The Ceiling by Room option inserts a ceiling inside the perimeter of a room provided all the walls are connected. Adding ceilings by room allows you to have different ceiling types in different rooms.
To insert a ceiling inside a room:
Make sure the location you want to insert the ceiling on is the current location in the building locations drop box.
Select Insert > Ceilings > Ceiling by Room, or click the Ceilings button on the Building toolbar and select Ceiling by Room.
In the catalog, select the ceiling type you want to insert.
Click inside the room that you want to add the ceiling to. The ceiling is added automatically. (If you want, you can continue adding ceilings to other rooms.)
Right-click and select Finish from the shortcut menu.