Hans,
I have installed the appropriate plugins and I followed the slopingCeilingGuide, and was able to make a roof, but on one side (I think it was the side on the ground before rotation) there is a rectangle at right angles to the roof that won't go away. Only the side toward the roof is visible; from the other side it's transparent.
The roof was created from a single sloping wall; it has no peak. ( /= )
How can I get rid of the rectangle?