Many consumers, builders and even some tile installers share the mistaken idea that natural stone tile products may be installed on the floor in the same way as is ceramic tile. In most installations this is completely untrue and will many times lead to a very costly replacement.
Many of these folks will tell you they have installed stone tile on a single layer of structural wood subfloor and backer board for years without problems. The main reason this poor choice is promoted comes from the desire to have the stone tile floor installed flush to the adjacent floor finish. While this is a nice idea as far as the flush surface goes, it is really a recipe for failure for the stone tile.
The 2015 TCNA Handbook has only one approved method for the installation of natural stone tile over wood framing with backer board. Method F250-15 STONE, as shown in the attached diagram, calls for the subfloor to be 19/32” tongue and groove plywood installed over wood joists at 16” on center with a 1/8” gap between sheets. The second layer, the plywood underlayment, is to be 15/32” for residential installations or 19/32” for light commercial installations which provides the needed strength and is the base for the backer board. These recommendations are based on the natural stone having adequate compressive strength, flexural strength and resistance to abrasion, but be aware that some stone tiles may not be appropriate for floor use.
The wood framed floor system must be in conformance with the applicable building codes prior to the natural stone installation. The maximum allowable floor member live load and concentrated load deflection is L/720. When an installation is to receive ceramic tile, the requirement is L/360. This means the stone tile floor structure must be twice as rigid (stiff) as the wood floor system required for a ceramic tile installation.
Additionally, the Marble Institute of America (MIA) prohibits installations of stone tile over single-layer wood floor systems under backer board since the joints between the subfloor panels cannot provide the necessary strength. Therefore, two layers of structural wood panel are required on floors to receive stone tile when backer board will be used as the substrate.
Before installing the next stone tile job over wood framing, read the entire TCNA Handbook F250 STONE detail along with the “Substrate Requirements” section in the front of the book on pages 30-31. This information, when put into practice, will yield better results without costly callbacks and failures.