An underdrain system for transferring forces and directing flow includes at least one underdrain block having a top wall, a bottom wall, and two side walls connecting the top wall to the bottom wall to define a block interior. An insert is disposed in the interior of the underdrain block. The insert includes force-transmitting buttresses spaced along an exterior of the insert that contacts the two side walls of the underdrain block and act to transfer downward forces acting on the top wall of the underdrain block and internal fluid forces acting on the insert to the side walls of the underdrain block. The force-transmitting buttresses can also direct hydraulic flow.