to horizontally extending angle members 17 as seen in fig2 on the outer facing surface of end rib 28 is a groove 33 which cooperates with a complimentary shaped tongue 34 on fillerboards 23 to interlock the floorboards and fillerboards in a manner which is best seen by reference to fig1 referring now to fig4 each of the fillerboards 23 is identically formed and includes a central horizontally extending portion 36 from which depends a rib 37 having an inwardly extending flange 38 and a vertically extending riser portion 39 referring now to fig5 each of the riserboards 24 is identically formed and includes a vertically extending riser portion 41 and a horizontally extending runner portion 42 a flange 43 extends vertically at the free end of runner portion 42 and serves as an abutment surface for contact with end rib 27 of floorboards 22 at the free end of riser portion 41 there extends offset from the plane of riser portion 41 a flange 44 which serves to overlap and thereby interlock with the riser portion 39 on fillerboard 23 mounting holes 45 are provided for mounting to understructure 12 the lengths of floorboards 22 fillerboards 23 and riserboards 24 may be varied according to the desired length of the bleacher seatboards referring now to fig6 9 the manner of installation of the decking system of the present invention will now be described as seen in fig6 the ends of a fillerboard 23 are installed into the inside corners formed by vertical angle members 16 and horizontal.