Thanks for your confidence, but truth is there are a bunch of other tile pros that devote lots of time and know their stuff.
I think it's safe to say that we all think 5/8" t&g plywood subfloor stinks. I don't think much of a builder that strives to build floors to the lowest standards allowable. However, 5/8" ply subfloor does meet specs as the subfloor. Of course the fine print also says, (or used to say) that the floor assembly must meet L360 deflection, which is again the most allowable deflection for ceramic/porcelain tiles. How does one know that? You don't.
As for your subfloor sheets I would add as thick a plywood sheet as possible, min. 3/8". I would then recommend Ditra membrane or one of Noble's sheet membranes. Alternatively a basic 1/4 or 5/16" concrete backer is fine too. Then the tiles.
I guess your eng. joists are I-joists. They are usually designed to rate no higher than L480, which is better than L360. Since the house will be inspected, we can gather you should be better than L360. Double check with the maker and tell them you want the floor's load rating should be at least 50/20 lbs per. sq. ft. Do not accept a deflection rating that calculates the load at the basic 40/10. (minimum code these days.)
1/2" cement board? 1/4 or 3/8 plywood and then 1/4" cement board?
If you can go with one of the above, why not go with 1/2" B/C ply and then Ditra.
I assume the min thickness is at least a 1inch or more for tile.
NO. The minimum ply thickness is 5/8", to me 3/4" is min. But... the more the better.
TILE GUY - retired- TROY, MI - Method & Product suitability consulting.
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