Before any work starts, we need to make sure that the surface is rigid enough to accept the stone flooring. The Ceramic Tile Handbook published by the Tile Council of North America (TCNA) explains that it is the responsibility of the project architect or engineer to determine if the substrate meets the proper stiffness and deflection criteria in accordance with the governing building code and the use criterion of the space. The substrate needs to meet the maximum allowable deflection for the finish and the installation system that will be installed. “Deflection” is the potential movement that the installation can experience when subjected to load and use. The suitable substrates for most electric heat flooring systems are: