A composite building material and a method for its manufacture are provided. The method comprises the steps of: providing a foamable mixture comprising a polymeric matrix material, a filler, a processing aid, one or more lubricants, a thermal stabilizer, and a blowing agent; foaming said foamable mixture to form a foamed composite substrate; optionally embossing the substrate; and coating the substrate with a polyurethane/acrylic coating to form the composite building material, wherein the polyurethane/acrylic coating is keyed chemically and/or physically to the foamed composite substrate.