A door sill has a plastic, or other water impervious, plank fastened on its underside. The bottom of the plank seats against the subfloor of a building. The ends of the plank protrude beyond the rest of the sill, forming short flanges under the lower ends of the attached door jambs. The jambs are fastened to the sill, and the flanges are fastened to the jambs, by nails, screws or staples. The plank is water impervious, thereby preventing rot of the jambs by water wicking up to, and into, the jambs. The plank also raises the sill, and therefore the entire door frame, up by the thickness of the plank, which keeps the bottom of the door from scraping against the upper surface of a finish floor material, such as carpet.