A decking system for placement on a subassembly having at least two spaced apart joists. A plurality of elongated tubular elements for placement in lateral interlocking engagement with each other transversely of and spanning the distance between the joists. Each of said plurality of elements having a top portion, a bottom portion, and first and second side walls connecting said top and bottom portions. The top portion extends laterally beyond the first side wall and has formed thereon a downwardly extending longitudinal flange. The bottom portion extends laterally beyond the second side wall and has formed thereon an upwardly extending longitudinal wall defining an upwardly opening channel adjacent the second side wall adapted to receive therein a watertight relation the downwardly extending longitudinal flange. The bottom portion having formed therein a plurality of apertures intermediate the second side wall and the upwardly extending longitudinal wall, adapted for the insertion of fasteners therethrough for engagement with the joists.