How do I correctly join roof planes?

Issue description

I am trying to join a dormer roof to the main roofing system, but I can’t seem to connect them properly as it looks incorrect on the inside but it’s fine outside.

Solution

1. Create the roofs as separate entities, and place them in the correct position on the plan. 
2. Select both roof systems and right-click and select Connect > Trim elements to Roof/Shell. A prompt will pop-up in the process. Click Use Roofs/Shells from current selection then click Trim to close.