the openings 50 through the upper surface of the slab assembly 16 to close the same obviously wiring or the like may be installed after a building has been completed the form of invention shown in fig4 is a modification of that shown in fig2 and like reference numerals designate similar parts in this form a vertical wall assembly 12 is erected and the horizontal floor ceiling slabs of assembly 16 are positioned to rest thereon but in this form the horizontal slabs do not extend outwardly beyond the vertical wall they actually terminate somewhat short thereof the upper edge of the lower vertical wall 12 and the lower edge of the upper vertical wall 22 are bevelled at their outer edge portions as shown at 54 and after the slabs have been placed in position suitable plugs 56 are positioned in the openings 10 of the floor ceiling slabs and concrete or the like 58 completes the outer surface of the building and extends to the bevelled portions 54 and is formed flush with the outer surface of the vertical wall slabs suitable reinforcing rods 60 being positioned therein to add strength to the completed structure if desired the concrete filler 58 may be provided with one or more openings 62 therethrough to provide external access to the openings 10 here again the.