The Pre-slab (Figure 1) is a structurally self-sufficient element of a floor slab which also works as a mold of the completed floor slab. A pre-slab corresponds to a cell of the structural skeleton's grid; it may have either rectangular or triangular shape. Includes a thin base made of reinforced concrete, steel joints (Figure 2-a)where columns can be connected, as well as trusses (Filigran), both perimetric and diagonal, ending at the joints (Figure 2-b). The pre-slabs are positioned in their place within the structural skeleton and connected to the supporting columns by screwing. Then the plumbing pipes (Figure 4-a), as well as the underfloor heating/cooling pipes (Figure 3). are installed. When all the pre-slabs, as well as the pipework of the floor have been installed (Figure 4-a), expanded polyurethane fills the triangular spaces formed between the trusses (Figure 4-b) while wire meshes are placed at the upper part of the floor. Concrete casting follows (Figure 4-c) which completes the construction of the floor slab. The concrete forms within each pre-slab both perimetric and diagonal beams that include the trusses as aramature. It also forms an upper layer reinforced by the wire mesh which has been placed there (this layer incorporates the potentially installed underfloor heating/cooling pipes). The floor slab constitute a "sandwich slab" 22cm thick.