A locking system for mechanical joining of floorboards, each of the floorboards comprising a body comprising plywood with several layers; a locking groove which is formed in an underside of and extends in parallel with a first joint edge at a distance from the joint plane, the locking groove having an opening, a bottom, and two side walls; a portion projecting from a lower part of the second joint edge and below the first joint edge and integrated with the body of the board; said projecting portion supporting, at a distance from the joint plane, a locking element for cooperating with the locking groove; said projecting portion being located entirely outside the joint plane as seen from the side of the second joint edge; and the walls of the locking groove comprise at least two layers of the body.