A method for protecting a building against flooding by use of a membrane connected along a first side edge to a float, and comprises the steps of arranging the membrane in a packed configuration at a first distance from the building, and fixing the membrane to the ground at a start location along a second side edge, allowing water to unpack the membrane and move the float toward the building to arrange a first portion of the membrane with a lower surface against ground in an area between the start location and the building, allowing water to raise the float a second distance along an outer surface of the building while further unpacking the membrane to arrange a second portion of the membrane with the lower surface against the building. The first portion forms a retention portion which is retained on the ground by water on an opposite upper surface.