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