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Unread 11-20-2020, 05:50 PM   #2
Veteran DIYer- Schluterville Graduate

STAR Senior Contributor

Join Date: Jul 2004
Location: Nashua, NH
Posts: 14,225
Most pros would not tile on 5/8" ply. The specs say it is okay (assuming the joist spacing is 16" OC) when new and properly installed...yours isn't. There is an industry procedure for tiling on two layers of plywood, but again, most pros don't like to do that.

Should you want to install your tile directly to two layers of plywood, the procedure calls for using a highly modified thinset.

The size of the room has nothing to do with whether the tile installation will be successful. You should check your joists for suitability by plugging in info in the 'Deflecto' tool in the blue bar at the top.

Plywood (subflooring) provides strength between the joists, the joists provide strength along them. You need both to have a successful installation.
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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