The invention relates to a slab formwork system comprising a plurality of formwork elements (98) which have on their underside girders (2) which can be coupled to heads (28) of vertical supports (90), in which slab formwork system the head of a vertical support has a lifting lock (44-51; 87) fixing a girder of a formwork element in the vertical direction. In addition, the head of a vertical support has fixing elements (52-66) which prevent a movement of the grider in its longitudinal direction.