Abstract: A system and method for protecting a building against flooding caused by the overflow of a liquid from a liquid drain line into the building to which the drain line is connected. A floor drain, connected to a drain line, is connected in a watertight manner to a vessel having a hollow interior which rises a preselected distance above the floor drain to permit a liquid overflowing the floor drain to rise in the vessel. The vessel is vented to atmosphere a preselected distance above the floor drain to permit liquid in the hollow interior to recede through the floor drain as the overflow subsides. The vessel may contain a drain hole therein near the floor which can be selectively opened or closed to permit a liquid on the floor to be drained through the floor drain. The vessel employed may be a conventional ceiling supporting column.