Horizontal formwork comprising a support base and a plurality of formwork panels which are supported on said support base, the support base comprising a plurality of basic grids, each basic grid comprising - four heads arranged at the vertices of said basic grid, each head being supported by a respective prop, - two main beams arranged parallel to one another, each end of said main beams being supported on one of the heads, - two edge beams arranged parallel to one another and perpendicular to the main beams, each end of said edge beams being supported on one of the heads, and - a plurality of central beams which are arranged parallel to the edge beams and supported on the main beams, the main beams comprising housings adapted for receiving an end of a respective central beam, and the formwork panels being supported on respective support surfaces (40, 50) of the edge beams and the central beams, wherein at least one basic grid comprises at least one main beam comprising a plurality of support sections for additional beams for supporting the formwork panels, characterized in that said support sections comprising a support surface for additional beams which is arranged below the formwork panels, so that said additional beams can be overlapped on the support surfaces of said main beams since a gap is created between the support surface and the formwork panel which allows arranging additional beams in said gap, the support sections thereby being configured for allowing said additional beams to be supported on said support surface projecting on both sides of the main beam so that one portion of each of said additional beams projects out of the basic grid and one portion of each of said additional beams projects towards the inside of the basic grid, extending the support base by means of the portion of said additional beams projecting out of the basic grid. a support base and a plurality of formwork panels which are supported on said support base, the support base comprising a plurality of basic grids, each basic grid comprising - four heads arranged at the vertices of said basic grid, each head being supported by a respective prop, - two main beams arranged parallel to one another, each end of said main beams being supported on one of the heads, - two edge beams arranged parallel to one another and perpendicular to the main beams, each end of said edge beams being supported on one of the heads, and - a plurality of central beams which are arranged parallel to the edge beams and supported on the main beams, - four heads arranged at the vertices of said basic grid, each head being supported by a respective prop, - two main beams arranged parallel to one another, each end of said main beams being supported on one of the heads, - two edge beams arranged parallel to one another and perpendicular to the main beams, each end of said edge beams being supported on one of the heads, and - a plurality of central beams which are arranged parallel to the edge beams and supported on the main beams, the main beams comprising housings adapted for receiving an end of a respective central beam, and the formwork panels being supported on respective support surfaces (40, 50) of the edge beams and the central beams, wherein at least one basic grid comprises at least one main beam comprising a plurality of support sections for additional beams for supporting the formwork panels, characterized in that said support sections comprising a support surface for additional beams which is arranged below the formwork panels, so that said additional beams can be overlapped on the support surfaces of said main beams since a gap is created between the support surface and the formwork panel which allows arranging additional beams in said gap, the support sections thereby being configured for allowing said additional beams to be supported on said support surface projecting on both sides of the main beam so that one portion of each of said additional beams projects out of the basic grid and one portion of each of said additional beams projects towards the inside of the basic grid, extending the support base by means of the portion of said additional beams projecting out of the basic grid.