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Unread 06-19-2006, 09:03 PM   #1
Delta_V
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Ditra/kerdi questions; Minimum tile size, Kerdi on ceiling, ditra over wood?

Hi there,
I've got lots of tile to set in the near future and I stumbled on this site while looking for info on ditra and kerdi installation. I'm hoping that some of you can answer a few of my preliminary questions. There'll be plenty more later.
First, a bit of background;
I'm the new owner of a 40 year old triplex that's had an absentee landlord and is in rough cosmetic shape so the tile is the least of my worries! First priority for me is completely refinishing the top two bedroom unit that I'll be living in (it's in the worst shape of the three) and eventually work on the others once the current tenants eventually leave. I've got some experience with DIY tiling and it's all gone well so far. I've got a very limited budget, but lots of friends who owe me favors. The bathroom and kitchen are original and must go. The bathroom is currently tiled in the tub enclosure, floor and four feet up the wall all around in pink tile with black trim. Very retro. Everything seems to be stable in the tub area with no evidence of water infiltration but I won't really know until it all comes off. Here is where the questions come in;
I'm planning on keeping and refinishing the cast iron tub which is in good shape except for a few worn areas of finish. I have a good refinisher lined up to do this for me.
My question is if the mud bed is solid, should I bother ripping it out or just strip the tile off and apply the kerdi over top? I'm using a mosaic tile in the tub enclosure.
Also, does the kerdi need to go on the ceiling of the enclosure as well?
With regards to the floor, I've got a octagonal mosaic with about 2 1/2" octagons and 1" black squares in between. Can I use a ditra membrane with this? Their website states a minimum 2" tile.
On to the kitchen. Original cabinets. Very rough. Gotta go. On top of that, the floor was re-tiled with a 12x12 ceramic that's nice enough but the grout lines are trying to disguise themselves as zebra stripes; All over the place. I got some very nice 12x24 porcelain to put down there and want to make sure I do it right. If I bring it down to the plywood, can I put the ditra directly over that or do I need to apply cement board before? What kind of joist spacing would I need to be safe with that? The kitchen butts up on to a load bearing wall on it's long axis and is only 9 feet wide. The existing very poor tile job is in good physical shape.
Any and all help would be greatly appreciated. I plan on being here for a while and don't want to re-do later it after already fixing someone else's mistake!
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Unread 06-19-2006, 09:25 PM   #2
jadnashua
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Use the "Deflecto" tool in the blue bar to determine the suitability of the floor. Note, you can't go by the room dimensions, you need to know where the joists are supported underneath.

Schluter won't warranty a floor on Ditra with tiles (the 1") that small. The tile needs to be big enough so it isn't sitting only on one of the plastic towers...it needs to be supported by the thinset around the towers and with their spacing, the 1" could end up being able to tip in a point load, ruining things.

Ditra is used in place of cbu, right on the plywood. Don't use both.
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Unread 06-20-2006, 06:20 PM   #3
John Bridge
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Hi Delta, Welcome aboard.

2x2 tiles are minimum for Ditra.

I would remove the old wall mud, but you can try stripping the tiles off.
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Unread 06-21-2006, 11:28 AM   #4
Delta_V
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Thanks!

Thank you both for the reply! A couple more questions though;
What about the kerdi on the ceiling? Overkill?

Also, for the kitchen, if the plywood has too much flex to it (I doubt it, but we'll see), what is the appropriate method to reinforce it? Thinset with another layer of plywood on top? SLC? I understand that with large tiles, a level surface is even more important.

I think I'll head out to the garage now and drool over my pile of very sexy tiles! Can't wait until they're in!

Thanks again,
Chris
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Unread 06-21-2006, 03:57 PM   #5
jadnashua
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Plywood is fine for between the joist deflection. At 16" spacing, 5/8" is the absolute minimum with a membrane, more is much better and required with cbu. The plywood doesn't help with deflection along the joists...their length, species, and height determines that. The average DIY'er shouldn't tile directly onto plywood. The prep has to be perfect, and you have to have a second layer to boot along with a super premium flexible thinset. CBU or a membrane isloates the tile so you can get away with one layer (except for stone tile).
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Unread 06-27-2006, 08:45 AM   #6
Delta_V
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Thanks for the replies;
What should I do if I need more strength? Add another layer of plywood before the ditra? Screwed down or thinset?
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Unread 06-27-2006, 09:17 AM   #7
bbcamp
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If you need a second layer of plywood, Schluter wants you to offset it from the joists by 4" or 1/4 the joist spacing, and staggered by 2 feet from the long seams in the original subfloor. Use screws only, 4 inches apart along the edges, 6" in the field. Leave a 1/8" gap between sheets and 1/4" around the room perimeter.
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Unread 06-27-2006, 11:03 AM   #8
Dan Clark
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Question Questions about second plywood layer

Bob,

Hi. I'm doing something similar. I'm replacing particle board with plywood - 3/4 T&G, then 3/8 plywood (offset), then ditra, then porcelain tile.

You said, "Use screws only, 4 inches apart along the edges, 6" in the field."

- Does this mean that you should NOT glue the 3/8" plywood to the 3/4" layer?

- Should you screw the 3/8" plywood into the 3/4" plywood only and avoid screwing the 3/8" plywood to the underlying joists?

Thanks,

Dan.
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Unread 06-27-2006, 06:55 PM   #9
John Bridge
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Yes to both questions, Dan.
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Unread 06-27-2006, 07:27 PM   #10
Dan Clark
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Talking Many thanks, John

John,

You're Da Man!

It never fails to amaze me how many ways we amateurs can screw up the tiny details (and the BIG details). You and this forum are an absolute god send!!!

While other forums merely provide information, you and this forum solve people's problems, reduce their confusion and ease their stress. You had the perceptiveness and vision to look past our technical issues and see our real problems - that these projects fill many of us with fear and trepidation.

It's not just the technical information (which is great). Your forum and books touch thousands of people and make their lives easier and better. You throw oil on our troubled waters. That's something that very few people ever accomplish in their lifetimes!

Bravo!!!

Regards,

Dan.
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