Abstract: A floor joist system for building construction comprising interconnected steel girders and joists. The girders may be castellated I beams having a plurality of large openings and regularly spaced vertical slots through its web, which extend between opposite side walls of a building or its foundation, supported thereby. Joists may be I beams of shorter height than the girders which have tongues formed from their webs which extend from their ends. The tongues of two joists are fitted into a single vertical girder slot from opposite sides of the girder, their tongues overlapping therein. The opposite ends of the joists are similarly fitted into an adjacent girder or rest upon a side wall of the building or its foundation. The girder slot is positioned and the side walls are adapted such that the uppermost surfaces of the joists and the girder(s) are coplanar.