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Old 07-24-2018, 03:28 PM   #16
Rigo
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I bought my Durock 1/4 Cement boards and hoping to get to it on Friday. I went with Durock, cause it says on their website that I can install it over my 3/4 plywood with my joists at 24 inches. Do I need to scrape off paint and sand my subfloor before I put down the Durock? I think I read on a post that I could just leave it. Also they say to use modified thinset, which I keep reading here I don't have to. Ryan I went to Contractors Direct for some Raimondi 1/8 Spacers they have really good stuff and prices.
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Old 07-24-2018, 04:58 PM   #17
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Quote:
Originally Posted by Rigo
I went with Durock, cause it says on their website that I can install it over my 3/4 plywood with my joists at 24 inches.
Help me find that, would you please, Rigo. I know of no industry accepted method of using a CBU underlayment over a single layer of nominal 3/4" plywood over joists on 24" centers. Manufacturer's instructions always trump industry standards, but I've never seen that one.

And I wouldn't recommend you do it.

As for the thinset mortar under the Durock, I know they do recommend a modified thinset mortar, but I have no idea why. Don't think they know, either. You'll note they also recommend the use of organic adhesive (mastic) in that same application.

My opinion; worth price charged.
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Old 07-24-2018, 05:40 PM   #18
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Hi Cx hope all is well, man I hope I read that right on the Durock website. Copy and past this link. Then click on thr 1/4 cement board specs. Sorry Im on my phone at work, wish I could get you there faster. The info is right where it says limitations. https://www.usg.com/content/usgcom/e...ent-board.html
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Old 07-24-2018, 06:20 PM   #19
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You read correctly, but I'm with Cx on this one. If you had 16 on center joist I'd say go for it. 24 oc is just too much for span for a single layer in my opinion. But, in the end we do preach to follow manufacturer specs and if you feel comfortable with that go ahead.
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Old 07-24-2018, 06:32 PM   #20
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Thx for looking that up Ryan. If you look at my first post, you will see my tile in the toilet area. Im gonna rip that out, I was able to see a cement board under that with no thinset. I have another bathroom with same type of tile and my laundry room has tile too all with backer board. I bought the house new 10 yrs ago and all the tile has help up well. I wasn't even concerned until CX pointed it out, good thing too cause I had no idea. I was gonna put wonder board. So what im getting at is, Im gonna go through with the project, since it has already been done, but gonna use thinset under the Durock. I will see for sure what the installer did when I rip the tile out. Also I have some 1/16 Raimondi spacers, can i try to sell them here? I dont need them, figured a pro on here can use them. I think its 250 spacers. Rigo
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Old 07-24-2018, 08:10 PM   #21
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Rigo, if you'll look at the main forums page you'll see a Buy/Sell/Trade forum listed. Please read the sticky thread there before you post.
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Old 07-24-2018, 08:30 PM   #22
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Thx CX will check it out. One more thing, my subfloor is covered in paint, do I have to remove it before the thinset and Durock?
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Old 07-24-2018, 08:50 PM   #23
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No.

[Edit] Now that I see where you got your information about using Durock over single layer subflooring over 24" joist spacing, Rigo, I understand your confusion. Please read that Limitations section again. Yes it says the minimum subfloor thickness is nominal 5/8ths-inch and yes, it says the maximum joist spacing is 24 inches on center, but read on and it also says the maximum deflection must be L/360 of the span.

You cannot meet that requirement while also using a single layer of 5/8ths-inch, or even 3/4-inch, plywood or OSB subflooring. To comply with the L/360 requirement for the subflooring you're gonna need a second layer. They know that. They apparently expect you to know that, but I don't know why.

James Hardie has been using that same confusing language for their product for many years, but I never noticed it in USG's literature.

My opinion; worth price charged.
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Old 07-24-2018, 09:02 PM   #24
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Thanks for clearing that up, Im gonna take out a ceiling speaker from my living room ceiling which is under my bathroom and look at the joists and see exactly what's going on under the plywood. I really dont want to install more plywood. Like I was telling Ryan, I have three other places upstairs with 1/4 backerboard with no thinset under and zero issues. Sorry if I sound hard headed or redundant, I know you're trying to make me do it the right way. Im already gonna have to jerry rig my transition piece dont want it any higher.
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Old 07-24-2018, 10:10 PM   #25
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You can certainly tile over whatever you like using whatever methods you like, Rigo. All we can tell you is where the smart money is betting.
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Old 07-24-2018, 10:17 PM   #26
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Thx CX, I will let you know what I see tomorrow.
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Old 07-25-2018, 01:32 PM   #27
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This is what I have going on under the floor. I don't know what to call this type of joist system. The joists are 24 inches, and there is osb in the pic but I have plywood in the bathroom. Guess they used both on the house. Do these joists help or hurt me?
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Old 07-25-2018, 01:35 PM   #28
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Those are called trusses. The Deflectolator won't work for trusses. Can you see any informational stamp or tag on the trusses? [Edit: the one rectangular stamp visible in your photos, on the underside of the top most horizontal member (chord) of the truss is likely a grade stamp put on the 2x4 prior to the truss fabrication, so it won't have any truss specific info. But it is worth checking.]

Or do you have the original design documents from when the house was built?

Cheers, Wayne
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Old 07-25-2018, 02:11 PM   #29
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Hi Wayne, thanks for setting me straight on what I have. I did see a stamp on a 2x4, just didnt take s pic.I will get back up there tomorrow and check. I was taking pics through a speaker hole so didnt have too much to work with. I saw some 2x6 or 2x8s just couldnt see a marking them. Last nigjt i looked through my house stuff last night, but couldnt find the plans or specs. Is there a different calculator some where for what I have?
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Old 07-25-2018, 02:28 PM   #30
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The wide chord on the top of those trusses helps you a little, Rigo, by reducing the between-joist span of your subfloor. I still wouldn't tile over a single layer of subflooring on that floor, though.

It would be exceptional to find an engineered joist system that did not meet at least L/360 deflection and in most cases it would exceed that. The only way to get detailed data is to contact the manufacturer.

My opinion; worth price charged.
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