opening panel.
Open the revit family with unattached voids and click create tab properties panel family category and parameters.
Add vertical or by face opening this is part of the revit pure basics package.
Determine the length of the void ideally in 3d view at wireframe mode extend this in order to intersect the roof surface that will be cut.
Click by face or vertical.
Use the join roof tool to join the dormer roof to the primary roof.