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Old 07-05-2005, 01:33 PM   #2
Da Home Builder
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Join Date: Jul 2001
Location: Boerne, Texas
Posts: 86,170
Welcome, Jon.

I'm gonna assume your joist spacing to be 16" on center judging by your results from the world-famous deflectometer. That being the case, you're not gonna meet the required L/360 deflection criterion for a tile installation. Close, but no cigar.

While you have the whole thing open, I'd suggest you sister those joists with additional pieces. Your sisters would not need to be the full depth or length of your existing joists, which would make the job rather easy in the grand scheme of things. Best to sister the full length if possible, though.

If you sistered with 2x6s, you could avoid having the sister touch the subfloor if you want, thus eliminating a possible source of squeaks. But you would also not gain any reduction in span of your subflooring (you'll be at the minimums for that also with only a single layer of 3/4 ply).

A further consideration would be the plane of the top of the existing joists. You could correct any un-flat/un-level condition while adding the sisters and kill a couple birds with that same rock. That would be my first choice in your situation, I think.

Ditra's advantage over CBU is ease of installation and thickness, or lack thereof. Its disadvantage is cost. Either will work in your project.

My opinion; worth price charged.

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