A plastic building product for placement on the wall of a building to provide a louver or window wherein siding abuts the product the product comprises a plastic body having a peripheral wall circumscribing the louver or window, an integral flange extending laterally from the wall for fastening the body to the wall of a building, and a movable flange memebr telescoped over the peripheral wall of the body. The flange member includes a laterally extending flange adapted to overlie portions of abutting siding or the like. The flange member and the peripheral wall include interengaging portions for selectively positioning the flange member at predetermined distances which respect to the flange on the body to accommodate siding of varying thickness.