A structural element constituting a panel for a floor, wall or roof of a space defining elongated box-shaped prefabricated section which includes a rectangular concrete slab with a peripheral rectangular frame of beams. A reinforcement network is attached to the frame beams. With I-beams constituting the frame beams, one side of an upper flange is cast within the slab, such flange being connected to the reinforcement network. Recesses are provided at the corners of the slab to receive upright extensions affixed to the top of the upper flanges of the I-beams, such extensions being adapted to receive for attachment columns or other components of the section. The recess provided for the extensions may be spaced therefrom and being filled in with a concrete or other plastic type material when the further component is received on the extension. The extension may extend above the slab without a recess. A covering material may be applied to the top of the slab and folded over its edges. An L-shaped edge piece may be connected to the slab at its upper edges, the vertical leg of the edge piece extending to the exposed edges of the upper flanges of the I-beam and the horizontal leg of the edge piece extending inwardly and being flush with the top surface of the slab. An angle piece member may be connected to the slab at its edges so it is received on top of the upper flange of the I-beam and its vertical portion extends upwardly from the exposed edges of the upper flanges of the I-beam. A further alternative is to furnish a wooden border member with its top flush with the top surface of the slab. Another alternative is to place a wooden beam on top of the slab which is rigidly connected to the I-beam, such wooden beam having a wall affixed thereto. Further, transverse I-beams may be connected to the web and lower flanges of the I-beam spaced below the slab to provide increased rigidity. The same structure is disclosed with channel beams. A wooden frame is disclosed wherein the frame beams are made up of two frame parts connected side-by-side with the outer frame part higher than the inner frame part and the slab received so as to be coplanar with the upper edge of the inner frame part, such upper edge being utilized to secure the reinforcement network. Transverse beams are located between the two longer sides which are the same height as the inner frame part so as to be received in the slab at the level of the reinforcement and beams in place is disclosed as well as a method for utilizing such jig apparatus.