The Metal Ceiling is the fourth unlockable ceiling. A ceiling provides protection from the elements, giving bonuses to hypo and hyperthermic insulation.
A Metal Ceiling requires placement on a Wall, Pillar, Foundation, or another ceiling.
Once the first Metal Ceiling is placed, subsequent Metal Ceilings can snap into place along the first one, allowing them to line up, provided that there is adequate room to do so.
A ceiling requires support from a nearby foundation or pillar in order to be placed. The ceiling can only be placed up to 2 ceilings away in any direction on the x or y-axis, and up to 1 ceiling away diagonally from a wall or another ceiling that is directly above a connected foundation or pillar. The ceilings on the far ends of this setup will not be able to support walls.
If a ceiling is destroyed, any building pieces that rely on it for support will be destroyed as well. If it was destroyed via the demolish option, no resources will be refunded for these extra building pieces.
Currently, you should not keep your creatures on ceilings. A bug can cause some of them to occasionally fall through the structure, forcing you to destroy part of the building in order to get them out.