Duradek Ultra Tiledek is a waterproof roofing and crack-isolation membrane designed for use under exterior tile or slate. Virtually identical to the company’s Ultra line, Tiledek is approved for use as a roofing membrane with a fabric laminated to its topside, on which tiles can bond securely with thinset mortar. The membrane allows for some movement in the structure without cracking the tiles, is part of three approved assemblies that are rated for residential or heavy-duty use, and is made of very dense PVC. duradek.com