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diy888 01-21-2009 01:47 PM

granite floor tile which mortar?
On the MAPEI site, the Kerabond info sheet says it is not to be used with natural stone: "Do not use for setting moisture-sensitive tile, natural
stone or their agglomerates" but the same document goes on to say that 100% coverage is required "when installing marble, granite and agglomerate tile floors." So I am confused.

Is the document making a distinction between natural stone wall tile and natural stone floor tile? Will Kerabond + Keralastic work with 12x12 granite floor tile? Is there a better mortar for this purpose?

Also, are the 1/8" gaps you leave between plywood boards on the underlayment (below the cementitious backerboard) considered "expansion or control joints", or are these gaps between the plywood something different? The Kerabond document says "Do not cover any substrate expansion joints or control joints with mortar or tiles."

The backer board manufacturer said the same thing. Are they referring to major expansion joints, not to the small gaps you leave between plywood sheets?


angus242 01-21-2009 02:02 PM

There are some natural stones that cannot be set using regular portland cement based thinset. Green marble and onxy are two that come to mind. You should have no problem using Kerabond/Keralastic. That's one of the best possible thinset combos you can use.

Don't worry about the 1/8" plywood gap. Yes, it is there for expansion purposes but it's completely different than an expansion joint.

Are you sure your subfloor can handle a natural stone tile installation? Most standard floor cannot without some form of added support.

diy888 01-22-2009 06:43 AM

Thanks again, Angus, for the clarifications.

I haven't found an independent comparison of isolation membranes. I bought 1/4" WEDI panels to go onto the plywood floor, but could use them elsewhere in the house. Would DITRA be a superior isolation membrane over plywood?

The floor passes muster dimensionally (nominal 2x10 joists, 16" o.c., 9.5' span) but it comes down to a judgment call about the condition of the wood.

If I say it's "unknown wood in good condition" the Deflecto calculator on the JohnBridge website yields L/735. If I say it's "unknown wood, kind of cracky and knotty" the calculator yields L/566.

So I have installed lots of blocking, have sistered the joists extensively, and have bolted 3/8" thick steel flitch plates on three of the joists, 3-5 ft in length, as wiring allowed, where some deep notches had been cut to route plumbing.

So it's a leap of faith, but the floor doesn't seem to deflect when I jump :)

angus242 01-22-2009 06:57 AM

Sounds like you've done your homework and are ready to tile.

My preference would be to use Ditra. You've obviously spent a lot of time and money on this project. Why "settle" at this point. There are many tile pros that rely on Ditra and I am one. Actually, that's the only underlayment I use. I will not do it any other way. It's very easy to install and you already have the proper thinset. Just use the Kerabond/Keralastic UNDER the Ditra and just Kerabond/water over Ditra. If you want to go one step further, get yourself a roll of Kerdi-Band and lay that over your Ditra seams and then overlap from Ditra to the wall. Now you have a completely waterproof floor!

diy888 01-22-2009 05:53 PM

My floor slopes as much as 3/4" over 10 ft. A customer service rep at the Schluter offices said the floor must be level, not just flat; whereas WEDI told me that although they do specify a maximum slope of 1/4" in 10', a slope greater than that shouldn't be a problem as long as the floor was stable and not moving any further. Does my sloping floor make DITRA really a no-go, or was the customer service rep quoting the spec I would have to meet in order to qualify for their guarantees/warranty?

Bob Mariani 01-22-2009 07:00 PM

The floor should be level. How are you going to get each tile to fit correctly next to each other if they are at a slope. Use self leveling compound first then the Ditra.

diy888 01-22-2009 09:29 PM

I can't use SLC, Bob, because the threshold between bathroom and bedroom is at the low end of the bathroom, and the SLC pour would make the difference in height about 2". The bathroom plywood is already almost 1" above the bedroom floor. Add to that the 3/4" SLC + 1/4" membrane + 10mm tile, and it's too much difference at the threshold.

I don't understand what you mean when you ask how am I going to put the tiles next to each other if the floor is sloped. The floor is flat, just not level. It's on one plane. Or are you referring to the wall tiles?

If so, I am going to leave one wall without tile, and the other wall will have a vanity that will hide the slop. I'm going to cut the tiles in the second row from the floor at an angle to match the slope of the floor: their tops will be level but their bottoms will match the slope. These tiles will be hidden from view behind the vanity where the difference would be noticeable.

JazMan 01-22-2009 09:31 PM

Although it would be nice, a floor does NOT have to be level, but it does need to be flat. When installing smooth, shiny or large tiles, you would like the floor to be flat "within 1/8" in 10 ft. AND 1/16" in 12" of the required plane".

If you also want the floor to be level, I suggest you find a pro that does "mud" work. He will place anywhere from 1" to 1 3/4" of deck mud to level the floor. This will of course create transition issues at doorways.


You posted before I was done. I fully understand the transition problem and sometimes you just have to live with an unlevel floor. Bob may have not understood the level/flat issue?

angus242 01-22-2009 10:51 PM

I think in a small room, the slope isn't so easy to notice. I like your plan about the wall tiles. That will be a good way to disguise the slope.
Make sure you caulk the floor to wall (+ tub/shower base) transition instead of using grout. You don't want accidental water pooling in a place that can seep beneath the tile.

diy888 01-23-2009 07:15 AM

Luckily, the shower is in an alcove area that I took over for the remodel. It is both level and flat. :) I used a WEDI Fundo base that I cut to a custom length and WEDI panels in the shower, with both sealant and their tape. BTW, for the shower and walls I chose WEDI over Schluter KERDI because WEDI panels are easier than Kerdi membrane for one person to apply without a helper.

Angus, you raise a question that has been troubling me. Schluter has high-tech coves that eliminate caulking where planes meet. Their literature discusses minimal movement where planes meet at wall-wall and wall-floor joints, and it says the rubber component of their coves is designed to accommodate that minimal movement and prevent tile and grout cracking. It says the unavailability of ceramic cove tile is one of the reasons they've developed these metal/PVC/rubber coves.

I have found some ceramic coves (quarter-cove and three-way inside corners) and would like to use them in this remodel to eliminate hard-to-clean corners. What I don't understand is why ceramic coves would not crack if they're cemented to both planes. Do ceramic coves get cemented to one plane only? Or do the new latex modified mortars accommodate the minimal movement and spare the tile?

diy888 01-23-2009 07:20 AM

jazman, I considered deck mud but the transition issue is the problem. The floor used to have deck mud, in fact, but I tore it out in order to reroute plumbing and relocate the shower and fixtures. I also wanted to sister the joists.

There used to be only one layer of subfloor of 3/4" tongue-and-groove planks and even then the old floor required a fairly steep marble stone transition. When I put down the new subfloor and underlayment, I used two layers of 3/4" plywood.

P.S. I bought John Bridge's book and was going to try deck mud myself, but hurt my neck and didn't think I could lug everything up to the second storey and mix everything myself. You really need a helper for that.

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