An improved cap for underdrains in gravity filters has a top surface and a bottom surface with a plurality of tapered screen members defining slots in the top surface. The slots provide communication with the filter bed without passage of fine grain filter media therethrough. The screen members are inclined with respect to horizontal to define a plurality of raised portions in the top surface, each raised portion defining a distribution chamber above the orifices in the distributor. The cap may be included in a capped filter underdrain block having a plurality of ribs and orifices on a top wall of the block. The cap rests on the ribs and cooperates with them to define distribution chambers above the orifices in the top wall of the block. The distribution chambers promote intermixing of air and water prior to introduction to the bed, and the ribs finely meter backwash fluids for even distribution of backwash fluids throughout the entire filter bed.