This invention generally pertains to a composite building material comprising a lightweight core with a thin fiber cement facing on one side of the core and a second facing material on the other side. The fiber cement facing that is used on at least one of the faces of the building material is 3/16" or less, more preferably " or less. The green fiber cement facing is preferably formed by a slurry-dewatering process to form a sheet that is in a plastic, uncured, state prior to manufacture of the composite. The composite building material is assembled in an uncured state and then cured.