Method of creating a watertightness barrier for a wall of a watertight and thermally insulating tank, comprising: arranging, on a thermally insulating barrier that has a flat support surface for the watertightness barrier, a repeating structure comprising alternately strips of sheet metal and elongate welding flanges connected to the support surface and projecting with respect to the latter so that the welding flanges run parallel to the strips of sheet metal over at least part of the length of the strips of sheet metal and so that lateral edges of the strips of sheet metal which are turned up with respect to the support surface are positioned against the adjacent welding flanges, for respectively a first and a second adjacent strip of sheet metal arranged on either side of a welding flange, watertightly welding the turned-up lateral edge of the strip of sheet metal to the welding flange using a straight welded seam parallel to the support surface along a first longitudinal portion of the strip of sheet metal, the straight welded seam being each time arranged beneath an upper ridge (32, 41, 43) of the turned-up lateral edge of the respective strip of sheet metal, characterized by the steps of: continuing each time the straight welded seam with an end portion (28, 29) of the welded seam which is deviated with respect to the support surface in the direction of the upper edge corner (32, 41, 43) of the turned-up lateral edge and which meets the upper edge corner (32, 41, 43) of the turned-up lateral edge, and producing a watertight edge corner welded seam between the upper edge corner of the turned-up lateral edge of the first strip of sheet metal and the upper edge corner of the turned-up lateral edge of the second adjacent strip of sheet metal along a second longitudinal portion of the two strips of sheet metal so that the edge corner welded seam watertightly meets the end portion (28, 29) of the respective welded seam of each of the two strips of sheet metal. arranging, on a thermally insulating barrier that has a flat support surface for the watertightness barrier, a repeating structure comprising alternately strips of sheet metal and elongate welding flanges connected to the support surface and projecting with respect to the latter so that the welding flanges run parallel to the strips of sheet metal over at least part of the length of the strips of sheet metal and so that lateral edges of the strips of sheet metal which are turned up with respect to the support surface are positioned against the adjacent welding flanges, for respectively a first and a second adjacent strip of sheet metal arranged on either side of a welding flange, watertightly welding the turned-up lateral edge of the strip of sheet metal to the welding flange using a straight welded seam parallel to the support surface along a first longitudinal portion of the strip of sheet metal, the straight welded seam being each time arranged beneath an upper ridge (32, 41, 43) of the turned-up lateral edge of the respective strip of sheet metal, characterized by the steps of: continuing each time the straight welded seam with an end portion (28, 29) of the welded seam which is deviated with respect to the support surface in the direction of the upper edge corner (32, 41, 43) of the turned-up lateral edge and which meets the upper edge corner (32, 41, 43) of the turned-up lateral edge, and producing a watertight edge corner welded seam between the upper edge corner of the turned-up lateral edge of the first strip of sheet metal and the upper edge corner of the turned-up lateral edge of the second adjacent strip of sheet metal along a second longitudinal portion of the two strips of sheet metal so that the edge corner welded seam watertightly meets the end portion (28, 29) of the respective welded seam of each of the two strips of sheet metal. continuing each time the straight welded seam with an end portion (28, 29) of the welded seam which is deviated with respect to the support surface in the direction of the upper edge corner (32, 41, 43) of the turned-up lateral edge and which meets the upper edge corner (32, 41, 43) of the turned-up lateral edge, and producing a watertight edge corner welded seam between the upper edge corner of the turned-up lateral edge of the first strip of sheet metal and the upper edge corner of the turned-up lateral edge of the second adjacent strip of sheet metal along a second longitudinal portion of the two strips of sheet metal so that the edge corner welded seam watertightly meets the end portion (28, 29) of the respective welded seam of each of the two strips of sheet metal.