These roofs consist of a single ply of PVC roofing membrane. PVC is classified as a weldable thermoplastic, meaning it can be hot-air welded. PVC and TPO are not compatible for welding with each other. Some roof system manufacturers report that the welded seam of their membranes are stronger than the membrane itself. Often, these materials are mixed with other compounds to enhance the material’s properties such as fire and UV radiation resistance. Although available in a variety of colors, PVC roofing is sometimes called ‘white roofing’ or ‘cool roofing’.