A thinset floor drain comprises a floor-mounted flanged pipe piece presenting a hidden thread inside a deep ring-shaped groove formed into the upper surface of the flange, and a second flanged pipe piece with an internal thread. The first piece is mounted pipe down in a perforation in the subfloor of a wet room with screws. The second piece is threaded from above into the hidden thread to form a clamp for a perforated sheet of membrane placed interstitially to the two flanges drawn together by the engagement of the thread mate, to achieve a watertight connection between the drain and the membrane. Because the thread mate sits largely below the subfloor tow which the floor drain is mounted, the upper surface of a grate assembly threaded into the mounted and clamped drain will present a low profile above the surface of the subfloor.