Abstract: A roof drain sump box (10) is utilized for positioning a roof drain (12) on a flat roof. The sump box is formed with a horizontally oriented flat plate (20) with a central opening (22) making a peripheral flange (24). The central opening has a width less than a distance between building roof joists providing attachment of the sump box onto the flat roof top of a building without altering the building structure. An angular transition portion (46) is integrally formed with the flange contiguous with the central opening. A vertically oriented downwardly depending step (48) is formed integrally with the angular transition portion and a horizontally oriented flat sump deck (50) is integrally formed with the step. The sump deck, is coplanar with the flange, and incorporates a roof drain opening (52) to receive the roof drain (12). The sump deck is positioned sufficiently below the flat plate permitting water to flow uninhibited into the roof drain without ponding.