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Unread 01-12-2021, 09:57 PM   #7
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Join Date: Jul 2001
Location: Boerne, Texas
Posts: 92,663
OK. And I had misread state in your drawing to be slate. I see you're planning only ceramic tile.

Bottom line: You gotta remove everything down at least to the board subfloor.

If you can stand to remove the board subfloor, that would give you the opportunity to flatten or/and level (only the flattening is required) the floor at the joist tops, the very best place to do that work. If not, you might consider a reinforced mortar bed of a minimum of 1 1/4" thickness with welded wire mesh in the vertical center. That would give you the very best substrate for your tile installation and allow you to make the surface near perfectly flat and level if you want it level.

If you don't want to do the mortar bed, you must, as Jim pointed out above, install a minimum of nominal 1/2" plywood over your board subfloor before you can do anything else in regard to flattening or leveling. It would be possible to use the SLC you mentioned. That material is not really self-leveling, but it can be caused to be level and flat if you work it correctly. And you could then use that as your tile substrate or you could add a tiling membrane of some sort if you prefer. Different manufacturers have different requirements for minimum thickness and reinforcement when installing the product. And each of them is exceeding proud of his product as reflected in the pricing.

My opinion; worth price charged.

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