Exterior building siding for aesthetic and protection of the building against wind, rain and solar energy by attaching each horizontal plank from the top of the wall downwardly that includes a starting strip where each plank is interlocked to the plank above it. Each of the horizontal planks includes a top wall portion that fits snuggly into an “L-shaped” groove above it so that the panels can be firmly locked together vertically in a downwardly fashion to prevent any wind uplift against the siding or moisture intrusion. In the preferred embodiment, an adaptor strip is bonded to a conventional siding plank for trap lock engagement. Each adaptor strip has a longitudinal recess to reduce mass and volume and a mid panel support for strength.