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Unread 08-07-2022, 02:16 PM   #4
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Join Date: Jul 2001
Location: Boerne, Texas
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Originally Posted by Stew
The new part didnt need sistering as the joist were cut to 8" with a 9' span.
Again, Stew, the joists were 2 and better before you ripped them down from whatever size they were to begin with. They're still Fir, of course, but not necessarily 2 and better.

You don't indicate the joist spacing. Most manufacturers of ceramic tile substrate materials will accept nominal 5/8ths" exterior glue plywood over 16" joist centers as their minimum requirement (which I consider borderline crazy), but your incorrectly oriented nominal 3/4" ply, presuming a 5-ply construction, will have barely more than 3/8" of material oriented in correct direction. I have no way to do meaningful testing of something like that, but I'd wager your incorrectly oriented 3/4" material would test less rigid than properly oriented 5/8ths" material.

Entirely up to you, though, and it's all about risk tolerance. Keep in mind, though, that when the plywood is tested as part of an entire tiled package to determine those minimums, it's done using new material, in perfect condition, near perfectly installed over joists with zero deflection. And the test needs pass only once.

My opinion; worth price charged.

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