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Unread 08-14-2022, 11:27 PM   #1
Snets
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Kerdi Drain Height: Wood vs. Concrete Vs. Kerdi Tray

So as we all know, when building a dry pack mud pan, the Kerdi drain minimum height, or the thickness of the mud under the drain is 3/4" for a concrete subfloor and 1" for a wood subfloor.

If you purchase a Kerdi shower tray, the drain will be 1/2" to 9/16" - ish above the substrate; either concrete or wood. It is supported by foam that is part of the tray.

Is that option available without using a full foam tray? Could one buy the smallest tray available, use the support for the drain from the tray and fabricate a custom mud pan around it?
I ask because with curbless showers, vertical requirements are everything in keeping proper slope and not notching joists/raising floors. Fractions of inches matter.

Last edited by Snets; 08-14-2022 at 11:34 PM.
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Unread 08-15-2022, 03:43 PM   #2
jadnashua
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The issue is, deck mud doesn't have all that much strength when it is thin when installed over wood. While the high density foam would support the drain, I'd worry about the stability of the deck mud that then mated up to it around the edges, and that 'cold' joint between the foam and the deck mud would be risky.
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Unread 08-15-2022, 04:38 PM   #3
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I think you might have some vertical movement where the foam meets the mud.

And we all know what that would lead to....
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Unread 08-15-2022, 09:47 PM   #4
Snets
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OK, thanks for the replies. As much as I do not want to use the 60" X 60" Kerdi tray, I'm afraid that's my only option without notching a small amount off of the joists - 3/8" notch over 60".

I'm at L/416 as it sits and the shower is in the center of the unsupported span. I do have standing access under the house If I decided to sister OR strongback the joists to do a mud pan.
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Unread 08-16-2022, 07:58 AM   #5
cx
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Quote:
Originally Posted by Snets
- 3/8" notch over 60"
That's not a notch, Snets, that's a rip. Either is verboten in the center third of your joist span. Will it cause a serious problem in your case? I dunno. I just know it's never a good idea unless you can sister or support the joists to properly accommodate such a rip. And if you can do that, I'd make the rip deep enough to do some real good, eh?

My opinion; worth price charged.
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Unread 08-17-2022, 09:57 PM   #6
Snets
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Quote:
Originally Posted by CX
That's not a notch, Snets, that's a rip.
OK, long notch, rip....either way, yes, not a good idea without sister-ing, 100% agree.

So let's dig into that a bit deeper should I decide to give myself a little wiggle room with a dry-pack mortar base:

I have an approximate 13.5'-14' unsupported span, with very easy access under the floor. I have not at this point investigated what all, if any electrical or plumbing would be in the way - I will take a closer look next week. If there is little in the way, sister-ing may be the easiest way to go.

What about strongbacking the joists with a 2x4? I have done some research and found THIS but it it not talked about much. What is the current opinion on this?
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Unread 08-17-2022, 10:34 PM   #7
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My current opinion would be the same as my previous opinion, which is probably expressed somewhere in that thread you linked, Snets. While the method is sound in theory, it's the practice of making it happen where I generally find fault.

As one of our recent members (pardon my rudeness in not remembering his name) states in his signature line, "In theory, there is no difference between theory and practice. In practice, there is."

When sistering joists, especially when sistering full length (except for bearing surface), it's easy to calculate the additional structure provided, presuming, of course, that the sistering is adequately done with mechanical fasteners and adhesive, which is also rather easily done. And while I've seen circumstances where I believe I could actually have added the 2x4 to the bottom of a joist successfully, there is a lot more room for error than in sisterirng.

It's your project, of course, and your decision whether you can make that method work and whether you can adequately calculate the end result. If you're starting with joists of a very good grade, making it possible (but certainly not assured) that you'll end up with the same or very similar grade of smaller joist, you may be able to say you've added as much thickness to the bottom of the joist as you've removed from the top, plus a calculable amount attributed to the width of the horizontal "strongback" piece, and prove to yourself that you're better off than you would have been just building atop the un-ripped joist.

It's a simple concept for sure, but I suggest it's not as easy to pull off successfully as it might appear on paper. I'd like to actually try it if I could stumble upon a situation where all the joists in question had a bottom sufficiently flat and square and clean and I had plenty working room, as you appear to have.

But if it was a customer's house, I think I'd still rather sister with joist material as tall as I could fit in.

My opinion; worth price charged.
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Last edited by cx; 08-18-2022 at 07:12 AM. Reason: typo
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Unread 08-17-2022, 11:08 PM   #8
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As always, excellent, thorough and well thought out advice; much appreciated.

I'm going to take a much closer look next week to weigh the options. This is exactly why I bring this up on this site, I am not willing to take chances in someone else's home - it has to be right, 100%.

Can you direct me to a post that describes nail/screw/glue pattern for proper sister-ing?
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Unread 08-18-2022, 09:22 AM   #9
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One thing that you might look into is the manufacturer drypacks like Laticrete 3701.

Oftentimes, they are stronger and can go thinner than a generic drypack. I looked at the data sheet for 3701 and I don't see a minimum number for going over wood subfloors so they probably want you to call in.

It won't be as easy to use but it might be a way of solving a problem.

Finally, I believe this very subject is on the agenda for a more thorough industry discussion next year.
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