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Unread 01-14-2021, 12:12 PM   #46
Davy
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Thanks for clearing that up, I've never used Warm up. Can you use the Warm up heated floor with mosaic tiles?
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Unread 01-14-2021, 01:16 PM   #47
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According to the rep, yes, because the system is designed to be covered with a layer of mortar or SLC. Need to do a bit more research tho.
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Unread 01-14-2021, 01:28 PM   #48
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The link I found shows the wire woven into a fiberglass mat like I talked about before. Maybe I didn't dig deep enough. Like you said, they do mention that you may need to mud the floor over the wires to get a flat surface.
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Unread 01-20-2021, 06:17 PM   #49
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Re the question of what kind of plywood to use on the subfloor underneath the CBU:

1) I didn’t encounter the spec about exterior glue ply until I had already put down regular 3/4” (not exterior grade) in one one of the two bathrooms. Would the recommendation be to remove it and start fresh? (Not necessarily an easy task because it’s thoroughly glued and screwed down...) Is there a product to prime it that will mitigate this problem? I’m curious whether the exterior glue recommendation is a hard rule, or whether folks ever use regular subfloor-grade plywood.

2) My local big box stores don’t seem to carry anything called “exterior glue plywood.” The exterior stuff they have seems to be pressure-treated and I have read elsewhere that that is not great for tile because it has moisture in it in addition to the chemicals used in the pressure cheating process and as it dries it may be more likely to warp or shift than the non-pressure-treated variety (or maybe that idea about pressure-treated was something I read that had to do with building a shower curb?) Am I not seeing the exterior glue stuff stuff at the Lowe’s because it’s a specialty item that I would need to go to a building supply store to get?
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Unread 01-20-2021, 06:29 PM   #50
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1. Matt, there is no exterior grade plywood. There are many grades of structural softwood plywood, identified by the quality of the two face veneers, such as AB, BC, CD, etc, and those are usually all exterior glue plywoods. Your local home center will almost certainly carry an AC or BC grade which would be fine for your application. You do not want anything with a face grade lower than C.

I don't know what a "regular subfloor grade plywood" might be and can't guess what you've got. Perhaps you could clear that up?

The exterior glue is a pretty hard and fast rule because if you don't have that the moisture in the thinset mortar you must use under the CBU can cause the top veneer layer of the plywood to delaminate. Not a good thing.

2. See #1.

You do not wanna use any treated plywood under a ceramic tile installation.

My opinion; worth price charged.
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Unread 01-20-2021, 06:32 PM   #51
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Quote:
Originally Posted by MesaTileworks
I didn’t encounter the spec about exterior glue ply until I had already put down regular 3/4” (not exterior grade)...
Can you tell us what the stamp or sticker on the plywood says? You could also take a look at the receipt.

Exterior glue plywood is extremely common at box stores. It is not a special order item.

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Unread 01-20-2021, 10:02 PM   #52
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Thanks very much, that’s helpful info! In that case I’m guessing that what I have is AB or BC, but I’ll take a look and see what identifying marks I can find. It’s definitely not pressure treated. I didn’t know about the distinction of the letter grades so that’s good to know.
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Unread 01-22-2021, 09:45 AM   #53
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So this is what’s available at my local Lowe’s. Several folks there gave me conflicting info about the grade of the veneer, what “exterior glue” meant, and what was and was not a suitable substrate for tile (I am never surprised when well-meaning employees at this store are not as knowledgeable as ya might hope but still…)

Do I understand right that the OSB tongue and groove stuff is NOT a good choice under tile? I believe from what I have read elsewhere that in spite of being called “subfloor,” it’s not recommended as subfloor for tile, unless I’m confused about what’s what here. Is what they are calling “plywood sheathing” the way to go, or do I need something else?




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Unread 01-22-2021, 09:53 AM   #54
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Matt, there is absolutely nothing wrong with OSB for subflooring. Some types of it are actually superior for the application in my estimation. The drawback to having it as the final surface of your subfloor when using a bonded membrane as your tile substrate is that some manufacturers of tile installation products don't like bonding to it with their thinset mortars. I, personally, don't like trying to bond to it using any kind of adhesive.

That aside, if OSB is what's available to you, it's adequate for at least the first layer of subflooring. If you plan a double layer, I'd use plywood for the top layer.

My opinion; worth price charged.
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Last edited by cx; 01-22-2021 at 10:51 AM. Reason: typo
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Unread 01-22-2021, 10:37 AM   #55
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Of the two OSBs pictured one is $48/sheet, the other is $36. The employee seemed hazy on the difference. Any insights there? Is one “glued” and one not (although technically both are made by pressing chips together with an adhesive, so hmmm...)

I was considering putting two layers down as a bit of extra insurance (I beefed up the joists but am still a little concerned about deflection). Is there any particular benefit to OSB instead of the sheathing for the first layer? Is the extra layer worth the extra time and expense?
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Unread 01-22-2021, 10:50 AM   #56
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I got no eye-dee what might be the difference between your 36-dollar OSB and your 48-dollar OSB, Matt. Either one is over priced to my thinking, but youins Yankees like to spend more dollars for your stuff than we do down here, seems like. Only way to know is to get what information you can about each and look up the specs. Try to find a grade stamp on the sheets. Or possibly get some actual identifying information from those tags.

The sheathing is likely a CD grade plywood and I'd avoid that.

As for two layer subfloor, I like it for tile. Look at it this way, using the Robinson floor tester (ASTM C627) to check for l/360 deflection over 16" joist centers, a nominal 5/8ths" subfloor can be made to pass sometimes; a nominal 3/4" subfloor will pass most of the time; a double layer of nominal 3/4" and 1/2" will pass every time. You are the only one can decide if it's worth the time and expense. It would be for me.

The added layer of subflooring will not improve your joist deflection.

My opinion; worth price charged.
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Unread 01-22-2021, 11:20 AM   #57
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Heh heh, well, I’m in Virginia so to us “yankee” usually means a little further north, but I guess maybe to someone further south, Virginia is too close to the Mason Dixon line to be truly southern. Regardless, lumber and other materials here have got *much* more expensive just in the last year. The housing and building market here is ridiculous in general.

I guess I’ll see if I can dredge up any other info on the grade of the sheathing, and check a local building supply store to compare. When you do a double subfloor with half inch and three-quarter inch, Which do you usually put down first? Does it matter?
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Unread 01-22-2021, 12:08 PM   #58
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Quote:
Originally Posted by cx View Post
The drawback to having it as the final surface of your subfloor when using a bonded membrane as your tile substrate is that some manufacturers of tile installation products don't like bonding to it with their thinset mortars. I, personally, don't like trying to bond to it using any kind of adhesive.
“Bonded membrane” = Kerdi or similar? Is this also true for putting down CBU as the underlayment, or is that more of an issue with sheet membranes?
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Unread 01-22-2021, 02:11 PM   #59
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Kerdi would not be used on a floor, Matt, but yes, you're on the right track. Products such as uncoupling membranes, crack isolation membranes, or similar that might be bonded using thinset mortar might prefer plywood, although manufacturers of those products indicate OSB as a suitable substrate. It's the mortar manufacturers who might have second thoughts. Or not.

Use of a CBU as your tile substrate would remove any consideration of the difference between OSB and plywood; either would work just fine.

My opinion; worth price charged.
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Unread 01-23-2021, 11:06 AM   #60
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A local building supply has Advantech and CDX. Any thoughts on how those fit into the plywood spectrum? If I do the two-layer route that you described, CX, would one of these be good for underneath and one for the top layer, and if so, which for which?
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