Abstract: Elongated siding panels formed from plastic have integral joint portions along their side edges which are adapted to fit together and interlock when the panels are mounted on a building wall in overlapping relation. A flange adjacent the top edge of the panels extends outwardly and downwardly to define a channel open at its bottom, and an upwardly directed lip on the bottom edge of the panels is adapted to project into and interlock with the channel on a panel mounted therebelow on a building wall. A plurality of inwardly directed protrusions, or dimples, are formed on the upwardly directed lip to provide an interference fit with entrance to the channel.