I think you're using the word "roof" to refer to ceiling. You are using walls to fine control the ceiling slope, but these walls are just control knobs, you don't want them visible. Am I right?
The best option is to throw away walls and ceiling and move to a real roof using the Roof generator plugin:
http://www.sweethome3d.com/support/foru ... read,11330
If you still prefer to use walls, look at the second point of this post:
http://www.sweethome3d.com/support/foru ... 1933#55677