Abstract: A structural panel is described which includes a thin substantially rectangular member of preselected length, width and thickness having a substantially flat upper surface and a peripheral flanged edge and supported on the lower surface thereof by a first plurality of evenly spaced transverse ribs and a second greater plurality of evenly spaced longitudinal ribs, and at least one pair of intersecting diagonal ribs extending from diagonal corners of the member, the panel being hot formed of a composite of plastic, about 20 to 25 weight percent comminuted cellulosic matter or glass fibers and about 3 to 5 weight percent flame retardant.