to engage shoulder 50 thus keeping that portion of plank 10 securely anchored against the joist 14 on the opposite sidewall 24 the longitudinal bead 32 is received within the recesses 42 of each of the plurality of clips 40 spaced along the length of each plank and anchored to the underlying joist 14 the recess 42 in turn is received in snap fit arrangement around the bead 32 to retain the sidewall 24 against the joists 14 the difference in height between the longitudinal stiffening members 20 and the outboard sidewalls 22 and 24 are compensated by the thickness of the tabular portions of the clips 40 it should be noticed that the spacing between the recess 46 and the cylindrical recess 42 are such that outside wall 22 may be positioned in a fixed spaced relationship with respect to sidewall 24 of the adjacent plank 10 to ensure even spacing between the adjoining planks the distance between each of the spacing bosses 36 and the clips 40 provides adequate room for each plank to expand and contract in a direction parallel to the longitudinal axis returning to fig1 and 4 through 6 the instant invention also accommodates for expansion between the ends of adjacent planks 10 referring to the referenced drawing figures a first end 60 of each plank 10 includes a tapered flange 62 positioned slightly lower than the upper surface 16 and slightly inboard of each sidewall 22 24 each tapered flange 62 is dimensioned to be received within a dimensionally close fitting.