Locking system, floorboard comprising such a locking system, as well as method for making floorboards

A locking system for mechanical joining of floor-boards of the type having a body, opposite first and second joint edge portions and a balancing layer on the rear side of the body, adjoining floorboards in a mechanically joined position having their first and second joint edge portions joined at a vertical joint plane, said locking system comprising:

• a) for vertical joining of the first joint edge portion of the first floorboard and the second joint edge portion of an adjoining second floorboard mechanically cooperating connectors in the form of a tongue groove formed in the first joint edge portion and a tongue formed in the second joint edge portion, and
• b) for horizontal joining of the first joint edge of a first floorboard and the second joint edge portion of the adjoining second floorboard mechanically cooperating connectors comprising
• a locking groove formed in the underside of said second floorboard and extending parallel to and at a distance from the vertical joint plane at said second joint edge portion and having a downward opening, and
• a strip integrally formed with the body of said first floorboard, which strip at said first joint edge portion projects from said vertical joint plane and at a distance from the vertical joint plane has a locking element, which projects towards a horizontal plane containing the upper side of said first floorboard and which has at least one operative locking surface for coaction with said locking groove,
• the strip forming a horizontal extension of the first joint edge portion below the tongue groove,
• wherein the first joint edge portion, within an area defined by the bottom of the tongue groove and the locking surface of the locking element, is modified such that said area differs from the rest of the first floorboard with respect to a material property, a material composition, or a structure of the balancing layer.