Abstract: Lateral member assembly for use in an underdrain lateral distribution system is made of plastics but is as capable as much heavier and more costly prior art metallic assemblies in supporting a bed of filter media which must be periodically backwashed with liquid and gas. Complementary elements, including a rigid extruded floor plate member having longitudinally extending side edge channels, and an overlying screen member having flange portions adapted to be retained by undercut recesses in the side edge channels, are adapted to be assembled to each other to form hollow lateral assemblies. In a preferred embodiment, the screen member is telescopically assembled to the floor plate member to cause the screen flange portions to interlock with the recesses in the side edge channels. The resulting assembly is sealed against leakage where the members are interlocked by a pair of flexible, integral plastic sealing strips which are co-extruded with the rigid floor plate member.