The Metal Foundation is the fourth unlockable foundation. A foundation is required to build any kind of housing/storage structure.
A Metal Foundation can be placed on even or slightly uneven ground. Once the first Metal Foundation is placed, subsequent Metal Foundations can snap into place along the first one, allowing them to line up, provided that there is adequate room and ground to do so.
A foundation provides the structural support to allow the placement of walls, ceilings, and other such objects, but only when the foundation is directly underneath them. It also supports up to two ceilings in any direction on the x and y-axis, and up to 1 ceiling diagonally. However, the ceilings on the far end will not support walls.
If a foundation 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.