After placing a new roof on top of four walls, there is a gap between the bottom of the roof and the top of the walls. Even after selecting Floor 2 (which I didn't do the first time) the roof is still not located correctly on top of those walls.
The shape placed to create the roof was positioned slightly above the top of the walls, therefore the bottom of the roof created from it was at the same elevation. Both ACS Plane and ACS Plane Snap locks were enabled, but since the Z coordinate at the top of the walls did not match Floor 2's elevation it resulted in the shape being placed at the floor elevation thereby creating a gap between it and the top of the walls.
In order to resolve this issue the ACS Plane and ACS Plane Snap locks should be temporarily disabled so that snapping to the top of the walls places the shape at that Z coordinate instead of the active Floor's elevation.