Warning: All parts of an extruded roof must face upward. Make sure that no part of the sketch lies below any other part of the sketch.
Issue: You have sketched an extruded roof in such a way that a part of the roof will lay under another part. Or you may have drawn vertical lines as part of your roof.
Solution: Sketch the roof edge such that lines join, but do not pass underneath one another. Also, remove any vertical lines in the sketch.