A.Tile expert Michael Byrne responds: Tiles generally crack for only two reasons: loss of bond or moving substrate. In this case, the substrate might be moving if the studs are spaced farther apart than 16 inches on-center, allowing that backerboard to flex.
Another possibility is that the backerboard was cracked prior to installation. Although cementitious backerboard often has minor cracks that don’t cause problems, over-stressing the boards during delivery or installation can cause more serious cracks. If the backerboard was installed over a layer of drywall, then water penetration through tile...
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