A flooring system comprising a plurality of laminate or wood veneer rectangular floorboard having a locking system for mechanical joining of such floorboards which have a thickness of 7 - 10mm and exhibiting an upper surface layer of 0.2 - 0.8mm and a 6 - 9mm body of fibreboard, opposite first and second joint edge portions (4a, 4b), a 0.1 - 0.6mm balancing layer on the rear side of the body, adjoining floorboards (1, 1') in a mechanically joined position having their first and second joint edge portions (4a 4b) joined at a vertical joint plane, said system comprising a) for vertical joining of the first joint edge portion of the first floorboard and the second joint portion (4a, 4b) of the adjoining second floorboard mechanically cooperating means (36, 38) 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 portion of a first floorboard and the second joint edge portion (4a, 4b) of an adjoining second floorboard mechanically cooperating means (6, 8; 14), which comprise a locking groove formed in the underside of said second board 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, said strip projecting at said first joint edge portion from said vertical joint plane and at a distance from the joint plane having a locking element, which projects towards a plane containing the upper side of said first floorboard and which has at least one operative locking surface facing the joint plane for coaction with said locking groove, and the strip forming a horizontal extension of the first joint edge portion below the tongue groove, wherein the locking surface of the locking element has a vertical extent which is at least 0.1 times and less than 0.2 times the thickness of the board, characterised in that the tongue groove depth as measured from the joint plane and inwards towards the board to a vertical limiting plane which coincides with the bottom of the tongue groove is less than 0.4 times the thickness of the board, and that the strip width as measured outwards from the joint plane to a vertical limiting plane which coincides with the outermost tip of the strip is less than 1.3 times the thickness of the board. a) for vertical joining of the first joint edge portion of the first floorboard and the second joint portion (4a, 4b) of the adjoining second floorboard mechanically cooperating means (36, 38) 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 portion of a first floorboard and the second joint edge portion (4a, 4b) of an adjoining second floorboard mechanically cooperating means (6, 8; 14), which comprise a locking groove formed in the underside of said second board 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, said strip projecting at said first joint edge portion from said vertical joint plane and at a distance from the joint plane having a locking element, which projects towards a plane containing the upper side of said first floorboard and which has at least one operative locking surface facing the joint plane for coaction with said locking groove, and the strip forming a horizontal extension of the first joint edge portion below the tongue groove,