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Unread 05-09-2020, 10:51 AM   #31
sd6000
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Cx, sorry I missed that question earlier.

The cracks seemed to be both parallel and perpendicular at different locations. The thinset wasn't adhered well to the floor either so it was loose from both tiles and floor in many places

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Unread 05-09-2020, 10:58 AM   #32
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The most common location of cracking due to subfloor deflection is over the joist tops. That's the primary reason for the requirement for a double layer subfloor for either direct to plywood bonding or for installation of any natural stone tile.

The cracking perpendicular to the joist is most usually associated with the plywood panel seams, especially if the seams are not T&G style or blocked from below (I wouldn't count on my observation of the WH closet to translate directly to my other subfloor constructions).

Having that strange plywood as your subflooring probably hasn't helped, either, but I can't really say it was a contributing cause without more knowledge of what it actually is. Were it my house, I'd replace it or at least add a layer of nominal half-inch exterior glue plywood with no face of grade lower than C over it. Then an appropriate tiling substrate and your tile.

My opinion; worth price charged.
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Unread 05-09-2020, 12:27 PM   #33
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I had pretty much the same stuff in my bathroom project floor. I called it short strand OSB for lack of a better description. No one I’d asked could give me any info on it. I did learn that it absorbs water and swells and doesn’t go back when it dries; hence the bathroom project. It’s not too sturdy under normal circumstances, we have it throughout the house under hardwood plank floors and it’s got just enough give to have squeaks all over.
I replaced it with 3/4” plywood in the bathroom and it’s the most solid floor in the house. I’d recommend you do the same.
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Unread 05-11-2020, 08:57 AM   #34
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Thanks for sharing cx! Your opinion is definitely worth price charged.
I will add 1/2 plywood. So obviously will stagger the joints from the existing layer, but does the orientation make a difference. In other words, existing plywood is perpendicular to joists, do I want to be same direction but staggering or perpendicular to current setup? Not sure if that is one of those "rotate the grain of the wood" for strength situations.
Last question is what the normal screw pattern would be?
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Unread 05-11-2020, 09:11 AM   #35
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There is an article in the Liberry (link above) that details the proper installation of subfloor plywood, Bryan.

But the quick answer is that the 2nd layer should be installed the same as the first; with the long edge perpendicular to the joists, and overlapping the seams of the first layer by, IIRC, no less than 6". 1 5/8" construction or deck screws will do the trick if attaching 1/2" ply to 3/4" ply. It is recommended to avoid screwing the 2nd layer into the joists.
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Unread 05-11-2020, 11:29 AM   #36
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Plywood has significantly different bending strength across the two axis...you want the strongest direction (the long one) crossing the joists, not parallel with them. Your goal is to minimize the deflection between the joists, so you want the strong direction that way.
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Unread 05-18-2020, 04:42 PM   #37
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All the advice is excellent, I thank you guys!
My plan is to build a mortar pan and use a kerdi drain, and kerdi the entire shower.
Waterproofing is important and I want to make sure I go everything correctly. So I have a few details that are not clear. The manual is somewhat generic.
How far outside of the shower must kerdi run? Such as on the half walls, kerdi the top and it's good, or must it run down the face of the external wall any distance?
Same question with the full height walls- The tile will extend past the shower glass a few inches. Is that far enough for kerdi? Will water wick farther than that?
What is the preferred substrate for shower walls? From my reading here cbu sounds like a pain to install kerdi over due to the thirsty nature of cbu. I see sheetrock mentioned alot. It's hard to change the old thinking that sheetrock doesn't belong in a shower!
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Unread 05-18-2020, 05:05 PM   #38
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Bryan, the reason you see drywall used as a backing material for kerdi in showers is that the manufacturer has jumped through all the necessary hoops to get an ICC Evaluation Report, essentially a waiver of the applicable building code. It permits the drywall to be used in a Kerdi Shower Assembly. You must build the shower exactly according to the manufacturer's instructions as set forth in the ER.

Some folks still don't like the use of drywall in that application and the Kerdi manufacturer accepts CBUs as an alternative. Up to you. I've personally built only one shower using CBU and a different membrane (USG Durock Shower System) only as a demonstration of the new product and found it easy enough to install that backer board (not as easy as drywall) and had zero difficulty installing the membrane over the CBU.

Either backing material you still gotta follow the manufacturer's instructions and comply with all other building code.

How far to waterproof outside the wet area? Again, up to you. A few inches beyond a fixed glass installation should be enough. Operative word is should. The wet area technically stops at the glass, but we know that's not always the case in real life.

My opinion; worth price charged.
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Unread 05-19-2020, 10:52 AM   #39
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Do do you typically use greenboard? Or does it matter since it's not a wet area once covered with kerdi?
What about the curb? Mine is made of 3 2x4s. Cbu on that for "stepping on strength"?


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Unread 05-19-2020, 10:56 AM   #40
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Got it. We have a plan.
Screw spacing should be what... Something like 6-8" on edges and 12 or so in the field?

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Unread 05-19-2020, 11:45 AM   #41
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Green board is unnecessary, and its coating might actually reduce the mortar bond.

If I had some CBU on hand I'd use it for the curb before covering with Kerdi, but plain old DW will work too. The tile or stone on top of the curb will distribute the foot loads just fine.
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Unread 05-19-2020, 11:47 AM   #42
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I'd say 4-6 at the edges, 8 in the field.

Is this the same project as your other thread, Bryan?
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Unread 05-19-2020, 03:34 PM   #43
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Thank you.
Yes, same bathroom as the other active thread. I posted separately since one is floor and one is shower, I consider 2 separate projects in one room. However I guess it is one project....

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Unread 05-19-2020, 03:36 PM   #44
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Thank you.

Yes, same bathroom as the other active thread. I posted separately since one is floor and one is shower, I consider 2 separate projects in one room. However I guess it is one project....

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Unread 06-04-2020, 01:32 PM   #45
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Schluter thinset? So they advertise if I use their thinset with kerdi or ditra, they will extend a lifetime warranty. Is that a gimmick or is that for real?
I also noticed they spec unmodified thinset with kerdi installations, unless you use their brand of modified.
Is it worth the little extra cost to use schluter thinset?

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