Locking system for mechanical joining of floorboards and method for production thereof

Floorboards provided with a locking system for mechanical joining of said floorboards, said floorboards having a body, said locking system comprising:

• for horizontal joining of a first and a second joint edge portion of a first and a second of said floorboards, respectively, at a vertical joint plane, a locking groove formed in the underside of said second board and extending parallel with and at a distance from said vertical joint plane at said second joint edge and a strip integrally formed with the body of said first board, which strip at said first joint edge projects from said vertical joint plane and supports a locking element, which locking element projects towards a plane containing the upper side of said first floorboard and which locking element has a locking surface for coaction with said locking groove, and
• for vertical joining of the first and second joint edge, a tongue which at least partly projects and extends from the vertical joint plane and a tongue groove adapted to coact with said tongue, the first and second floorboards within their joint edge portions for the vertical joining having coacting upper and coacting lower contact surfaces, of which at least the upper contact surfaces comprise surface portions in said tongue groove and said tongue, wherein the upper and lower contact surfaces are essentially plane-parallel and extend essentially parallel with a plane containing the upper side of the floorboards,
• the locking element comprises an upper edge which is closest to a plane containing the upper side of the floorboards, the upper edge being located in a horizontal plane which is positioned between the upper and the lower contact surfaces, and
• the thickness of the strip is varied.