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Unread 01-09-2020, 06:59 PM   #6
jadnashua
Veteran DIYer- Schluterville Graduate

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Join Date: Jul 2004
Location: Nashua, NH
Posts: 13,478
That second layer of ply should NOT be attached to the joists, so those longer screws were a mistake. The top layer should only be attached to the first layer, and that first layer must be installed properly.

If that first layer was really 1/2", that's a problem, as building codes call for it to be at least 5/8". Note, that's a nominal value, after sanding, it's slightly less than 5/8", but more than 1/2".

Then, the ply must be installed with the grain crossing the joists, not along them (not clear how yours was installed).

An oak veneered ply is not a great tile underlayment material. It's very unlikely that the glue is moisture resistant, and as a result, not particularly stable.

Regardless of the size of the room, without knowing the joist structure, it may not be suitable for laying tile.
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Jim DeBruycker
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