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Old 09-19-2012, 12:17 PM   #1
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Marble tile


Hello!

I am planning on installing marble tile in my 2nd floor bathroom. The area I am installing tile is small (about 7' by 5'). I currently have 2 x 8 joists that are 16" on center. The joists run about 10' from the outer wall to the inner wall. The subfloor is 3/4" hardwood slats (the house was built in 1929). Should I add another layer of 3/4" plywood over the subfloor, then durock, then SLC?

I am concerned about deflection. Is 5' by 7' a large enough area to be concerned with cracking of grout/tile? The remaining 3' of the joist span is taken up by the bathtub.

Thanks!

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Old 09-20-2012, 04:36 PM   #2
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Hi,

Since this is an old house, it's likely the joists measure an actual 2x8 instead of 1.5x7.25. If so, and the joists are in good shape, that part of the equation should be fine for marble. If the joists are not a full 2x8 you may have a problem. Do you happen to know the species and grade? Is the 10' span a guess or actual to the inch?

You definitely need the layer of underlayment grade ply, 3/4" is very good. Then install your favorite concrete backer and then the SLC as necessary. May wanna consider Ditra membrane instead of concrete backer.

How far out of level is the floor? Just wondering how thick the SLC will be. You noticed I mentioned Ditra as an alternative to concrete backer, I'd consider it. It only adds 1/8" total, where Durock will add about 3/8" if you choose the 5/16 or over 1/2 with the thicker board. If you do go with Ditra, the SLC is done over the ply, before Ditra.

Jaz

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Old 09-20-2012, 05:14 PM   #3
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Originally Posted by JazMan View Post
Hi,

Since this is an old house, it's likely the joists measure an actual 2x8 instead of 1.5x7.25. If so, and the joists are in good shape, that part of the equation should be fine for marble. If the joists are not a full 2x8 you may have a problem. Do you happen to know the species and grade? Is the 10' span a guess or actual to the inch?

You definitely need the layer of underlayment grade ply, 3/4" is very good. Then install your favorite concrete backer and then the SLC as necessary. May wanna consider Ditra membrane instead of concrete backer.

How far out of level is the floor? Just wondering how thick the SLC will be. You noticed I mentioned Ditra as an alternative to concrete backer, I'd consider it. It only adds 1/8" total, where Durock will add about 3/8" if you choose the 5/16 or over 1/2 with the thicker board. If you do go with Ditra, the SLC is done over the ply, before Ditra.

Jaz
The joists measure 2x7.5, and they are oak joists, so they seem pretty solid. I got the 10' span because the room ends on an outside wall, and the joists measure just shy of 10' from that wall to the other side of the room (which is supported by a wall in the kitchen below.

My intention is to install a heated floor system under the tile. I currently have 3/4" hardwood and a 1/4" layer on top of that (which was under the floating floor that was there). I was only thinking of doing more wood then thick durock for stability. However, if it would make more sense to only add another 1/2" of ply, SLC, then a ditra membrane, I will go that route. I figured the SLC will help to level the floor, and cover the heating coils at the same time. I'm not sure how much you can put over those though. Too much sub-floor and I will have to step up into the room!
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Old 09-20-2012, 07:46 PM   #4
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OK, 2x7.5 Oak joists. Do you know what type of Oak? Oak is not as stiff as Doug Fir or Southern Yellow Pine. I don't think 2x7.5 Oak, even the stiffest which is Red Oak will take you to L720 deflection at close to 10 ft. I believe your floor has too much bounce for marble tiles. You're likely to have cracking with a 10 ft. span. If the span was 8 ft. it might be ok. Be warned and consider a nice porcelain tile instead.

Is your hardwood "the" subfloor? Are they fastened directly to the joists? Normally hardwood is not the subfloor and has to be removed.

Depending on what your subfloor is, how wide the planks etc, you can go with 5/8" and Ditra. That would save you lots of height, cuz SLC usually ends up being a lot thicker than people thought it would.

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Old 09-25-2012, 04:41 PM   #5
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It appears the subfloor is the same wood (oak) as the joists. They weren't using OSB or plywood sheeting from what it looks like, but rather boards that are 12" by 3/4" and attached to the floor joists. So would you recommend another 1/2" of OSB on top of what I have, then SLC, then Ditra?

Unfortunately I jumped the gun and special ordered marble for the floor before thinking about support. I just figured with the house being old and having used quality building materials, combined with the small space, that it would be ok.
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Old 09-25-2012, 08:50 PM   #6
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You need to understand a few things. Marble and many other natural stone tiles are much softer and weaker than manufactured tiles and so require a stiffer subfloor system. The joists are one equation, the other is the subfloor which gives you stiffness between the joists.

The deflection of these two parts is quoted as a fraction. The basic minimum standard, (maximum deflection) for modern homes other than sleeping rooms is L360, (1/360). However with natural stone tiles the industry standard is L720 or better. I know of few homes that will meet that standard without making adjustments to the joists and subfloor.

You're thinking the joists and subfloor planks are Oak. Not impossible, but very unusual. So, I don't know what to do with that and you're not able to give a definite answer. I need answers, otherwise we're just wasting time.

And no, I would not use 1/2" OSB over 12" planks, if we assume the rest is ok to proceed.

Jaz
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Old 09-25-2012, 09:21 PM   #7
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Red oak has a modulus of elasticity of about 1.2 million psi. The L/D ratio for 2 inch wide x 7.5 inch high red oak joists spaced 16 inches on center spanning 10 feet is about 675, which is pretty close to the desired 720. The system will be a little stiffer than 675 assuming you use high quality 3/4 inch plywood appropriately fastened to the joists. So you will not be all the way to L/D = 720, but you will be close. Use of Ditra would provide a slippage plane which would minimize the potential for cracking of the tile or grout, but not eliminate it. Use of small format tiles (say 6 inch) will also reduce the potential for cracking versus use of large format (12x12 inch) tiles.
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Old 09-25-2012, 09:52 PM   #8
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My plumber mentioned he thought they were oak (he is also a woodworker). If it helps I can take a photo of the joists in my basement, which are the same material and same subfloor.
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Old 09-25-2012, 10:03 PM   #9
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All,

Because these are old joists measuring a full 2"x7.5 instead of 1.5x7.25 I also agree he is very close to min. specs. If all other aspects are fine I'd go ahead, but I'd try to find a mortar that claims to work well with natural stone even at L480 instead of the industry's L720 spec. I believe Laticrete is where I'd research.

Jaz
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Old 09-25-2012, 10:22 PM   #10
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So would you reccommend slc, ditra, then tile, given my sub floor is 3/4"? Ditra looks like a great choice.
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Old 09-25-2012, 10:35 PM   #11
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Yes, we recommend Ditra, but what happened to the 3/4" underlayment?

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Old 09-25-2012, 10:49 PM   #12
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I'm sorry, I'm confused.I thought you said earlier when I mentioned putting osb on top. What would you reccommend I put down in between the sub floor thats in there now and the tile?
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Old 09-26-2012, 08:33 AM   #13
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Quote:
Originally Posted by Ocelot View Post
I'm sorry, I'm confused.I thought you said earlier when I mentioned putting osb on top. What would you reccommend I put down in between the sub floor thats in there now and the tile?
So, what are you saying?

You said;
Quote:
So would you reccommend slc, ditra, then tile, given my sub floor is 3/4"? Ditra looks like a great choice.
Your subfloor is the old planks, you think they're 12" Oak. You need to add an underlayment, I recommend ext. glue underlayment grade plywood. Being that the planks are so wide you probably need the thickest you can stand. You said you were gonna add 3/4", I think that's good.

Jaz
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Old 09-26-2012, 12:28 PM   #14
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These are shots of what my joists look like in the basement. The actual bathroom is not on this floor, but the joists are the same condition upstairs.

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