Abstract: A tool for forming a tapered seat in wood subflooring for a drain fixture having a predetermined thickness comprising an inverted cup shaped cutting saw having a top and a depending circular skirt having saw teeth formed on the lower terminal edge of the skirt, and an adaptor mounted on the top of said stall saw, at least one cutting blade mounted on said adaptor having a cutting edge disposed angularly to relating to the axis of rotation of said saw and detent means limiting the cutting depth of the tool to form the tapered seat in the subflooring.