An underdrain block for a filter system comprises a housing that has of a top wall, a bottom wall, a first end wall, a second end wall, a first side wall, and a second side wall. The end walls and the side walls connect the top wall to the bottom wall. The bottom wall forms two lateral rows of support legs configured to straddle infrastructure of the filter system. One or more snap-locking mechanisms are located on the side walls so that adjacent underdrain blocks are interconnected by the locking mechanisms when snapped in place. In one embodiment, the first side wall comprises one or more tongue lugs and the second side wall comprises one or more groove lugs, each groove lug defines a groove for mating with the tongue lug so that adjacent housings are interconnected by the tongue-in-groove locking mechanisms when the tongue lugs are snapped into the grooves.