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Unread 12-01-2022, 11:11 AM   #31
GalaxyWide
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Deck Mud/Slope/Floor

TL;DR: Can I lay fortified deck mud with embedded lath to a minimum 1/4" thickness? Mud to be used only for structure, no interaction with water.

I've run into a slight snag with final tile height and the thickness of the mud bed. We've decided to do the drain at the entrance for a lower final height, which we have no issue with design/visually.

The issue (that's present regardless of drain location), is that with the drain recessed into the subfloor and sitting on top of the joists (the joists are NOT being notched, per code they cannot be as the shower is mid-span), the drain is only .25" above the subfloor, meaning any mud bed would taper to only .25" thick there. I'm seeing 3/4" generally given as a minimum mud bed thickness, which doesn't work unless I change something. However, the Mapei deck mud lists 3/8" as the minimum, and the Laticrete fortified mud says it can be spread to a feathered edge. Keep in mind, I'm NOT using the mud bed for anything other than structural support, as it will have membrane over it and the drain is a separate assembly.

My floors are 1 x 8 diagonal subfloor (3/4" actual thickness) over 2 x 8 joists. Any mud bed will include a plastic sheet under it, and stapled down metal lath/wire mesh


Options that I can think of:

1. Use a mud that specs a sufficiently thin minimum and call it a day, the mud is only there to put tile on. Easy, simple, seems like it ought to be fine.

2. Cut out all the subfloor in the shower area, install a new, dropped subfloor flush with the joists and put mud on that, allowing for a 1" minimum thickness. Ton of work, and seems like I would lose a lot of stiffness from the long subfloor boards.

3. Suck it up and get a foam shower base, most of them taper to .25". Seems a bit ridiculous to pay $200-$500 for a chunk of what is essentially insulation foam or Styrofoam, but it's a possibility.

4. I've seen someone do a single slope pan with a sheet of 3/4" plywood supported with wedges or a slope cut into the joists. In my case the joists run parallel to the shower, so supporting such a sheet well enough would be tricky. Would also require removal of existing subfloor in the shower.


Right now I'm leaning towards #1, for obvious reasons - if the thin edge of the bed is well supported and braced under the subfloor to prevent defection of the 1 x 8's, it really seems like the mortar bed won't be under much stress and should be plenty strong in compression, plus with the large (12x24) floor tiles any loads will be rather well distributed. Does this seem even remotely like a good idea, or am I totally off in left field picking flowers?
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Last edited by GalaxyWide; 12-01-2022 at 11:12 AM. Reason: Added TL;DR to small novel
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Unread 12-01-2022, 01:37 PM   #32
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Quote:
Originally Posted by Scott
2. I see now, thinset/mediumset/thickset/etc are all methods of installation,..
Well, except for the "mediumset." No such thing in the tile industry.
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Can I lay fortified deck mud with embedded lath to a minimum 1/4" thickness?
Well, you can, of course, but not if you want to come even closer to following tile industry standards.

Technically, there is no specification in the tile industry for a shower pre-slope because said pre-slope is part of the plumbing. But you're correct that the tile industry doesn't recognize a floor mortar bed thinner than 3/4" for any installation. The industry does, however, require that you have such a slope under the waterproofing of the shower receptor, as do the plumbing codes.

For a mortar bed over nominal 1" sawn board subfloor, though, only a reinforced mortar bed is recognized, which must be a minimum of 1 1/4" thick with welded wire mesh in the vertical center. That also presumes the boards are T&G, and perpendicular to the joists, which are spaced no more than 16" on center.

So, what's a fella to do? Whatever he thinks will work in his application, I guess. I can tell you what some of the standards say, but you hafta decide what to do, or not do, with the information, eh?

Picking the flowers is always optional, far as I can tell.

My opinion; worth price charged.
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Unread 12-01-2022, 02:48 PM   #33
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Ah you got me, I was thinking of "medium bed" which does appear to be a term.

I appreciate the info, even though it seems like the answer is "no, but no one can agree on why".

Given that my subfloor boards are not T&G, not perpendicular to the joists, and have decent (over 1/4") gaps between them, I wonder if putting down a sheet of nice 1/4" plywood over them and then bringing the mortar down to a feathered edge might be best. It's going to have Ditra-Heat on top of it (yes, I know, I decided against a heated shower floor but I've since changed my mind back. I want a warm floor dammit! ), so it won't be bonded directly to anything above it, just there for support.

Maybe I'll think more about this, and the best way to integrate all the layers.

It just occurred to me I could bring the mortar down to a feathered edge, then put the Ditra on that, butted up to the drain and save myself 1/4" of height vs putting the Ditra on top of the drain body. Of course, that part wouldn't be heated, but you have to keep the cable away from obstacles anyhow.

Or maybe it would be better to replace the subfloor in the shower with 3/4" BC or better plywood, thereby eliminating all the gaps between the subfloor boards. Less work than recessing it, that's for sure!
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Unread 12-01-2022, 03:30 PM   #34
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Quote:
Originally Posted by Scott
Ah you got me, I was thinking of "medium bed" which does appear to be a term.
Nope, not a term, not a method, not a thing in the ceramic tile industry today. Think of it as a Dodo bird. Gone. Strike from your memory.
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Originally Posted by Scott
I appreciate the info, even though it seems like the answer is "no, but no one can agree on why".
Pretty well agreed in the ceramic tile industry literature that floor mortar shall be no thinner than 3/4-inch. In your house, things may be different.

No 1/4" plywood anywhere in your floor package, please. The minimum the industry accepts is nominal 1/2" exterior glue plywood over sawn board subfloor. And even that requires the perpendicular orientation and T&G edges, but I'll give you a pass on your subfloor under a shower if you don't cheat on any of the rest of it.

Raise the drain or lower the floor would be my advice.

My opinion; worth price charged.
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Unread 12-01-2022, 03:57 PM   #35
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The term seems to have some use, see here for CBP's page on their VersaBond LFT - it calls it a "medium bed mortar", though it's unclear what that means besides that it's thicker than a "thinset mortar" (which as already pointed out, is of course an abuse of terminology).

I appreciate the advice as always, I'll do some more thinking and staring at it and see what I come up with. I may have gotten myself a bit of wiggle room by moving the Ditra from on top of the drain to next to it, in addition to the main floor having a minimum height of about 1.22" (15/32 ply, 1/4" ditra, 3/8" thinset, 5/16" tile).

As much as I'd love to drop the floor, the shower is just about dead in the middle of the joists so they cannot be notched (I thought about trying to sister them, then notch, but couldn't find ANY reference to such a thing and overall it seems like a terrible idea and I didn't feel like breaking out the statics textbook to do deflection calculations. As it is, I have to repair an illegal notch already!) and therefore the top of the joists is as low as I can go. Meanwhile, the rest of the house is just 3/4" above the subfloor, and I'd like to avoid a large step up (though I accept that some is inevitable).
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Unread 12-01-2022, 06:17 PM   #36
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Quote:
Originally Posted by Scott
The term seems to have some use, see here for CBP's page on their VersaBond LFT - it calls it a "medium bed mortar", though it's unclear what that means besides that it's thicker than a "thinset mortar" (which as already pointed out, is of course an abuse of terminology).
And they're not the only manufacturer who still likes to slip that term into their advertising, Scott, even though they most certainly know better. Don't know if they think it's cute, or if they're still mad at Gobis, for getting the description changed to Large and Heavy Tile Mortar because people were getting the idea that there was actually a medium-bed method of setting tile, or just what other problem they might have. But I can assure you that you will find no reference to medium-bed anything in the official tile industry literature except an occasional parenthetical mention that it was formerly called medium-bed. 'Nuff of that.
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Originally Posted by Scott
(15/32 ply, 1/4" ditra, 3/8" thinset, 5/16" tile)
Unlikely that you'd end up with that much thinset mortar, but I suppose you could if you tried real hard and used the right mortar.

Dropping your subfloor between the joists is still an option. Yes, lotta work, but nearly all remodeling begins with the recognition that it'll be a lotta work, eh? And what about adding plywood on top of the bathroom floor and making a transition to the outside flooring? Have we covered that somewhere?

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Unread 12-06-2022, 12:17 PM   #37
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Interesting to know about the "medium-bed" term, seems like it's a bit of a contentious issue in the tile industry and I had no idea. At any rate, I'll just call it all mortar and specify the trowel type from now on.

I suppose I'll be closer to 1/4" of mortar, since given a .5" x .5" trowel that averages out to .25".


I've decided that dropping the subfloor in the shower is unavoidable, and while I haven't chopped out the structural planks yet I'm committed to doing so.

I remain undecided on bed/slope method - mud seems to be the accepted method, but effectively pouring concrete in the middle of the house seems a bit strange to me - especially given the weight of the bed and it's position midspan on the joists. I'm considering doing the slope with plywood and wedges instead, since that's effectively identical from the tiles' perspective to plywood over joists.

My current dilemma is that I already have 2 sheets of 15/32 BC plywood that I would like to use, but specs and installation guidelines (as well as common sense) dictate a minimum of 5/8" plywood as a single layer and I would feel better with 3/4". I'm thinking that 2 sheets of 1/2" plywood, glued and screwed together, ought to be significantly stronger than even 3/4" plywood and would work fine for both the subfloor and the slope (should I choose to go that route). Should I be putting in supports between the joists to prevent any deflection of the (now dropped) subfloor?


I have decided against dropping the entire bathroom floor, since the finished height as-is will be acceptable and I'd really rather not have to mess with shortening the cast iron toilet pipe
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Unread 12-08-2022, 08:53 AM   #38
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If you are determined to use the 1/2" ply, Scott, and gluing it together, you'd want the glue to cover the entire sheet of ply. You don't want to use ribbons of adhesive because there's a chance the top sheet of ply will sit on top of the ribbons resulting in voids between the sheets. You don't really need glue at all, just proper spacing of the screws.

But. It will be a challenge to get the screws to bite and hold in 1/2" plywood, it's just too thin. I wouldn't use 5/8" either, 3/4" will be best.

Not quite sure how you intend to implement the wedge idea, but the only way I would is by ripping down some 2X4's, which in itself might be a challenge; finding some that are straight enough, and not twisted. And then there's securing them to the top of the joists - the thin end of the wedges won't have much thickness and thus be prone to splitting.
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Unread 12-08-2022, 09:20 AM   #39
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Quote:
Originally Posted by Scott
I suppose I'll be closer to 1/4" of mortar, since given a .5" x .5" trowel that averages out to .25".
That would presume that you held your notched trowel perpendicular to the backing material, Scott, which you won't be doing. If you really need to know the resulting mortar thickness, the only real method is to set a couple of your tiles using your mortar and your trowel and measuring the resulting height.

The industry standards call for a minimum of 3/32nds" of mortar under the set tiles.

Only disagreement I would have with Dan's advice there is that I wouldn't use the two sheets of half-inch plywood without a full spread of glue, regardless your fastener schedule. But, as you can see, that's up for debate. With thicker plywood as the first layer, I'd be content to rely upon testing done by these guys, who say they can achieve "almost" the same rigidity without glue.

There is also the Appendix - Limitations and Explanations to ANSI A108.11, Installation of Cementitious Backer Units, wherein it says "3. CBUs cannot be used on floors that are sloped to a drain." I've never had that one explained to me, but you might keep it in mind.

[Edit] I asked one of our resident gurus about the reasoning behind the prohibition of use of CBUs on shower floors and he couldn't remember the rationale, either. Maybe one of the other old timers will weigh in and 'splain us.

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Unread 12-15-2022, 11:59 AM   #40
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32x60-pre-waterproofed, single slope to linear drain,(honeycomb/foam- 235psi) list price $275.00
Not sure why you couldn't use CBU on a single slope to drain however..
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Unread 12-15-2022, 12:28 PM   #41
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Eric, who's shower slope are you quoting there? If it's Noble (I assume it is), their 32x60 is end drain only and I need side drain.

It does appear that their 64x64 (SKU: 506464101) would work, however, and at *only* $272.55 (plus ?? shipping) it's at least less than the hydroblok one. I assume there's no reason it wouldn't work with my Ebbe Inni drain?

I might actually price compare their full kit too, at $782 it might wind up cheaper than having to buy a full roll of hydroban membrane when I only need enough for the shower pan. I'll have to see if hydroban liquid and Noble are compatible, since I still want to use that for sealing/peace of mind/extra waterproofing.
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Unread 12-15-2022, 12:48 PM   #42
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Subfloor

You guys do bring up some good points about plywood, sandwiching or not. The more I think about it, the sillier it sounds...and I think I have a better alternative.

I already picked up some 1x8 and 1x10 boards I was going to use to replace some broken/cut out piece of the existing subfloor, but I don't see why I couldn't just cut them to fit between the joists and use that as the new, recessed subfloor (think slats on a bed, multiple cut pieces placed in between the joists and supported with ledger boards screwed and glued to the joists). Should be at least as stiff as 3/4 plywood, and likely stiffer. Getting them to screw down at the edges without splitting will be a task, but nothing some washer head screws and pre-drilling can't fix. Plus, once they're glued down the screws will largely be for show anyhow (can you tell I like glue? It smells great!).

A large part of my reluctance to get a thicker sheet of plywood is that I don't have a good way to move it...I don't have a truck, and I'd rather not have to mess with trailers, rentals, paying for delivery, etc if I can help it, especially for just one sheet of plywood! I know, lame reason, but it's where I'm at.

The project has stalled out a bit due to other things coming up, but I'm slowly getting more and more sick of using our tiny second bathroom as a main bathroom so I'm sure I'll get back to it soon.
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Unread 12-15-2022, 03:43 PM   #43
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Wink

Yes Noble Co.

list $320.64 --32''x60" custom made ,Honeycomb pre waterproofed, single slope.(slopes in one direction ) Freight paid
And Hydroban is compatible.
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Unread 12-16-2022, 07:13 AM   #44
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Scott,

I think using 3/4" plywood is going to yield a more stable, maybe even stiffer, sub floor than will using those 1X sawn boards. For the same reason it is recommended to install a minimum of 1/2" ply over a sawn board subfloor; the sawn boards are too unstable with temp and humidity changes. But if you are going to cover them with another layer of ply then they would work.

You might check with whatever store has the plywood you'd need (3/4", no face grade lower than C, EXP1/Exposure 1 rated) to see if they have a panel saw. They'll cut the sheets into more manageable sizes for a nominal fee.
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