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Unread 11-20-2003, 09:09 PM   #1
regalan47
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Do I need more underlayment?

One last question before I start tiling the kitchen and bathroom floors. This is a 1953 ranch with 16" centered joists. The flooring is 3/4" planks 3 1/2" wide. I plan on putting down (with thinset) 1/2" CBU. The kitchen tiles are 12 x 12 porcelain and the bathoom tiles are 2x2. PLEASE tell me I don't need to add 3/4" more of underlayment - I really do not want the additional height.

Thanks,
Al
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Unread 11-20-2003, 10:35 PM   #2
Dog paws
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Hi Al

Yes, you need to put down plywood. You should not put the CBU over plank.
Give the pro's your joist size and span and they will reply.
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Unread 11-21-2003, 06:29 AM   #3
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You don't need 3/4" plywood, 1/2" is plenty with 1/4" CBU. If you were willing to spring for Ditra, you could use 3/8" plywood and keep your floor at the same height you were planning.

We really do need to check the joists, too.
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Unread 11-21-2003, 11:43 AM   #4
regalan47
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Thanks for the input gentlemen and I will be posting joists sizes later. Familiar with CBU but not with Ditra - tell me about it. I have gone on line to examine the product and would like to know how it compares/replaces CBU.
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Unread 11-21-2003, 12:17 PM   #5
bbcamp
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Ditra does the same job that the CBU does: both separate (decouple) the inevitable movements of the wood subfloor from the rigid tiles. CBU has a cost and availability advantage. Its everywhere and fairly cheap. Ditra can be found, but most don't know about it, and it is fairly expensive. CBU is heavy, Ditra is light. CBU is hard and messy to cut, Ditra cuts with common scissors. Both products must be embedded in thinset. Ditra is 1/8" thick, CBU is 1/4" or 1/2". Ditra can be used to waterproof a floor. Ditra is orange, CBU isn't.
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Unread 11-21-2003, 12:41 PM   #6
regalan47
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So bbcamp - which would be the best way to go in terms of preventing the cracking of grout/tiles? The 3/8" of ply & ditra or the 1/2 ply with CBU. Concerned about cost but more concerned with quality of the job. (and cutting installing Ditra sounds a helluva alot better than CBU which I find to be a pain to cut.)
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Unread 11-21-2003, 01:03 PM   #7
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Unread 11-21-2003, 01:10 PM   #8
bbcamp
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I believe both solutions to be technically equal. Pick one and have at it!
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Unread 11-22-2003, 05:37 PM   #9
regalan47
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bbcamp - I have decided to go with 1/2" plywood to be safe and Ditra - viewing the instructions I just thinset and place down the Ditra - it appears that you can tile immediately once the Ditra is down - is that the case?
Thanks to you and all others who provide help!
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Unread 11-22-2003, 06:12 PM   #10
bbcamp
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Yeah, you can tile immediatly. Don't rush it, though.
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Unread 11-22-2003, 08:07 PM   #11
John Bridge
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Hi Al,

Make sure you install the Ditra with a thin set that is recommended to go over plywood. I usually use Versa Bond from Home Depot because it's readily available, and it's good.

Schluter recommends you set the porcelain tiles over the Ditra with unmodified dry set mortar. Custom Master Blend is what I often use. Don't mix it stiff. Keep it loose enough that it will contact the backs of the tiles and cling to them. Lift a tile every once in a while to make sure the thin set is covering the back and sticking well.

You could also use the same Versa Bond you use to install the Ditra, but you would then wait about a week before grouting. This would be to make sure the polymers in the mix have an adequate chance to air dry.

I'm glad you chose Ditra. It is hands down the best way to go.
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Unread 11-22-2003, 09:59 PM   #12
regalan47
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John & bb you are the best - "And away I go!"

Thanks
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Unread 11-24-2003, 07:13 PM   #13
Chris3
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Regular thinset

John I am also putting 13" x 13" porcelain over Ditra. Is there any disadvantage in using non modified thinset? Is it worth waiting a week to use the versabond for tile?
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Unread 11-24-2003, 07:25 PM   #14
Rd Tile
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I know Ditra says to use un-modified on top for setting tile, but I have a big problem trusting anything but modified thinset holding to porcelain, even ceramic.
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Unread 11-24-2003, 08:05 PM   #15
Chris3
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To many choices

Would you use Versabond, Flexbond or I see Custom has a thinset just for porcelain.
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