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Old 09-29-2009, 04:16 PM   #1
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Slate subfloor specs question?


Does anyone happen to know what the Tile Council's specs are for natural stone (slate) installation? I read somewhere that for ceramic installations, they say you need a 5/8 in ply subfloor, but then i read somewhere else that for natural stone that it's 1 1/4 inch - although I'm assuming that's with some sort of cement board. Also i read that the deflection for slate should be L720....thanks so much for your feedback!

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Old 09-29-2009, 08:01 PM   #2
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Slate subfloor specs question?


Hi Britt,

The recommendation is L720 max deflection for the joists and double plywood subfloor that is at least 1 1/8". That is the very min. for the subfloor thickness. Most of the time people start with a 3/4" sub. and I recommend adding 1/2" underlayment plywood for a total of 1 1/4".

Over this you add the cement backer or a membrane such as Ditra or one of Noble's membranes.

Do you know the deflection of your joists? The lack of enough strength between the joists, in the subfloor is the major cause of tile failures.

Jaz

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Old 10-05-2009, 10:50 PM   #3
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Slate subfloor specs question?


a 14 by 9 kitchen area. I have particle subfloor and have added 1/2 in plywood already as the rest of the dining room, living and hall are being done in pine tongue and groove. I live in a mobile home (16x76) early 1990 model. I didn't think of the weight issue. Am I going to be ok?
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Old 10-06-2009, 08:13 AM   #4
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Slate subfloor specs question?


Nope.
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Old 10-08-2009, 11:58 PM   #5
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Slate subfloor specs question?


should i just stick with putting the tongue and groove in instead then? what will happen with the tile? seems like the weight would be evenly distributed enough. I don't want to ruin anything.
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Old 10-09-2009, 06:07 AM   #6
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Slate subfloor specs question?


Slate requires the same rigid floor as ceramic tile, a min of 1 1/4" of very stiff subfloor. And a tile backer as the last layer. If you don't have that, the grout and possibly the slate will crack in short order.
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Old 10-14-2009, 08:07 PM   #7
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Slate subfloor specs question?


i don't understand "deflection". i did have someone who is experienced in laying floors look at it and because it is not 14x7 straight through but rather, with 3' and 5' sections that would not be tiled because of cupboards that i would be ok. i would still like to be sure that i am not in danger of ruining my floor joists. input?
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Old 10-14-2009, 09:15 PM   #8
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Slate subfloor specs question?


OK, so let's start with you telling us the type, size, spacing (24") and unsupported span of the joists. I've never seen a mobile home that even came close to L720 deflection, heck hardly any regular house meets L720 either.

However it is believed not meeting L720 wouldn't be the end of the world for slate, since the deflection between the joists is more important than that along the joists span. That raises another problem with your 24" o.c. spacing, not good but that could be workable with the added plywood and maybe some Ditra or Ditra XL?

One big problem is your particle board sub. Most people would tell you it has to go....too late now I guess? You could still go ahead as long as you realize you'd have a high risk installation that no one will guarantee.

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Old 10-14-2009, 09:38 PM   #9
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Slate subfloor specs question?


down. I plan on adding 1/2 in "Durock" next gen cement board that is suppose to be the "lightest weighted" prior to the slate tiles (which are on the thin side anyway). Its the weight I'm afraid of. But it will be laid out approx 4 ft wide in front of sink and cupboards and then wider when going under the stove and frig areas to the length of the room around 14 ft. Do I have a 60/40 shot at success? maybe 70/30?
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Old 10-14-2009, 09:41 PM   #10
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know i hit them at 24 in apart.
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Old 10-14-2009, 10:41 PM   #11
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Slate subfloor specs question?


The problem is that the combination of joists, particle board, Durock, and slate is quite heavy, and without specific knowledge of the size of your joists it is impossible to say whether the joists would be strong enough to support the load, let alone whether the slate will crack due to deflection. Strength of the joists and deflection are different criteria, strength refers to whether the joists will break, deflection refers to how much they move under load. Both are important. I strongly suggest you crawl under the mobile home and measure the width and depth of the joists before you put down the slate, this project could be serious trouble if the joists are undersized.
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Old 10-14-2009, 11:04 PM   #12
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Slate subfloor specs question?


can remove some of the insulation under there to see. But if I am under there and doing that, I would like to add support then, no matter what is already there. How do I do that? I'm thinking just cut up some 2/4's and attach them crossways? (meaning it would look like a ladder or "H"'s when done) would that work? Its almost winter and........
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Old 10-15-2009, 04:55 PM   #13
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Slate subfloor specs question?


Quote:
I'm thinking just cut up some 2/4's and attach them crossways?
That won't help, if the joists need strengthening, you will have to sister them or cut the unsupported span down to where they will meet specs for stone by placing a cross beam mid span under them.
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Old 10-15-2009, 10:28 PM   #14
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Slate subfloor specs question?


I would have gone with the wood option. Is there somewhere else you can use the slate? A hallway or even outside, depending on the color.
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Old 10-24-2009, 04:28 AM   #15
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Slate subfloor specs question?


as I don't want to ruin my foundation (ie: floor supports) just because i think the slate would be gorgeous there. Hmmm, but i really want it So, thanks for the recommendations and know how. How long do you think it will take me to strengthen it up? Its the cutting of all my insulation out to get to the joists that botheres me. I know good ol duct tape does not stick under there on that plastic. I think sistering will be the easiest if I can't do it how I was thinking.

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