Room-over-room designates methods allowing level design which transcends Doom's ordinary engine limitation of one floor height and one ceiling height at any given location on the (x, y) plane. Creating a room above another room is normally impossible, though the effect can be faked to a limited degree in the ordinary Doom rendering engine through various tricks, such as moving sectors, as in Industrial Zone's south-east building, or via self-referencing sectors. Through advanced source port features, notably 3D middle textures, 3D floors and portals, more fully convincing effects are possible.
The earliest implementation of room-over-room, provided by the RORDoom source port in 2000, allowed sectors to overlap. The implementation also found its way in the experimental build of Eternity called rorEternity. This approach is not in general use now.
Doom 64 simulates a room-over-room effect by changing the floor and ceiling heights of sectors via macros, depending on the direction from which the player approaches an area. This effect is noticeable while using a no-clipping cheat in Doom64 EX, since the sectors will not change properly. Such an approach is incompatible with multiplayer games; however, Doom 64 itself had no support for multiplayer.