Here's the way i use to do roofs. It's really clean and no brush goes through each other. To start, make a set of hollow walls: Then, after making them higher, go down to 4 units and make the roof plate, and with vertex editing make it diagonal. Now select the walls and with the clipping tool follow the vertices of the plate and clip off the walls that go through it. Now duplicate and flip the plate and clip off the walls on the other side. The end result: Enjoy!