A screw fixing comprising a screw (9) having a nut (1) threaded on it, the screw head and the nut each having a bearing surface facing the other, and a channel (5) having at the free ends of the limbs of the channel inturned flanges (6) directed towards the base (8) of the channel, the flanges defining a slot (10) which is open at both ends, at least one of the flanges having teeth (7) in its free edge directed towards the base of the channel and the bearing surface that engages it having parallel ribs (4) that interlock with the teeth to prevent the screw and nut sliding longitudinally in the channel.