The earthquake hazard reduction of historical buildings often requires the strengthening of existing floors in order to obtain stiff diaphragms, able to transfer the horizontal action to the shear resistant masonry walls. Past experiences showed that a suitable technique consists in overlaying the floor with a thin concrete slab, although the dead load and therefore the seismic action are significantly increased. Since the thickness of the slab cannot be reduced because of the minimum concrete cover required by the steel mesh, the substitution of the ordinary reinforcement with steel fibres is proposed in this paper. The removal of the mesh allow for a reduction of the slab thickness and, consequently, of both the dead load and the seismic action. The efficiency of the proposed solution is evaluated by means of nonlinear finite element analyses.