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Unread 11-21-2005, 07:29 AM   #1
joe14580
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Underlayment question

Hi guys, noobie here. I wish I’d found this site sooner, it might have saved me a little time as I think I might have made a mistake. I’ve visited the library and thus tried to do my homework, and also read through the forum, but I still have a few (simple) questions. It’s regarding underlayment.

My plan is to install a tile floor in my bathroom. The joists are 8” set on 16” centers. There is existing ¾ plywood over the joists. The distance from the top of the existing ¾” plywood to the top of the existing toilet flange is 1 ¼ inches. Here are my noob questions:

1.I assume the final floor, including tile, can be level with, or slightly below, the toilet flange. Or should the toilet flange stick up above the final floor a bit and if so how much ?

2.We have thus far installed a ¼ “ piece of Luann (frequently nailed but not glued) on top of the ¾ “ plywood and plan to install ½ “ Hardibacker on top of the Luann. The Hardibacker would be set onto the Luann with thinset. Would this be an acceptable subfloor for placing ceramic tile ?

3.What is the nominal/average thickness of ceramic floor tile ?

4.Assuming the Luann was a mistake, what would you recommend for the underlayment if you had 1 ¼” to “play with” (the 1 ¼ " assumes the finished tile floor can be level with the toilet flange per question # 1 above).

Thanks so much in advance for your help to this clueless noob.

Joe
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Unread 11-21-2005, 07:55 AM   #2
Shaughnn
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Hello Joe,
The luan needs to come up and be replaced by another layer of plywood for structural strength. I'm not to fluent in the "Deflecto-babble" stuff so I'll let someone else confirm what grade and thickness, though I believe it'll be 5/8" plywood. "Average" tile thickness is a funny duck. Some tile is only 1/4" and others can be nearly 1/2". If you *plan* for 5/8" for Hardibacker, thinset beds and tile, you should be close with a common floor tile. If that's too much height, you can use Schluter's DITRA product instead.
The toilet flange shouldn't be much higher than the finished floor, but if it's lower and flange extender or extra wax rings can be used to seal the differance.
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Unread 11-21-2005, 10:27 AM   #3
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Welcome, Joe.

What Shaughnn said.

Well, 'cept for the "Deflecto babble" comment, of course.

Additional structural plywood would be the way to go unless you're intersted in doing a nice mud bed. Always better, that.

I would suggest you go out and buy your tile first, then you won't hafta guess about the thickness.

The terlit flange should optimally sit directly on top of the finished floor.

My opinion; worth price charged.
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Unread 11-21-2005, 01:28 PM   #4
joe14580
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Red face

Oh, I'm most grateful to the most wise and benevolent respondents for taking the time to answer my noob-ish queries. As commanded, I'll rip out the 1/4 " Luann and replace with plywood.

I have one last question(well, ok, maybe two):

1. If I go to Home Despot or Lowes, what plywood should I ask for ? In noobing around these forums it seems I want to avoid "CDX" style of plywood as that is used for roofs ? But what should I ask for ? What is the name of the "right stuff". Is it interior or exterior ?

It seems to me that what I'm hearing is that I should install the thickest plywood available for structurally integrity on top of the existing 3/4 " plywood. Then top that off with 1/2 " (or 1/4 " if they make it since this is for a floor only) Hardibacker and that the Hardibacker would be set with thinset material. And then of course the tile on top of that. All while staying equal or slightly less than the 1 1/4" I have to play with.

I infer that since Hardibacker does NOT provide structural support, the idea is to get as much plywood down as one can. Thus accordingly, if 1/4 " Hardibacker is available, use that instead of 1/2 " (or use 1/4" DuraRock if that is availalbe) since the thickness of the Hardibacker for floors is not really much of an issue ? All's that matters is that one NOT set the tile directly on top of the plywood.

Does this wayward noob have it straight ? Sorry to be so anal, but I've already (nearly) screwed this job up already !!!

Muchos Thankos.
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Unread 11-21-2005, 01:49 PM   #5
MarcusEngley
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You want ACX or BCX ply, plugged and sanded -- I used two layers of 3/4" ACX, b/c it was readily available. You don't want to go thinner than 1/2" I think, or it doesn't do you any good.

Can't really comment on the Hardi, but I think you're correct about 1/4" on the floors. Make sure it's well thinsetted and screwed...
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Unread 11-21-2005, 01:51 PM   #6
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Yep, you're right on target, Joe. Minimum thickness of structural plywood is 3/8ths inch (nominal), but I never recommend anything under half-inch. Beyond that, you can mix and match any combination of thickness of plywood and underlayment to get your desired thickness.

For plywood you must select a grade with no face lower than grade C (A, B, or C is OK), and it must have exterior glue. You don't need Exterior rated plywood, but you can use it. You need at least an Exposure 1 rated plywood. At Homers and such you're likely to encounter AC or BC wood with an Exposure 1 rating and either is good. Choose the least expensive material that meets the minimum rating - this is one area where the minimum is, indeed, good enough.

My opinion; worth price charged.
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Unread 11-21-2005, 06:13 PM   #7
joe14580
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Thanks once again to all you guys that took the time to respond. I'll be all over this job like ugly on an ape tomorrow. As suggested, I also went out tonight and found the tile I'm going to use so now I have all my dimensions.

Happy Turkey Day or if you're Canadian - Happy Moose Day
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