Erez,

1 - Delete the low wall and replace it with a box that has the length, height and thickness of your wall. Apply colour(s) and/or texture(s) if needed.
2 - Use Split wall to make "support" points for the ceiling.
3 - Make sure that the ceiling corners are on the exact spot of these points, using Edit room points from the AdvancedEdit-plugin.

It's really simpel.

Hans