If you have just a roof- make a room, give it a roof, demolish the walls and floor again- that doesn't count. Floor patterns applied to the ground don't count either. Also, you can place fences as fences (rather than place walls and then make them fences) and they won't count.
Oh, and your levitating roof left over from when you demolished the room out from under it-- it will look less silly if you place some columns to hold it up.