GRK RT Composite screw is a specially designed fastener for attaching composite and cellular PVC materials. The RT Composite has a reverse threading under the head of the fastener that helps the screw head disappear beneath the surface and eliminate mushrooming when installing Composite materials. RT Composite Screws have a Climatek Coating for installation in both interior and exterior conditions where exposure to moisture is intermittent and is code-approved for use in treated lumber.