The system used to form slabs on large surfaces includes a grid structure comprised of vertical braces, support bolsters, primary beams and cross beams, all of which rest on the bolsters, with the cross beams closing the grid and providing a stiffness to the structure that allows assembly of the entire grid structure and the later positioning of the formwork panels. The bolsters provide support so that the beams can descend to a stripping position allowing to recover, before the concrete sets, both the beams and the panels. The longitudinal beams are provided with supports for the panels, constructed such that they allow the panels to hang vertically during the stripping operation. The cross beams end at a profile made of rubber or the like providing a means for sealing the grid, as well as for absorbing any possible clearances originated during the assembly of the panels.