Schluter-Systems requires the use of an unmodified thin-set mortar meeting the requirements of ANSI A118.1 to install ceramic (including porcelain) and stone tile over DITRA-HEAT. Portland cement-based unmodified thin-set mortars are dependent on the presence of moisture for hydration in order to gain strength. Since DITRA-HEAT membrane is impervious, it does not deprive the mortar of its moisture. This allows the cement to properly hydrate, resulting in a strong, dense bond coat.
In general, we don’t recommend the use of modified thin-set mortars to set tile over the membranes because these mortars must air dry to cure properly. When sandwiched between two impervious layers such as DITRA and porcelain tile, drying takes place very slowly through the open grout joints. [According to the TCNA Handbook for Ceramic, Glass, and Stone Tile Installation, this drying period can fluctuate from 14 days to over 60 days, depending on the geographic location, the climatic conditions, and whether the installation is interior or exterior]. Therefore, extended cure times would be required before grouting if using modified thin-set mortars between DITRA and ceramic tile, including porcelain tile. If extended cure times were not observed, the results could be unpredictable. This is even more important to consider in exterior applications that are exposed to rain, as there is the additional concern of latex leaching.
Exception: Certain moisture-sensitive stones, e.g., green marble, or resin-backed tiles may require special setting materials. Please consult your stone supplier and Schluter-Systems for more information. For an acid-resistant covering, use an epoxy adhesive to set and grout the tile.