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Unread 10-09-2019, 08:21 PM   #4
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Join Date: Jul 2001
Location: Boerne, Texas
Posts: 90,285
If your joists are of good grade and species, the structure meets code and tile industry deflection requirements for a ceramic tile installation, but you're certainly not overbuilt. That said, you'll be adding a good bit of dead load to the floor with your installation.

Your subfloor, as described, should be adequate as well and your proposed between-joist blocking just isn't gonna buy you enough of anything to be worthwhile in my opinion. Any builder who would start a subfloor with half-inch, square edge plywood is very unlikely to have glued it to the joist tops. With that in mind, it's at least possible that if you elect to remove the ceiling below you could raise the center of those joists a bit and sister them to perhaps alleviate a bit of your center area sag. May be more work than it's worth, but that would be up to you.

I doubt that removing that hearth will make any notable difference, but it's not a bad idea, anyway. Surely the fireplace is not also supported by the floor framing, right?

I would want a look at the joist structure just to see why the sagging took place at all before I committed to adding more dead load to the floor.

As for the flattening above the existing subfloor, if you plan to use a CBU as your tiling substrate, you must do all your flattening or leveling on top of the CBU. If, instead, you elect to use a membrane of some sort as your tiling substrate, you must do all your flattening or leveling on the subfloor before installing the membrane. Either method is perfectly acceptable.

And for tiles the size you're planning to use, the industry standard for flatness is no deviation from intended plane of more than 1/8th" in ten feet nor 1/16th" in two feet. That's a very, very flat floor and you'll be glad to have it when you start setting those big tiles. The only two ways I know of to actually achieve that degree of flatness is with a properly executed SLC installation or a mud bed, but you can try whatever you think will work for you.

My opinion; worth price charged.

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