I have created a perimeter on the first floor. Then created a second floor. Then created a smaller perimeter for the 'floor and a half '. I build the roof and it looks good except all the walls are missing on the floor and a half level. I thought you needed walls to make a roof unless you used room dividers. I didn't use room dividers and got the same effect somehow. I would prefer to see the walls. Any thoughts as to what tick I may have erroneously ticked? Thanks!