A prefabricated building product structure made of sintered expanded polystyrene includes a composite assembly formed by aggregating steel to the sintered expanded polymeric mass, preferably of high density, and by embedding therein rod-like steel elements, preferably pre-assembled and forming a cage of electrowelded elements. A method of related production includes positioning the rod-like steel elements, preferably pre-assembled forming a cage of electrowelded elements, in the mold for the expansion of the polystyrene, pouring in the mold incoherent granules or pearls obtained from the polymerization of the styrene, with grain size and volume appropriate for the attainment of a high density finished product, expanding and sintering or amassing the incoherent polystyrene through contact with water vapor at a temperature higher than ninety degrees centigrade, and trapping the armor in the monolithic mass thus obtained to attain a solid geometry, according to the mold, of armored sintered expanded polystyrene.