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Unread 10-21-2020, 04:21 PM   #1
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Join Date: Oct 2020
Location: Queens NY
Posts: 17
Slate Deflection in Condo and 1/2" vs 1/4" CBU?

Hi Everyone,

I kept getting really good info from searching google on this forum, so thought I would ask a quick question...

I'm re-tiling the kitchen in my condo, I want to do Natural Slate, 12x24x3/8" (can cut down to 12x12 if it's necessary)

It's a sealed condo floor, so no way to get under the subfloor to take a look at what the joists are made of. It was build in 1991 in Queens, I do know that the joists are wood or more likely engineered lumber, 19.2" on center, with 3/4" ply on top. The kitchen is around 90"x94", so not a huge span. Unfortunately the Deflectolator is not terribly helpful here without joist details.

Doing an IRL Deflectolator with my laser level, I see pretty much no difference in the laser height on the wall setting it on the middle of the floor with no load vs. 2 people standing next to it (400lbs!). Maybe 1/32?, 1/64 or less than 1/2 the width of the laser beam. Valid test? Seems pretty solid just by feel.

The old floor had 15/32" underlayment which had rotted from leaks and was covered in glue from a a terrible tile job, so I pulled it up and replaced it with 19/32" CDX, glued and screwed every 8" in the field and 2" on the perimeter with expansion gaps. Going by TCNA Handbook F250-16 for natural stone as a guide....

Next step is concrete board...

So my questions are

1. Is there any reason to go with 1/2" CBU vs 1/4"? Seems like most people say it does not add rigidity. I have 1 3/8" of plywood on top of the beams, and will already have 3/4" to transition to the wood floor using 1/4" CBU. I'm also thinking Hardiebacker for sentimental reasons.

2. Still safe to go with the 12x24 natural slate tiles? Is there any huge advantage to avoid future cracking if I run them parallel vs. perpendicular to the joists? Or cut them in half to 12x12?

3. The floor is pretty much flat, but is not level, sloping around 3/8" over 8 feet. Is this a major issue other than having to shim the cabinets up after installing the tile wall to wall?

4. Any other advice?

Thanks in Advance!

Last edited by risotttto; 10-21-2020 at 04:41 PM. Reason: flatness vs level
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