Make 5 cubes, let all drop into world center.
Make all cubes positive.
Pick # 1 and pull up Y size control to make the roof.
On each of the 4 remaining use the Resize X and Z control to pull the 'walls' to the outside.
Select all, Group and make it positive (this is the room).
Any object you make next as a negative and group with any wall of the room (group1) will cut out an area.
See illustration.