Kit for strengthening and lightening floor and roof framing, the kit comprising at least one device for strengthening and lightening floor and roof framing, a prefabricated beam,a mesh,molds, reinforcing rods, and concrete to be laid on site, wherein the device comprises: • a profile comprising holes (J, t, p) and projections, • two screws, • a pin, and • holes and projections for reinforcing rods; wherein: • the profile is made from a tensile-strength resistant material, folded into a "U" shape, according to the used prefabricated beam, • the inner wall of the "U" shaped profile base, has the shape and length of the base of the used prefabricated beam cross-section, • the inner walls of the side sections adjacent to the profile base, have the shape of the adjacent sides to the base of the prefabricated beam cross-section up to a height where said prefabricated beam section is widest; and the inner walls of the side sections adjacent to the profile base have the same length as the length of the adjacent sides to the base of the prefabricated beam from the base of the prefabricated beam up to said height, from said height, the profile sides continue being perpendicular to its base until reaching a height over its base which will be equal to the distance from the prefabricated beam base to the determined plane, in order to affix the mesh which will be integrated to the compression slab, at said height, the profile ends are folded to form anchorages or will have added elements to constitute anchorages; • the holes (J, t, p) and projections are located in both profile sides; • the two screws are symmetrically housed facing to each other in holes (t) at both profile sides, located at a height from the profile base plane, equal to the distance from the base of the prefabricated beam cross-section up to the level where said section is widest; the screws have a length so that they cover the profile thickness having a remaining segment protruding in order to settle the side edges of the molds, and • the pin comprises a substantially straight rod with a minimum length sufficient so that its ends are affixed in the holes (p) of the profile sides, both sides having said holes (p) to affix the pin, the pin remaining parallel to the profile base plane and at a distance from this plane, equal to a height of the prefabricated beam cross-section, such that when the profile is installed on the prefabricated beam, the pin is affixed touching the prefabricated beam upper section avoiding the displacement of the profile downwards of the prefabricated beam. • a profile comprising holes (J, t, p) and projections, • two screws, • a pin, and • holes and projections for reinforcing rods; • the profile is made from a tensile-strength resistant material, folded into a "U" shape, according to the used prefabricated beam, • the inner wall of the "U" shaped profile base, has the shape and length of the base of the used prefabricated beam cross-section, • the inner walls of the side sections adjacent to the profile base, have the shape of the adjacent sides to the base of the prefabricated beam cross-section up to a height where said prefabricated beam section is widest; and the inner walls of the side sections adjacent to the profile base have the same length as the length of the adjacent sides to the base of the prefabricated beam from the base of the prefabricated beam up to said height, from said height, the profile sides continue being perpendicular to its base until reaching a height over its base which will be equal to the distance from the prefabricated beam base to the determined plane, in order to affix the mesh which will be integrated to the compression slab, at said height, the profile ends are folded to form anchorages or will have added elements to constitute anchorages; • the holes (J, t, p) and projections are located in both profile sides; • the two screws are symmetrically housed facing to each other in holes (t) at both profile sides, located at a height from the profile base plane, equal to the distance from the base of the prefabricated beam cross-section up to the level where said section is widest; the screws have a length so that they cover the profile thickness having a remaining segment protruding in order to settle the side edges of the molds, and • the pin comprises a substantially straight rod with a minimum length sufficient so that its ends are affixed in the holes (p) of the profile sides, both sides having said holes (p) to affix the pin, the pin remaining parallel to the profile base plane and at a distance from this plane, equal to a height of the prefabricated beam cross-section, such that when the profile is installed on the prefabricated beam, the pin is affixed touching the prefabricated beam upper section avoiding the displacement of the profile downwards of the prefabricated beam.