Floorboards (1, 1') provided with a locking system for mechanical joining of said floorboards (1, 1') having a body and preferably a surface layer on the upper side of the body and a balancing layer on the underside of the body, said locking system comprising: for horizontal joining of a first and a second joint edge portion (4a, 4b) of a first and a second of said floorboards (1, 1'), respectively, at a vertical joint plane, on the one hand a locking groove which is 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, on the other hand, 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 projects towards a plane containing the upper side of said first floorboard and which has a locking surface for coaction with said locking groove, and for vertical joining of the first and second joint edge (4a, 4b), on the one hand a tongue which at least partly projects and extends from the joint plane and, on the other hand, a tongue groove adapted to coact with said tongue, the first and second floorboards (1, 1') within their joint edge portions (4a, 4b) for the vertical joining having coacting upper and coacting lower contact surfaces (43, 45), of which at least the upper comprise surface portions in said tongue groove and on said tongue, wherein the upper and lower contact surfaces (43, 45) are essentially plane-parallel and extend essentially parallel with a plane containing the upper side of the floorboards, the lower contact surfaces comprise surface portions in said tongue groove and on said tongue, and the thickness of the strip varies over its width perpendicular to the joint plane, characterised in that the lower plane contact surface of the lower contact surfaces constitutes the upper side of the strip and is largely positioned outside the joint plane, and that the upper edge of the locking element, which upper edge is closest to a plane containing the upper side of the floorboards, is located in a horizontal plane, which is positioned between the upper and the lower contact surfaces (45, 46), but closer to the lower than to the upper contact surfaces. for horizontal joining of a first and a second joint edge portion (4a, 4b) of a first and a second of said floorboards (1, 1'), respectively, at a vertical joint plane, on the one hand a locking groove which is 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, on the other hand, 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 projects towards a plane containing the upper side of said first floorboard and which has a locking surface for coaction with said locking groove, and for vertical joining of the first and second joint edge (4a, 4b), on the one hand a tongue which at least partly projects and extends from the joint plane and, on the other hand, a tongue groove adapted to coact with said tongue, the first and second floorboards (1, 1') within their joint edge portions (4a, 4b) for the vertical joining having coacting upper and coacting lower contact surfaces (43, 45), of which at least the upper comprise surface portions in said tongue groove and on said tongue, wherein the upper and lower contact surfaces (43, 45) are essentially plane-parallel and extend essentially parallel with a plane containing the upper side of the floorboards, the lower contact surfaces comprise surface portions in said tongue groove and on said tongue, and the thickness of the strip varies over its width perpendicular to the joint plane, characterised in that the lower plane contact surface of the lower contact surfaces constitutes the upper side of the strip and is largely positioned outside the joint plane, and that the upper edge of the locking element, which upper edge is closest to a plane containing the upper side of the floorboards, is located in a horizontal plane, which is positioned between the upper and the lower contact surfaces (45, 46), but closer to the lower than to the upper contact surfaces.