In simplest terms, “deflection” can be defined as bending. As you can imagine, many floor finishes can bend when the substrate deflects. But not ceramic tile; too much subfloor deflection, or bending, can be the cause of cracked tile. This is why ceramic tile installation standards require a substrate that is rigid enough that it bends less when loads are applied. When wood framing (floor joists) and wood sheathing (plywood or OSB subflooring) will provide the support and rigidity for a floor tile installation, pay attention to joist spacing and subfloor sheathing thickness, which significantly affect the amount of subfloor deflection between the joists, the location of greatest concern for tile installations.
The TCA Handbook for Ceramic Tile Installation outlines requirements for successful installation of tile, including maximum joist spacing and minimum subfloor thickness. The wider the joists are spaced, the more the subfloor attached to them can bend under loads. Thus, the majority of Handbook floor installation methods limit joist spacing to 16” on center. When joist are spaced wider, Handbook methods require a second layer of subflooring to ensure a rigid enough substrate.
For subfloor thickness requirements, some Handbook methods allow 19/32” thick subflooring (commonly referred to as 5/8”), while others require 23/32” thick subflooring (commonly referred to as ¾”.) In many cases, either thickness may be used, but the thicker subflooring must be used for a higher level of service (more traffic and heavier loads.) To be sure your installation will stand the test of time under expected conditions, check the subfloor requirements for the Handbook method you are going to use, as well as that method’s service rating, found in Floor Tiling Installation Guide in the front of the Handbook.