A method of creating a watertightness barrier for a wall of a watertightened thermally insulating tank, involves steps of: arranging a repeating structure including alternately a strip of sheet metal and an elongate welding flange connected to the support surface, so that the turned-up lateral edges of the strip of sheet metal are positioned against the adjacent welding flanges, welding the turned-up lateral edge to the welding flange using a straight welded seam along a first longitudinal portion, continuing the straight welded seam with an end portion which is deviated in the direction of an upper edge corner, and producing a watertight edge corner welded seam along a second longitudinal portion of the strip of sheet metal such that the edge corner welded seam watertightly meets the end portion of the welded seam.