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Unread 10-21-2020, 04:50 PM   #3
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Join Date: Jul 2001
Location: Boerne, Texas
Posts: 91,866
Welcome, S.

If you don't put that geographic location into your User Profile the information will be lost before we leave this page. And it's frequently helpful in answering some types of questions.

The requirement for joist deflection for a natural stone installation is L/720, exactly double the requirement for ceramic tile. If your structure was built without natural stone in the original plans, it's very un-likely that your joists meet that requirement. If you want to install your stone tiles anyway, that's entirely up to you. The information you gained from your deflection test tells you nothing at all useful in determining the design deflection of your joist system

The Deflectometer will not help you with LVL joists even if you could see them. The size of your room to be tiled has no bearing at all on the joist requirement. It's the unsupported span of your joists that matters and you cannot determine that unless you are on good terms with your downstairs neighbor.

For you subfloor: Method F250 requires joists spaced no wider than 16 inches on center. In fact, I think you'll find the Natural Stone Institute doesn't have an installation recommendation for any wider joist spacing. Once again, you wanna install over what you've got, that's up to you.

Your CD grade plywood does not meet the requirements of either the ceramic tile or natural stone industry requirements. We'd also rather that you had not glued the subfloor layers together.

Did you use a full spread of wood glue or something out of a tube?

Did you orient your second layer of plywood with the strength axis perpendicular to the joists? That would be the same direction as the first layer.

1. There is never a reason to use the thicker CBU in a floor installation unless you just want to raise the finished floor height. I find it difficult to get sentimental over Hardiebacker, but...........

2. See above. No, it makes no difference in which direction you orient your tiles.

3. Your tiles don't give a rat's patooti whether your floor is level, they care only about flat. The larger the tiles, the more they care. The industry standard for flatness for tiles the size you have is no deviation 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 come time to set tiles.

4. Consider re-thinking your tile selection. Lots of stone-look porcelain tiles these days that are not unpleasant to the eye and would be a safer choice in your application. But you can certainly install what you have if that's your choice.

Your Condo Association is OK with your new floor covering?

[Edit] Jim's not really faster, he just typed less than did I.

My opinion; worth price charged.

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