So, you've got 2 roofs, drawn separately as they have different eaves heights, thicknesses and angles, and you want to join them together. Well the normal thing to do is to draw one roof, then the other, then use the join roofs tool to join them together. Easy.
What if you want to cut out the underlaying part of the main roof though? Simple. Use the dormer roof opening tool, select the roof you want to cut, then the roof that joins into it, and it will cut the opening in the main roof perfectly and neatly joining the 2 roofs.
No sketching lines or messy roof junctions. Brilliant!
(of course, you can also use the dormer roof tool for what it's intended, which incidently its very good at too!)