Box elements (4-104-204) made from extruded cellular sheets of polypropylene and/or other suitable waterproof plastic materials, filled if necessary, are disposed in the space between the metal lattices (3) projecting from the reinforced concrete slab (1-2) of a floor, and are fixed over the whole length of this space, with or without interruptions, and act as lightening elements. The box elements are downwardly open, are provided with transverse closing and stiffening walls (8) fixed by mortising and/or other suitable means and suitably distributed over the whole length of the said elements, which are preferably also provided with small lateral openings (7-10) which prevent the entry of the floor finishing casting but which allow the water to flow freely, so that after the setting of the said casting the floor no longer releases liquid components.