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Old 10-07-2004, 09:53 AM   #1
Amos K.
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Request input on kitchen underlayment

Hi,

First time poster here. I have seen a number of posts on closely related questions but couldn't be sure how to apply the answers to my specific problem, so here goes:

I am getting ready to tile a kitchen floor. The subfloor is 3/4" plywood on 2x10 joists spaced 16" O.C. and spanning about 9 feet. It feels rock solid based on my unscientific walking around, stomping and bouncing tests. The tile I like is 3/8" thick, which complicates the problem slightly.

I am trying to determine what is best to put under the tiles to find the happiest medium between two competing objectives: adequate or better stiffness and movement isolation, plus minimizing the difference between the finished floor surface height and the height of adjacent 3/4" plywood -- looking for as close to zero difference as possible and definitely less than a quarter inch.

First, what additional build-up should I expect from the thinset securing the backerboard and the tiles themselves?

Second, here are a few options I had considered -- any recommendations pro/con?

a) 1/4" exterior grade plywood, secured with face grain perpendicular to the subflooring, then ditra, then the tile. Total height: 3/4" plus the thinset. Height difference from hardwood: whatever the thinset adds.

b) No more plywood, just 1/4" concrete backerboard on the existing subfloor, then tile. Total height, 5/8" plus thinset. Would this put it just about even with the hardwood?

c) The max stiffness option: 3/8" additional exterior grade plywood, no backerboard, then the tile. Total height, 3/4" plus just enough thinset to set the tiles and none for backerboard.

If I had to go ahead with no info, my choice would be option b. I have had one installer tell me I needed 3/8" plywood over the subfloor plus 14" backerboard, producing about a 3/8" step up in the end, which was not acceptable. I hope and suspect he was being excessively careful. 1/4" thick tile is an option I would certainly consider before abandoning the project, but if I can use the tile I have, that would be ideal.

Thanks in advance for your inputs.

Amos
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Old 10-07-2004, 10:10 AM   #2
Bill Vincent
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If it were me, it would be option "a", but substituting 1/4" CBU for the plywood, and then the ditra. The thinset will only add about a shy 1/8", and if you hold your tile back about 3/16" from the hardwood, you could just caulk that joint afterward.
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Old 10-07-2004, 06:01 PM   #3
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Welcome aboard, Amos. Get registered and get the benefits.

I didn't read "Ditra" in your question, but Bill commented on it. Ditra is a mat/membrane product from Schluter Systems. It will take the place of the backer board over your plywood subfloor. http://www.schluter.com

I would still like to see the 3/8 additional plywood. It might be overkill, but I like it. You can step your new tile floor back down to the wood floor hieght with a matching wood reducer. The difference won't be that great and certainly not enough to stumble on.
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Old 10-07-2004, 06:18 PM   #4
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Well, Bill got hold of me in the back room out of public view, fine gentleman that he is, and pointed out that I missed Ditra in option A. I owe ya one, Bill.
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Old 10-07-2004, 06:26 PM   #5
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Old 10-07-2004, 08:25 PM   #6
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Yep, several ways to do it. Only thing I would mention you not consider is the 1/4 inch plywood option. There is no real structural value to 1/4 inch plywood no matter which way you lay it, and it can cause you flatness problems without giving you any benefit. Well, OK, if you glue it down really well it will add rigidity, but it still ain't worth it. See disclaimer below.

Option B is actually above industry standards for ceramic tile, which I think the others are assuming you have. If your tile is natural stone, so state, because that changes the rules a lot. As John said, most of the pros here don't like to see you use only the minimum standards.

My opinion; worth price charged.
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Old 10-08-2004, 03:24 PM   #7
Amos K
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Thanks

Thanks to all for your input. This is a great forum and I appreciate getting so much help so quickly. Yes, it is ceramic tile. I will forget about the 1/4" plywood option.

Based on your input, my two favorite options are now:

b) option b as described in previous post, and, since the safety margin over minimum industry standards is small, don't allow any elephants in the kitchen.
d) 3/8" plywood with Ditra, then exchange my 3/8" thick tile for 1/4" to mitigate the surface height difference.

One final question: the "shy 1/8" for thinset: is that meant to be the total of the thinset under the CBU and the thinset under the tile, or do I need to double that figure two account for the two applications?

Thanks again and have a great weekend...

Amos
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Old 10-08-2004, 04:19 PM   #8
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Sorry about that, Amos-- double it.
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Old 10-08-2004, 06:48 PM   #9
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I wouldn't let that little bit of transition height dictate my tile selection, Amos. It's really not a big deal and it's very easy to do against a hardwood floor.

My opinion; worth price charged.
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Old 10-14-2004, 02:50 PM   #10
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Thinset + Ditra: less than the sum of the parts?

Hi again,

I won't be able to put off this job much longer without it delaying the completion date for the kitchen, but am still going back and forth on the options, because that's how I am.

In the case of Ditra, should I still add 1/8" both below and above the Ditra to account for the contribution of thinset to the finished floor height? I would think with all the cavities built into Ditra, the same rough amount of thinset might end up being effectively thinner with that than with CBU. In case it's not already obvious, I'm firmly committed (and you are welcome to consider it irrational) to keeping the finished floor surface height within 1/4" of the adjoining hardwood. So I'm trying to figure out if using Ditra might actually leave me enough room to consider the additional plywood that a couple of you have urged me to use.

And... if I end up needing a reducing strip, do I hold the tile back some from the hardwood and slip in a block of wood in between to anchor the strip to, or do people generally just tile right up to the hardwood and then fasten the reducing strip to one or both finished floor surfaces somehow?

Thanks again,

Amos
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Old 10-14-2004, 03:10 PM   #11
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No. I would add a shy 1/8" for above the Ditra, and nothing for below. By the time you're done installing the Ditra, you don't have much more than the thickness of a piece of paper between the Ditra and the subfloor, if it's done right.
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Old 10-14-2004, 03:54 PM   #12
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Cool. That would give me a shy 1/4" of height difference at the transition.
Thanks for the info.
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Old 11-09-2004, 08:43 AM   #13
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Now for the layout...

And the winner is… 3/8” plywood, then Ditra, which I finished installing Sunday night. No problems except I apparently guessed wrong as to what is meant when Schluter says to use a “fairly fluid consistency” of thinset. It took a lot more work than I expected to make the Ditra turn gray and to iron it out flat. Now I have marked a couple of perpendicular sharpie lines and am faced with the next set of stupid questions. At this point the topic shifts from underlayment to layout. The short versions of the questions are:

(a). Do I lay the tile – which will be in a diamond pattern – to follow a natural centerline in the room, and allow a tiny piece to go right in the middle of a doorway, or do I make sure the pieces along visible walls and in doorways are a reasonable size and forget about alignment with the sink and window?
(b) The tile is being laid over the whole floor, and cabinets will be installed over it. For layout purposes, should I treat the line where tile disappears under a cabinet as if it were a wall, or should I not care how much of a full tile pokes out from under a cabinet?

Here’s the picture. The room is a small rectangular kitchen with the cabinets forming an L along the south and east walls. The cabinets are not currently in place, they will be installed over the tile. The tiles are to be installed in a diamond pattern, and are 14” square (19.8” corner to corner) including an allowance for grout. There is one doorway to the room and it’s through the west wall. I marked one line from the centerline of that doorway back to the east wall. The north wall is 86.5 inches (4.37 diagonals) from that centerline, and has a window. The south wall is 48” (2.42 diagonals) away from the line and will have the sink base backing up to it. By luck, the centerlines of the sink and of the window at the opposite side of the room will be aligned perfectly, so that seems like a natural place for the other axis to go. But there’s the rub: measuring forward from that line to the where the doorway threshold will be, it’s 41.5” or 2.1 diagonals. Which means that there would be a little tiny triangle of tile right in the middle of the doorway. The distance back from that centerline to the east wall is 50” or 2.53 diagonals, but I could just skip all the 0.03 tiles because that wall is completely hidden and even if it wasn’t, then shoe molding could cover it up.

So… if it were your kitchen, what criteria would be most important to you, and what would you designate as the center of the room for layout purposes?

Thanks, and sorry for the long winded question…

Amos
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Old 11-09-2004, 11:56 AM   #14
Amos K
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Someone pointed out that I could shift the whole pattern by one half diagonal and end up with two little triangles in the doorway, but flanking the centerline instead of being on it... That's twice as many little pieces, but somehow I like it better than one that's in the middle of the doorway.
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Old 11-09-2004, 05:17 PM   #15
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Do what you like, Amos. If it looks good, it's right.
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