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Old 06-14-2018, 04:34 PM   #1
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Want to install 1" hex marble tile, Deflecto says no natural stone

Hello all,

Remodeling small 1940s bathroom, and hoping to install 1" hex marble tile (3/8" thick) (can't post link but it's Carrara (Carrera) Bianco Hexagon Honed 1" Marble Mosaic Tile from theBuilderDepot.com, Item #CBH-1X1-HEXH.)

My results from this site's Deflect-o-meter tell me I can't install natural stone but I could do ceramic tile (Inputs: Doug Fir joints, 5.5"x1.5"x8' joists; Results: deflection of 0.229 inches, "This translates to a deflection of L / 420. Since the maximum deflection for tile is L / 360, and for natural stone is L / 720, your floor is rated for Ceramic tile, Congratulations!")

1. Does this mean I can't use the above-listed hex marble tile?

2. If not, can I beef up the joists by sistering, allowing me to use this marble tile?

3. Also my plan had been to replace the decaying 5/8"x7" planks (see pic) with 3/4" (actually 23/32) tongue and groove plywood, then our tile guy is installing Hardibacker as underlayment. Is this sound?

What am I missing here? Any advice welcome. Thanks in advance.
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Old 06-14-2018, 05:24 PM   #2
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Welcome to the forum.

1- correct

2- yes, or add a load bearing beam below to reduce the span

3- the tile bible prohibits tiling over plank subfloor. If its in good shape, and well secured (I'd screw them down) you still need to add plywood to the top before the ceement board. Got room (height) for all of that?

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Old 06-15-2018, 08:39 AM   #3
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Thanks for the tips and the welcome. I don't have room to keep the planks, otherwise tile would be 3/4" high where bath floor meets the hallway wood floor at door.

Is there a structural concern with removing the planks, or is it just that it's obviously less work to leave them in place?
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Old 06-15-2018, 09:34 AM   #4
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Welcome, Jake.

Not usually. While it's possible the removal of large areas of subflooring could weaken the structure, your small area should pose no such problems.

With your boards being only 5/8ths-inch thick, and not being oriented perpendicular to the joists, removing them is a good plan. The minimum thickness of plywood required over board subfloors is nominal 1/2-inch, but your floor wouldn't qualify for that.

Nominal 3/4-inch plywood would satisfy the requirements for Hardiebacker if your joists are spaced at no more than 16 inches on center.

My opinion; worth price charged.

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Old 06-15-2018, 03:48 PM   #5
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If you insist on the natural stone tile, you have to do two things:
- beef up the joists to achieve the required minimum of L/720
- achieve a TWO layer subfloor (your current planks do not meet the requirements for the first layer), with product meeting the industry requirements.

Then, whatever you decide to put on top of that second layer of ply is up to you, whether it's a membrane or cbu. There's an article in the 'Liberry' that describes the minimum specs of the materials and the installation method required to make it work.

There is one way to keep the buildup lower, but it's a lot of work, and isn't officially recognized. That would be to recess your first layer so it is even with the tops of the joists on adequate blocking/cleats. The ply's orientation would need to be between the joists, so you'd be working with 4' sections, not 8'. THen, install a second layer of ply over the entire floor. If there are any gaps between the sheets, it won't work right. That first, recessed layer would need to be at least 5/8", and personally, I'd want the second that same, and preferably make both of them thicker. Getting a screw to hold in 5/8" ply is tougher, so 3/4" would be better for the first, recessed layer.
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