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Unread 10-11-2019, 10:47 AM   #2
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Join Date: Jul 2001
Location: Boerne, Texas
Posts: 92,826
Welcome, Bob.

The tile industry standards require that you install a minimum of nominal 1/2" exterior glue plywood with no face of grade lower than C on top of a sawn board subfloor of a minimum of nominal 1" (3/4" actual) thickness over joists spaced at a maximum of 16" on center before a ceramic tile installation substrate and tile.

But that presumes your boards to have T&G edges and be laid perpendicular to the joist structure, neither of which you have. I would recommend you use at least nominal 5/8ths" plywood over your boards, presuming that your joists are on 16" centers. Is that actually equivalent to the standard requirement? I dunno, but I'd do it.

Or, of course, you could remove what you have and start with a first layer of nominal 3/4" T&G plywood. That might meet minimum requirements depending upon your joist spacing.

My opinion; worth price charged.

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