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Old 11-12-2004, 05:46 PM   #1
Randy B
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Subfloor questions

I just found your board and have spent the afternoon reading through much of the past postings. That has answered most of the questions I had...
Here is my situation... I have a kitchen/diningroom/entry area of about 700 sq.ft. that I would like to tile with ceramic tile. Using your Deflcto calculator (great little piece of software) I find that the 2"x10", 16" oc, 13' span floor joists will work for the ceramic tile. However, there is only a 1/2" thick plywood subfloor over the joists. (I am midway through a major house remodeling and the old underlayment was a hodgepodge of particle board and damaged plywood... it had to go) The finished floor height of the adjoining rooms is 1" above the existing subfloor. I plan to place another 1/2" layer of CCX or better plywood on top of the existing subfloor using the recomendations found in the linked article "Position of Underlayment to Prevent Cracked Tile and Grout" at least as far as placement goes... Above the additional plywood I plan to use 1/4" CBU set in thinset and screwed to the plywood. Above this I believe that the appox. 1/4" ceramic tile and thin set should bring the finished floor height very close to the existing floor height.
My first question is whether or not to glue the two layers of plywood along with the screws I intend to use. I know that it would be a much stiffer and hence, better substrate for tile... yet I read on this board that the two layers of plywood should be only screwed or nailed.... Did I get that correct? If so, what is the thought behind that recommendation?
Secondly, would 1/4" plywood and 1/2" CBU be as effective a solution for my situation?
I am a middle aged homeowner/DIY with 20+ years of unintentional experience in home repairs...
Thanks in advance for you help...
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Old 11-12-2004, 07:13 PM   #2
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No adhesive needed between the sheets of plywood, also I would use AC or BC exterior grade.

Never use 1/4" plywood in a subfloor or anywhere there is tile.

To keep the height down, you can use the 1/4" backerboard or save about another 1/8" by using a sheet membrane.
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Old 11-12-2004, 09:45 PM   #3
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I'm not gonna agree fully with RD here. I'd suggest you glue the bejesus outa them two layers of half-inch ply in this application.

The reason for the specifications you read for plywood underlayment is to both provide the maximum strength and rigidity in that second layer, and to somewhat isolate that second layer from the movements in the floor joist structure. In some applications, both of those are important factors. I don't believe this is such an application. Keep in mind that we have had this argument on these forums many, many times and a full consensus may never be had. You're getting my view.

I don't like to see anyone start a subfloor with half-inch plywood. It's just not good for any flooring application. Adding a second layer of half-inch plywood will produce a subfloor that is stronger and more rigid than the minimum requirement of a single layer of 5/8ths inch plywood (for a CBU/ceramic tile installation), but not as stiff as a first layer of 5/8ths inch plywood and another layer of 3/8ths inch, which we frequently recommend as our acceptable minimum subfloor. I have no test data to support that theory, but I'd be happy to wager dinero on such a test. If, however, you glue and screw your two layers of half-inch plywood together, you will have essentially a single-layer one inch plywood subfloor, which is more than adequate for a CBU installation.

I'm not sure where RD got the part about 1/4 inch plywood, I didn't see that in your post anywhere. I don't know if that's because I missed somethin' or if RD's makin' stuff up again. We've counseled him on that in the past.

Your 1/4 inch CBU installation over that subfloor would be fine. Be sure to install the CBU per manufacturer's instructions, including the thinset under the CBU and the joints taped and bedded with more thinset.

My opinion; worth price charged.
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Old 11-13-2004, 05:48 AM   #4
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Quote:
Secondly, would 1/4" plywood and 1/2" CBU be as effective a solution for my situation?
CX, third sentence from the bottom.
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Old 11-13-2004, 08:08 AM   #5
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1/2" plywood subfloor
I'll go along with Kelly(cx) here and say that you need to laminate th 2 -1/2" sheets together to come up with a decent single first layer of flooring.
I would then consider using the Ditra sheet membrane RD mentioned.

1/2" + 1/8" =5/8" + tile and thinset of 3/8" puts you right where you need to be and you have the added benefit of an engineered uncoupling membrane and the ability to waterproof the installation to boot.
Theres a time and a place for all things.Nows the time for Ditra
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Old 11-13-2004, 10:39 AM   #6
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Oh, that quarter-inch ply.

He wasn't really gonna use it, though.
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Old 11-13-2004, 12:56 PM   #7
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I was just at a job that had 1/2" osb as the sub floor. They had 1/2" ply on that with vinyl glued to it. I removed the vinyl which took most of the day. I thought screwing the two together would have stiffend it up it didn't. Had to tear out the ply and put 3/4" in then ditra. Same joists as Randy but less of a span. Also had to have a carpenter raise the door.
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Old 11-15-2004, 06:57 AM   #8
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I'm a DIY'er with a similar situation. I'm tiling a bathroom and I have to replace a portion of 1/2" plywood subfloor that is rotten. I've got 2x8 joists on 16" centers with a 7" span. I will be using Ditra and my desired finished floor height only gives me room for 1/2" plywood on top of the subfloor. Should I also glue and screw the two layer's of 1/2" plywood together? If so, what should I be using to glue them together?

Also, If I am using Ditra with 5/16 porcelain tile, what can I expect the total thickness of the thinset>ditra>thinset>tile to be?

Thanks in advance.

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Old 11-15-2004, 07:03 AM   #9
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Yes, liquid nails Subfloor adhesive or PL400 for glue.Use lots!!

Finished measurement will be very close to 1/2"(strong) for ditra +tile + thinsets.(strong means it may be up to 1/16" over or so which really shouldnt be a concern )

Randy B from minnesota,welcome aboard!! For a second there i thought maybe you were Randy Block( a good tileman up north here whom ive had the pleasure of working with on a couple projects).
So, where ya from Randy??
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Old 11-15-2004, 08:51 AM   #10
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Todd, we are nearly neighbors... I'm in Duluth...
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Old 11-15-2004, 10:09 AM   #11
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Hi All,

Might as well get my two bits in.

I agree the two layers of half-inch should be glued and screwed together. Screws about ever eight inches in both directions. Don't need to screw into the joists with the second layer (installed according to the instructions in the Liberry).

Either quarter-inch backer or Ditra over the top. If you use Ditra, make sure you read the Ditra Handbook on the Schluter site as it pertains to porcelain tile over the membrane.

http://www.schluter.com
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Old 11-15-2004, 11:37 AM   #12
Leo427
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Question

Would it still be necessary to glue the two if the subfloor (first layer) is 5/8" and the second layer is 1/2"?

A 1/4" cbu would go on top.

Last edited by Leo427; 11-15-2004 at 11:42 AM.
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Old 11-15-2004, 11:43 AM   #13
bbcamp
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No, not required. But if you were using backerboard, glue the plywood together. If you are using Ditra, don't. The only reason for not glueing the plywood when using Ditra, is that Herr Schluter doesn't want glue there. You want Herr Schluter to warrenty your installation, so you do what he says.
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