Abstract: Apparatus for preventing flood water entering a building through a wall opening has a first member for permanent attachment to the wall and second member detachably fixed to the first member. The first member has a frame having first fixing mechanism and two upright members for permanent fixing to the wall around the opening and a third member bowed away from the building and having a flat upper surface. The second member has a curved member with circumferential flanges and a second fixing mechanism engaging the first fixing mechanism. Two flanges extend outwardly from the curved member, and third flange is perpendicular to flanges and to the curved member. The flanges fit flat against the members of the frame through a gasket with the third flange extending horizontally on top of the third member forming a step.