From the Super Mario Wiki
Roof is the name of a room cleared in Area 4 in Luigi's Mansion. The player can traverse the mansion's roof through an elevator in the Clockwork Room. The key to the Armory and the entrance to the Sealed Room can be found here. The player is also transported here for the final battle against King Boo in the Bowser suit, though here it is not labeled on the map, as the edges are engulfed in flames.
Ghosts on the Roof
Ghosts on the Roof in the PAL Hidden Mansion