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Old 05-15-2014, 02:35 PM   #1
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Advise: best subfloor for moasic marble?


Renovating a small 100 year old 6x10 bathroom on second floor. It was gutted back to joists / brick wall. New plumbing was run, and I'm ready to install new subfloor.

Joists are true 8x2, 16" oc. Wife wants to install mosaic marble on floor. My original plan was: 3/4" plywood + 1/2" plywood + ditra.

But mosaic tile is too small for Ditra, so I'm back to the drawing board. My tile installer suggested dry pack. Would 3/4" plywood + 1/2" drypack and then tile work?

What other options do I have? I was planning on installing subfloor tomorrow.

Thanks.

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Old 05-15-2014, 04:53 PM   #2
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Advise: best subfloor for moasic marble?


Subfloor grade material not less than 3/4" tongue and groove and then dry-pack not less than 1-1/8" thick would work.

Two layers of plywood, the first 3/4" T&G subfloor and the second not less than 1/2" Underlayment Grade Exposure 1, then 1/4" cement board would work.

Two layers of plywood, the first 3/4" T&G subfloor and the second not less than 3/8" Underlayment Grade Exposure 1, then 1/4" of Self Leveling Compound would work.

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Old 05-15-2014, 05:38 PM   #3
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Advise: best subfloor for moasic marble?


I don't have enough space for 1/4" cement board or self leveling.. the gap from the bathroom to the hardwood floor is already big.. I can't support >1" gap after tile.

Is there an alternative to Ditra that will work with tiles smaller than 2"x2" and is available to us Canadians? The local home depot sells redgard. Will that work over 1 1/4" wood floor?
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Old 05-15-2014, 05:59 PM   #4
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Advise: best subfloor for moasic marble?


Quote:
Originally Posted by leonki View Post
I don't have enough space for 1/4" cement board or self leveling.. the gap from the bathroom to the hardwood floor is already big.. I can't support >1" gap after tile.

Is there an alternative to Ditra that will work with tiles smaller than 2"x2" and is available to us Canadians? The local home depot sells redgard. Will that work over 1 1/4" wood floor?

Redgard is not an alternative to DITRA. Redgard is a waterproofing membrane only.

Cement board, deck mud, or self leveling compound will provide the "tooth" you need to attach the stone tile. The mosaics are going to require about a 1-1/8" total substrate thickness minimum, and that's stretching it.

The absolute minimum for deck mud would be 3/4" thickness over 3/4" T&G subfloor but that's a minimum that most tile installers would frown on.

You could stick your neck out and use 3/4" T&G subfloor with 1/2" cement board but that doesn't meet anybodies minimum codes anywhere. Your floor structure also requires twice the deflection strength of that required for ceramic tile. I think you probably have that, since your floor joists are full-dimension joists and only 10' long. A double layer of plywood is also a requirement but you are tossing that out the window with your requirements.

Confused yet?
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Old 05-15-2014, 07:41 PM   #5
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Advise: best subfloor for moasic marble?


I'm learning.

So, assuming I don't want to do any mud work over the subfloor, and stick to either adding more plywood, or some man made material which is screwed, glued or mortared into place, I was wondering if the following will work:

3/4" T&G subfloor + 1/2" hardibacker board (mortar between hardi and plywood, and then screwed down into subfloor as suggested by manf.) and than mosaic tile.

Or does one have to have 1 1/4" wood MINIMUM no matter what they use on top of it if they want to use natural stone?
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Old 05-15-2014, 08:33 PM   #6
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Advise: best subfloor for moasic marble?


Quote:
Originally Posted by leonki View Post
I'm learning.

So, assuming I don't want to do any mud work over the subfloor, and stick to either adding more plywood, or some man made material which is screwed, glued or mortared into place, I was wondering if the following will work:

3/4" T&G subfloor + 1/2" hardibacker board (mortar between hardi and plywood, and then screwed down into subfloor as suggested by manf.) and than mosaic tile.

Or does one have to have 1 1/4" wood MINIMUM no matter what they use on top of it if they want to use natural stone?
You can do that. Keeping in mind it doesn't come up to any minimum specs. In reality...The only one you have to please is yourself...erh-a, I mean your spouse.

Okay let's re-write the handbook! May the tile-Gods look down on you with understanding and compassion!

Now it's 3/4" tongue and groove exterior grade plywood. Not treated plywood, not CDX plywood, but exterior grade Exposure 1 plywood. Glued and screwed to the floor joists.

Then 1/2" cement board, set in fresh thinset combed in one direction and screwed to the plywood using a fastener-schedule of screws every eight inches in both directions in the field of the boards, and every four inches along any board perimeter edge or seam. The cement board screws should be intentionally missing the floor joists.

Any seams in the cement board shall be taped with alkali-resistant mesh-seam-tape and joints filled with thinset.

Here's a small hint about installing the seam tape.
OPTION #1. If you will wait to install the seam tape using thinset until just ahead of placing the tile you won't have a hump along the seams to deal with.
OPTION #2. ( This is my suggestion) You can use a spray adhesive to spray the seams and after it tacks a little install the seam tape over the seams. Don't use any thinset at this time. Wait to address the seams with thinset until you are placing the tile. Just ahead of placing the tile squish/force thinset into the seam/joint as you are spreading the thinset to set the tile. Comb the thinset in one direction only.

The seam tape has a tacky-back and will stay in place in the spray adhesive giving you time to cut and fit and cut and fit the tiles as needed.

I assume the tiles are matted to a web backing. The adhesive used to adhere the tiles to the web is water-soluble. Once it gets wet the adhesive will begin to disintegrate. Tiles will drop-off if you dilly dally then you will be placing individual tiles one at a time. That sucks!

But, it is also a good thing...because any tiles that don't match the layout and marry-up with adjacent tiles when placed on the floor can be easily moved and shifted to correct any irregularities in the tile match.

Don't expect all of the sheets to match-up perfectly - they won't. But this is where you adjust individual tiles after the tile-to-mat adhesive has loosened a little. This is your only opportunity to get the tiles placed correctly and fine tune the layout.

Keep a wet sponge and bucket of clean water at your side at all times. When thinset purges up between the tiles quickly clean it with the wet sponge being sure to clean the grout line deep enough so-as to receive grout later. Keep all tile away from the walls leaving a 1/4" expansio-gap around the room.
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Old 05-15-2014, 08:52 PM   #7
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Advise: best subfloor for moasic marble?


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Originally Posted by Bud Cline View Post
You can do that. Keeping in mind it doesn't come up to any minimum specs. In reality...The only one you have to please is yourself...erh-a, I mean your spouse.
If the tile cracks after 6 months, she won't be that happy. But at the same time, I was warned .. she doesn't want a big step between the hardwood floor on the entire 2nd floor and the bathroom. She's fine with a small transition.

That being said, I put priority on not cracked tile vs transition. So I'm trying to figure out the best application (and order). As far as I understand it, based on reading, and your great suggestions. I can also do the following:

- 3/4" T&G + 1/2" plywood + 1/4" hardi

If I go this route, what do you think the final thickness (joist to top of basketweave mosaic tile) will be? Rough estimate?

Hardi's web site suggest that it can be installed on top of 3/4" T&G and then tiled. The secret that I think I clued onto is that it's only rated for ceramic install! Because I'm going with natural stone, the subfloor has to be doubled and thicker (1 1/4" min). Am I finally getting it right?

Is it true than that hardi does not add to the strength of the subfloor? (i.e. same as ditra doesn't add strength).
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Old 05-15-2014, 08:57 PM   #8
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Advise: best subfloor for moasic marble?


almost forgot.

After I'm done with the subfloor, and before I install tile, I need to install bathtub into alcove. Should that be done first (i.e. tub sitting on 3/4" T&G) or should tub sit on top of 1/2" plywood or should tub sit on top of 1/2" plywood + 1/4" hardi?
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Old 05-15-2014, 09:24 PM   #9
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Advise: best subfloor for moasic marble?


Chapter Two

No cement board offers any structural benefit what-so-ever. Your structural improvements are in the plywood. In this case, good solid plywood with no voids hidden in the center between laminations.

Floor deflection is NOT your friend. Floor deflection of NTE 1/360 is required for ceramic tile. Floor deflection of NTE 1/720 is the requirement for natural stone.
The age of your structure and the floor joist dimensions may well have you where you need to be, or very close. I can say this only because I am familiar with 100 year old structures using full-dimension lumber and having small bathrooms. Worked on too many of them.

Natural stone floors also require two layers of plywood. Here's the kicker; that two-layer-requirement is for larger format stone tiles, not necessarily stone mosaics. Stone mosaics are going to offer a little more forgiveness when it comes to deflection. This is why I am so willing to quickly back away from the specifications a little. The mosaics are the one major thing you have going for you.

Okay...overall thickness from joist surface to tile top ???? Based on your earlier laundry list......Two Inches.

About the tub...
If'in it were me - I would place the tub on top of everything, including the Hardiebacker. The reason is...if you should get a seriously wet floor, you wouldn't want that water running in-under the tub and finding its way into a pocket somewhere down under. Should water get under the tub it would be slowly wicked into the Hardiebacker depending how much water there was and then have an opportunity to evaporate without damaging anything. Of course if a lot of water gets loose then it's insurance-time anyway.
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Old 05-15-2014, 09:58 PM   #10
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Advise: best subfloor for moasic marble?


Bud, I can't begin to thank you for all your help. You're like a tile encyclopedia and very much appreciated!!!

The reason I asked about the tub is to see if it buys me more time. More time for the wife to shop for tile (hope she changes her mind and goes all ceramic!) so I can focus on installing tub, finish plumbing, outside wall installation, hang drywall, and only then (fast forward 3 weeks as I'm doing this as a weekend warrior) I can decide do I add 1/2" ply + hardi, or do I go hardi only and then pay the tile guy to do the tile.

I'm not so worried about water proofing under tub, as most tub installer know, there a 9"x12" opening under the tub drain for plumbing after the tub is set. If water runs .. it runs into that BIG HOLE!!

It amazes me how every tile store and tile installer gives me a different answer. Makes it VERY DIFFICULT for the home owner / DIYer as myself. Thank goodness for the internet .. as I've spend the last 12 hours reading forums, I keep on reading the same thing from many different people .. and I always bet the odds.

So.. worst case scenario (i.e. thickest floor to do it right). I'm looking at:

3/4" T&G + 3/8" ply + 1/4" hardi + the tile ..

The thing that's working against me:

2nd floor has really old hardwood floor (you know, the 2" wide planks?) it was sanded so many times, that I don't think it's even 1/2" thick sitting on top of 3/4" plank boards (which I removed in this bathroom).

So the actual height difference between bathroom floor and hardwood floor is on the order of: 3/8" + tile ..

is this too much? or have you ever ran into something like this before?
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Old 05-15-2014, 10:27 PM   #11
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Advise: best subfloor for moasic marble?


Matching new tile installation elevations to existing floor covering elevations is something that installers strive for every day. However, sometimes it just isn't in the cards. Something you might consider since you are apparently trying to install a bathroom floor that is somewhat a period motif, you could consider a 4" marble threshold in the door way.

This would accomplish a couple of things. It would allow your elevations between the two floors to vary without being unsightly. It would basically remove any trip-hazard. It would add to the decorum. It would subconsciously tell users to "step-over" at the entry and would say be ready for a slight change. A 3/8" variance in a floor surface is a lot actually but if it is aided by using a threshold it becomes a non-issue.
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Old 05-16-2014, 10:45 AM   #12
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Advise: best subfloor for moasic marble?


In the old days when bathroom floors were always mudded, there was usually a good 3/4" or more difference in height from the hardwood hallway. The mud could be made as thin as 3/4", but we added a marble threshold which was about 3/4". So the top of the threshold was from 1 1/4" to 1 1/2" above the subfloor. I said 1 /1/4" because the mud under the thresh was often scraped lower to minimize the height.

With today's thinner installation methods it's possible to make the transition much closer. 3/8" is nothing and since you have two different types of flooring, a marble threshold would solve the transition and look good, as Bud mentioned.

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Old 05-16-2014, 11:14 AM   #13
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Advise: best subfloor for moasic marble?


There are many transition types, ranging from "hard" steps ups to angled wood transitions to "newer" products from companies like Schluter in angled metal, such as the Reno series.
http://www.schluter.com/137.aspx

I don't see any reason you can't put mosaic over Ditra, and at only 1/8" it will be lower than any HardiBacker. To make the installation cleaner, you can take one pass with the thinset to fill in the holes, then let it dry or at least set up, then comb your tile thinset. To avoid getting a lot of thinset squishing up, I comb out the thinset, then "knock down" the ridges with the flat side of the trowel to create a smooth layer of thinset without high spots to get into the mosaic joints. For example, if you're using a 1/4" trowel, you would end up with a flat 1/8" of thinset to set the mosaic in. Therefore the total height on top of the subfloor is going to be about 1/4" including the tile setting thinset. You can't match that with backer board.

By the way, I usually use the thinset leftover from laying the Ditra to fill in the waffle, assuming I'm not tiling until the next day. Keep in mind that you use a modified thinset to set Ditra over wood subfloor. You normally shouldn't use this same thinset to set the tile, since that should be unmodified. But the reason you use unmodified is that the thinset normally can't dry when sandwiched between the Ditra and the tile. But if you're leaving it to dry for a day before setting the tile, it's OK. It makes use of thinset you otherwise would have wasted and saves a bit of time when you install the tile by not having to make a first pass filling in the waffle.

Last edited by jeffnc; 05-16-2014 at 11:21 AM.
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Old 05-16-2014, 12:15 PM   #14
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I don't see any reason you can't put mosaic over Ditra...
Because Schluter Systems says: "ABSOLUTELY NOT"!

Here's why: The feature that makes DITRA work so perfectly as an isolation-underlayment is the waffle-channels that exist in the DITRA Mat.

The bottom of the DITRA is clad with a "scrim" (if you will) that will not allow thinset to get into the bottom side of the channels. This is what gives DITRA its ability to flex laterally and isolate the tile from the substrate. On the topside the waffled squares are filled with thinset when installing the tile thereby creating a solid column of compression strength between the tile and the substrate. To maintain this compression benefit a tile must cross-over a minimum number of filled squares. Mosaic tiles do not meet that minimum size criteria and could easily rock and become dislodged due to the open channels (underneath/underfoot) when walked upon. You are already fudging the minimums as it is. To use DITRA would be a huge mistake.

In fact I am surprised and disappointed that a seemingly experienced guy would even suggest it would be okay to use DITRA under mosaic tiles when it is clearly against the manufacturer's recommendations. And for good reason. The fact that natural stone mosaics are intended is even more reason not to use DITRA in this particular application.
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Old 05-16-2014, 12:30 PM   #15
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Advise: best subfloor for moasic marble?


Jeff,

Filling the waffles of Ditra is something some of us have advocated for years when setting small tiles. I've never had to set small tiles over Ditra, (chose CBU instead), but have filled the squares to use up left over thin set at the end of the day. Usually near the wall under appliances, small amounts. But Schluter does not like the idea, and I wouldn't feel 100% confident to suggest it with a guarantee. Do it at your own risk, it'll probably work most of the time in small spaces.

However I don't agree with your method of troweling thin set over Ditra and then going back to knock down the ridges. The amount of thin set above Ditra's surface will not be different because of the holes/squares. You use the same trowel that is compatible with the tiles you're using whether it's on a flat surface or Ditra. So try using a different trowel Ditra does not change that.

I know you referred to the 1/4" trowel as an example, but I doubt I'd use a 1/4x1/4" for mosaics. Plus 1/4" trowel does not leave adhesive ridges that are 1/4" high. 1/4" wide yes, but not high.

Jaz

Opps, I type slow. Now I will read what Bud thinks.

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