Dave,
the top of the building will be above eye level so it does not have a top. If you look at the top of the walls, you can see that I painted it black so if you did see it, it looks like a roof. On a previous layout the building was lower so I cut a roof piece for the opening then drew and cut out a circle for the post. I then cut it in half and set it on some wood going around the top of the building walls.