Abstract: In one embodiment, a siding plank assembly comprising an interlocking feature that allows the siding plank to be stacked with other siding planks in a manner such that a uniform and deep shadow line is created. The interlocking feature sets the gauge of the exposed plank face and allows for leveling of the plank during installation. The siding plank assembly comprises a main plank section securely bonded to a second piece that produces a deep shadow line. In preferred embodiments, the second piece may be a fiber cement butt piece or a plastic spline that is bonded to the main plank section. A lock structure may also be included on the fiber cement butt piece or plastic spline.