A shower pipe for a rotary drum filter includes a fiberglass pipe body of rectangular cross section with two rows of spray holes and with the holes of one row being staggered with respect to the holes of the other row. The upper row includes a separate diffusion flange for each hole. The lower row includes a continuous diffusion flange which is reinforced and rigid to resist deflection and serve as a striker to limit the thickness of pulp stock passing upwardly by the flange. The lower flange includes a flexible flange extension which extends to the pulp mat on the drum to limit heat loss from the spray of washer fluid. Different diffusion flanges on the same pipe define different spray trajectories to prevent overconcentration of spray impinging the mat.