Abstract: A deck plank in the form of an elongated plastic channel having a mounting base, upright side walls terminating in cap engaging locking means and intermediate support means between the side walls, at least the upright side walls having a honeycomb structure, and a longitudinal generally planar plastic cap for closing the channel having edge formations adapted to engage and interlock with the side wall locking means. In its preferred form the entire channel is of a honeycomb structure, the channel wall cap engaging locking means are arcuate sockets and the cap edge formations are depending arcuate legs adapted to snap into the wall sockets.