Put your cursor immediately outside a perimeter wall. ( not on the wall but just ouside of the wall)
You will get a light blue border around the perimeter, click on it.
It will turn to a solid orange border, you will also notice that the Tool Options box is opened on the right side of the screen.
Click on the Options button. Remove all the checkmarks available to you.
Now go to the roof tab and delete all the roof sections.
The auto roof will not come back anymore, for that level.
For the second floor do the same.