Abstract: A lost mold element for manufacturing reinforced concrete flat slabs, which is intended, when used together with other mold elements, for creating voids and for allowing the molding of beams. The mold element is made of a recycled plastic material, having a shape of a hollow box, substantially of a flat prismatic shape, and preferably rectangular or square, which is formed from two half boxes including an upper half box and a lower half box joined together removably. Each mold element has side or lateral walls with profiles that define together with other side or lateral walls of other mold elements an I-profiled channel or I-degenerated profiled channel for molding beams. Further, between the side or lateral walls are arranged steel rod attachments and distribution supports for securing the steel rods, and for reinforcement of the I-profiled or I-degenerated profiled beams. Such a mold element can be used in the building industry.