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Unread 07-27-2021, 02:22 PM   #14
Veteran DIYer- Schluterville Graduate

STAR Senior Contributor

Join Date: Jul 2004
Location: Nashua, NH
Posts: 14,964
There is an acceptable way to lay tile directly over TWO layers of plywood, but that also requires a highly modified thinset material. But, the industry also calls for an expansion joint for all tiled surfaces. On a floor, depending on the size of the area tiled, and any environmental issues (that could be uneven heating from the sun, or in your case, maybe from the heating duct), they also can call for an expansion joint there, plus it calls for an expansion joint at any change of plane or materials...that means that on a floor, you need an expansion joint all around the cannot just butt up against a wall or other material without a 'soft' joint in between. That can be a literal air gap (usually if you can cover it with say the baseboard trim), or maybe a caulked joint, or an engineered one in the field, or against another material.

Your structure might be fine, but if the materials used were not sufficient, or they did not abide by the industry standards about expansion joints, that can cause a tiled floor to fail.

What holds a tile down is the interlocking crystalline spikes that the cement grows as it cures. Those spikes, create a tangled, interwoven mesh that pokes into microscopic imperfections. Movement beyond certain limits can literally shatter those crystals, and it can be gradual, so over time, the bond becomes too weak to hold things together. In the right circumstances, a modifier can cushion those crystals, but if the stress is too great, things will still bend enough to break them, loosening the bond. The modifier also can act somewhat like a glue, augmenting that bond, but the primary bond is still from the cement.
Jim DeBruycker
Not a pro, multiple Schluter Workshops (Schluterville and 2013 and 2014 at Schluter Headquarters), Mapei Training 2014, Laticrete Workshop 2014, Custom Building Products Workshop 2015, and Longtime Forum Participant.
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