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Unread 07-31-2020, 11:32 AM   #1
johnssc1
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OSB Underlayment over Plywood

I'm redoing a master bath. The subfloor is currently 3/4 ply over 2x10 16 OC, but a very short span of 5 ft max due to an I beam in the middle

I have extra 1/2 osb. Can I use this as underlayment for ditra?

I've seen mixed information. It may be more water resistant than ply, but it may flex more. With the short span, does it make a difference.

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Unread 07-31-2020, 12:11 PM   #2
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If you're talking adding a second layer, then yes, you can add that. IF you're talking about removing the 3/4" and replacing it with the 1/2", no, you can't. The minimum thickness ply required is 5/8", and that assumes a nominal 16" OC joist structure. That is considered the minimum acceptable for use with a ceramic tile or actually for any flooring. A natural stone tile requires two layers. The Ditra will attach to either OSB or plywood, assuming the thinset you use is also suitable.
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Unread 07-31-2020, 01:03 PM   #3
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A second layer, Of course. I just wanted to make sure there wasnt another issue with osb I wasnt aware of

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Last edited by cx; 07-31-2020 at 02:13 PM.
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Unread 07-31-2020, 04:29 PM   #4
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Welcome back, Steve.
Quote:
Originally Posted by Steve
...but a very short span of 5 ft max due to an I beam in the middle
The I-beam in the center of the room has nothing at all to do with the unsupported span of your joists. If you have a center support beam, you need to know the unsupported span of the joists on either side of that beam, regardless what they're supporting.

Even more of a consideration is the treatment of the joists where the meet/overlap on top of the support beam. Do you have visual access to that area?

The choice of subflooring has nothing at all to do with the joist span or deflection. The added layer will help with the subfloor deflection between the joists, which is a good thing, but completely independent of the joist deflection.

And while I'm always in favor of a second layer of subflooring under a ceramic tile installation, I would rather see you use plywood if you're planning to bond to it with thinset mortar. Schluter considers OSB fine for a backing for Ditra, but some thinset mortar manufacturers don't recommend bonding to it with their products. I personally don't recommend bonding to it with anything, but that's a different discussion.

My opinion; worth price charged.
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Last edited by cx; 10-31-2020 at 08:25 AM. Reason: typo
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Unread 10-08-2020, 06:39 AM   #5
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Freestanding tub floor flange and ditra

I have a simple question that i cant find asked anywhere else. Maybe you will know

When installing a freestanding tub floor flange, does the ditra go over the flange, or does the flange go under the ditra? I can see reasons for each method.

Flange link for reference

https://www.homedepot.com/p/Freestan...TD35/206503209

Thank you
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Unread 10-08-2020, 07:21 AM   #6
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The very limited instructions suggest installing before tile, and that it's compatible with their membrane. I would tend to think it's meant to install to your prepped for tile floor, or, over the ditra. It's probably best that you contact this drain manufacturer and ask them. Any leak damage in the future should be their liability.

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Unread 10-08-2020, 08:46 AM   #7
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This part of your master bath project, Steve?

For your flange installation I'd first wanna know if you're trying to make the installation waterproof, as in making a wet room as opposed to just plumbing in the tub.

While the manufacturer indicates that the flange "can be installed with waterproofing membrane, there is no indication of what membrane nor how the membrane might be attached to their "epoxy coated metal deck drain."

If it's just to be a drain installation, I'd be inclined to install it before installing my Ditra. If you're trying to make a wet room installation, I think it would need to go on top of the Ditra and then be covered with Kerdi or KerdiBand. Seems like you'd need to pre-fill the Ditra waffles in that area and let that cure before installing the drain and you'd need to bond the Kerdi to the drain flange with Kerdi Fix or similar.

My opinion; worth price charged.
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Unread 10-23-2020, 03:46 PM   #8
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Kerdi Bench Over Shower Tray

So after setting the shower base in, now the wife has changed her mind and wants one of the corner benches.

How important is Schluters rule to put the bench down on the subfloor, instead of the shower tray?
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Unread 10-23-2020, 05:22 PM   #9
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The tray is sloped, the bottom of the bench is not, so getting the slopes and fit right is harder. It's not an insurmountable problem IMHO, though.
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Unread 10-23-2020, 05:28 PM   #10
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Steve, I've combined your three threads on this bathroom project here so folks can see what you're working on and what's been previously asked and answered. A moderator can give it a more generic title any time you'd like to suggest one.

The reason Schluter wants that bench set on the floor is because it won't fit into the corner if it's tilted like the shower tray. And, unfortunately, the top of the bench will not be properly sloped to drain even if it is set per their instructions. I don't have one of their benches on hand, but we're told that, like their curbs, the top is not properly sloped.

You can avoid both problems by using one of the floating corner benches available online. Doesn't matter what your floor is on accounta the bench doesn't touch the floor.

I've used a good number of the Better Benches available from Inovis and find them useful and they also don't infringe upon your floor space, which gives a bit of a larger feel to the shower.

My opinion; worth price charged.
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Unread 10-23-2020, 06:03 PM   #11
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Thanks for the tip. This actually sounds like a better idea overall, and I don't lose floor space

I decided for the other question on the flange to just wait, since we don't have the tub yet I don't know how far to space the drain from the faucet. Luckily I can get to the plumbing below, so ill just drill the pipe through the floor. I'm not making a totally waterproof room

Yes this is all part of the same project. It all started with a quranteen demolition, but im getting pretty close to actually getting the tile
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Unread 10-23-2020, 06:12 PM   #12
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I'll second the use of the Better Bench.

Make sure you install it and fill it with mud before you waterproof. You want it waterproofed just like the rest of the shower.

And while you can shape the mud to make the slope on top, I prefer to screw in the back corner, then push down just a little on the outside corners to get the slope. The seat has just enough play in it that you can do that.

I'm assuming that you've already put the wall board in place, so you've got a couple of choices for securing the bench where it doesn't have lumber to screw into.

1. You can cut out some of the wall board (maybe) and put a 2x4 or 2x6 in with screws, then replace the wall board.

2. You can use some good anchors designed for securing to sheetrock. These are my current favorite. You can buy them in small packages so they're usually around $1 per anchor and screw. But they claim to hold over 200 pounds in sheetrock.

You'll also have the tile underneath the bench to help hold it in place.
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Unread 10-23-2020, 08:10 PM   #13
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These are good for 300# https://www.wingits.com/products/win...or_fastener_48
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Unread 10-27-2020, 06:56 AM   #14
johnssc1
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Kevin, ifs kerdiboard, but I do know where the studs are. Is it better to trace and notch out the board. Or can I compress it as is?

Another small question. The drywall isn't level, and I'm ending up with a gap between the kerdiboard and drywall. My plan is to put a kerdiband over the transition, and extend the tile about 1/2 inch onto the drywall.

Can i use hot mud here? Or should I make some sticky mortar to fill in the gap?
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Unread 10-27-2020, 10:01 AM   #15
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Looks a bit large for straight mud. I would put a piece of sheetrock in there and then mud and tape over that. If you are going to run your tile past that gap then I would use a strip of kerdi band and skip the mudding and taping. Either way that gap needs a piece of sheetrock rock or kerdi board
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