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Repeatedly during inspections and troubleshooting investigations, I find ceramic tile floors with long crack lines that follow the joints in the plywood subfloor or underlayment below. Sometimes these cracks form a perfect trace of every joint throughout the room. This cracking is caused by the edge rotation of the plywood sheets as they flex under load. Ceramic tiles, being inherently very stiff but not particularly strong, simply crack along these flex lines. The problem can be prevented by increasing the stiffness of the subfloor and eliminating the flex points. A double layer of plywood, glued and screwed, and with all joints staggered, can do just this (see Figure 1). Stiffness vs. Depth The success of this technique derives from an engineering principle that