A molded plastic skid base for a resin building is formed with all of its openings facing downward and with runners integral with a deck that forms a floor of the base. Replaceable wear plates cover the bottoms of the runners and have truck loading notches molded at their ends and a longitudinal channel in which fastener heads are positioned. Stake down slots are provided in the ends of the runners and wear plates in a diagonal orientation so as to be aligned toward the center of the building, below the base. The floor of the base is a grid formed of diagonally oriented ribs, at least some of which may be shorter in their center than at their ends, is crowned in the center and has openings which taper upwardly. A solid sheet may be placed over the grid to close it. Bosses for receiving the wear plate mounting fasteners are positioned at the same locations along the sides of the base as the fastener mounting locations for attaching the building side walls to the base, to provide a greater thickness of material at those locations for the building sidewall fasteners to penetrate. Edges of a septic tank opening formed in the base are slanted downwardly and rear corners of the opening are triangular in shape. Downwardly opening openings are also formed in the base into which weights and weight covers may be fixed.