Abstract: An automatically self-erecting barrier which prevents ingress of flood water into a building. A float is provided which when inactive may serve as a doorstep, and which when active rises with the flood water to extend a barrier across a lower part of a doorway into the building. The barrier is guided at its lateral edge, and vertical channels may be fixed to the building on either side of the doorway. The barrier may be sealed to the channels by inflatable sealing elements.