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Old 08-28-2013, 06:01 PM   #1
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Wet bed or thin set with Ditra for stone tiles


Which is better? The tiles are 16"x16", 3/8" thick slate. Joists, 3"x10", are spaced 16" span about 18'. We have tongue and groove solid wood subfloor diagonally. Trying not to raise the floor too much. It butts against hardwood floor in the adjacent room. I am told we could bury heating cable in 1/2" mortar bed and put down the tiles on top or put down 1/2" plywood, thin heat mat and ditra. If I am concerned about the stability of the floor, which would be a better choice? Thank you.

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Old 08-28-2013, 06:09 PM   #2
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Wet bed or thin set with Ditra for stone tiles


Sorry, if you have 3x10 floor joists 18 feet span--you need to beef up the framing or switch to a ceramic tile.

My deflection chart shows the deflection is 392---

safe deflection for ceramic is 360---for natural stone you need 720

check those joist sizes and unsupported length---then we can talk about the floor deck and backer---

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Old 08-28-2013, 06:14 PM   #3
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Wet bed or thin set with Ditra for stone tiles


Thank you. Unfortunately, that's the size of joists. How do you suggest to strengthen the floor?

Last edited by Igghy; 08-28-2013 at 06:23 PM.
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Old 08-28-2013, 06:22 PM   #4
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Wet bed or thin set with Ditra for stone tiles


You need to add more floor joists or a support beam to shorten the span.

Few residential floors can support natural stone without adding extra strength to the floor----
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Old 08-28-2013, 06:26 PM   #5
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It is the kitchen floor and the garage is below the kitchen. We can't add a support beam. Do you suggest tearing down the subfloor and double the floor joists?
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Old 08-28-2013, 07:43 PM   #6
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Wet bed or thin set with Ditra for stone tiles


Igghy,

You need to switch to a nice porcelain tile or do a lot of extra work to meet minimum specs for slate tile.

3x10 timber joists spaning 18' should be fine for ceramic/porcelain, but not slate as Mike said. Then it sounds like you have 1" nominal planks laid diagonally. This is your subfloor. You'll need to add min. 1/2" underlayment grade ply, then the warming wires and finally Ditra. You can not install wires or self leveling cement over the planks. Some bad info there.

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Old 08-28-2013, 09:15 PM   #7
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Wet bed or thin set with Ditra for stone tiles


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You'll need to add min. 1/2" underlayment grade ply, then the warming wires and finally Ditra. You can not install wires or self leveling cement over the planks. Some bad info there.
Seems to me you are saying the same thing as he was actually told to do, based on how I read his post.
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Old 08-28-2013, 09:34 PM   #8
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Seems to me you are saying the same thing as he was actually told to do, based on how I read his post.
Thank you,Jaz---I was hoping you would look in on this one---You know I am no familiar with the Schluter products.
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Old 08-28-2013, 11:00 PM   #9
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Thank you for adivices. Very helpful. My builder didn't warn me about joists needing enforcement and i already got the slate tiles. I cannot return them without losing good money. What would be the best way to enforce joists? If I tear out subfloor planks and double the number of joists, would that be sufficient?
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Old 08-29-2013, 06:23 AM   #10
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How old is your house?

is the floor structure open below so you can see the joists?
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Old 08-29-2013, 06:51 AM   #11
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It's built in 1953. Below is the garage with closed up fire retardant ceiling. far end of it is showing where I took joist measurement. We had to enforce joists last year to remedy termite damage at the far end. It is not a big area, about 8'x18'. Thank you.

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Old 08-29-2013, 06:56 AM   #12
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Thanks---yes, sistering in floor joists will be the answer---let me see if I can get a deflection rating ----
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Old 08-29-2013, 07:00 AM   #13
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589 is the deflection with a sistered in 2x10----

not quite enough----
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Old 08-29-2013, 07:07 AM   #14
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Wet bed or thin set with Ditra for stone tiles


Note that these aren't hard and fast numbers - like if your deflection is below X your floor will crumble, and if you're above you're fine. It's more of a risk factor. You might be OK, you might not. Lots of people have tile installations that are going strong that didn't follow recommendations. And many do not.
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Old 08-29-2013, 07:41 AM   #15
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Maybe we can get away with 589 then? I'd get worried whenever the weather turns nasty. I tend to worry about thses things.

Could we use 3x10 to sister in to double the strength? Is 2x10 true measurement or is it more like 1 3/4" by 9.5" in practice? Or can we add 3x10 between existing joists istead of sistering in. That will half the spacing and double the strength? Does it work that way?


Last edited by Igghy; 08-29-2013 at 07:57 AM.
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