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Unread 03-19-2022, 12:02 PM   #1
PlumbFool
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Pre-slope DEEP trouble

Hi,

First time posting.... Since a picture is worth a thousand words, I'll spare you the long story and show you what I'm dealing with. Please see picture.

Question 1: The deepest cavern is about 2". My plan was to thinset as best as possible, then dry pack to even everything out. I figure I have to keep the thinnest parts wet until the 2" deep mud sets up to avoid cracking. Am I on the right track, or do I have to jack-hammer the whole concrete pad and start over?

Question 2: I had to build a little 4 inch bump-out (like a narrow bench) to accommodate a 2 inch horizontal drain pipe (another long story). Now there's a 1" gap between the bottom of the PT plate and the original shower floor. What's the best way to close that gap before pre-sloping?

Question 3: There's a 2.5" gap between the "bench" and the curb to accommodate the marble threshold (lower left, utility knife points to it). Should I just cover the space with Durock and wall tile, or slope the gap to the drain?

Thanks in advance for your help - your expertise has already helped me immensely.
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Unread 03-19-2022, 12:37 PM   #2
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Welcome, Caryn.

1. I would patch those holes with a concrete mix of some kind, using a slurry of thinset mortar or pure Portland cement as a bonding agent on the old concrete.

I would want to know how you plan to waterproof your shower, and in particular how you intend to make the receptor. That construction joint (I'm guessing) could be a significant problem.

2. If you actually plan a mortar pre-slope, I would just let the mortar fill that gap as it sees fit.

3. Sorry, can see no such gap in your photo.

My opinion; worth price charged.
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Unread 03-19-2022, 01:10 PM   #3
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Thanks CX.

How deep should the thinset slurry be, just enough to cover all the rough surfaces? And if I fill in the deep gaps with Portland (I was planning on using Sand Mix), I assume that I then thinset over everything after it sets when I cover the rest of the pan, to ensure adhesion of the pre-slope mortar?

I was planning on Redgarding the whole thing for waterproofing. The potential for shifting around that construction joint is why I paused. What do you recommend?

Also, I know the little gap to the curb is hard to see. It's just a continuation of the plate to the curb, with the vertical bench support on one side and the curb on the other.
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Unread 03-19-2022, 01:14 PM   #4
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Or, do I do it all in one shot? Slurry the deep holes, fill with Portland, then immediately thinset the entire shower pad before the Portland cures, followed by mortar pre-slope? Seems like letting the deep holes cure before continuing would reduce the possibility of shrinkage later on....
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Unread 03-19-2022, 03:13 PM   #5
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Again, how do you plan to build your receptor? Sometimes called the pan. It's the waterproof portion that will hold water up to the top of the curb level.

You indicate you'll use RedGard. Do you still plan to use a "pre-slope" as you're indicating, then make a second mortar bed on top of that? Or do you plan to use the "divot" method, where you would make a single sloped bed with a "divot" at the drain and bond your tile to the RedGard? Or.....other?

I would not want to bond anything to that floor. The only way I can think that you might isolate that construction joint (I'm assuming again that's what you've got there) from your shower floor tile would be with an unbonded, reinforced mortar bed, preferably only as your pre-slope for a traditional receptor.

Entirely up to you, of course.

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Unread 03-19-2022, 10:33 PM   #6
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Hi CX,

Sorry, wasn't sure what you meant by "receptor".

The plan is to use thinset and mortar for the pre-slope, followed by Oatey PVC shower pan liner, (including over the curb), then mortar bed over that. The Redgard will come after the shower pan is finished.

I'm not sure whether that's a construction joint; I demoed the previous 48 " fiberglass shower surround to expand the shower to 55", and had to break up the level concrete that was under the wall I took out in order to create a 1/4" slope. The bathroom was tiled prior to the surround installation apparently, so I gave up trying to figure out what previous owners did and I'm just trying to make sure I fill in the deep spots properly.

Would you recommend metal hardware cloth to reinforce the top mortar bed? I read that it's not really necessary, but maybe in this case it couldn't hurt?
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Unread 03-20-2022, 08:39 AM   #7
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The very safest method, if that is a construction joint, would be, as I mentioned earlier, to use an un-bonded, reinforced mortar bed of a minimum 1 1/4" thickness as your pre-slope. That requires a welded wire mesh in the vertical center of the mortar bed (hardware cloth is never a suitable reinforcement for any mortar application).

Over that, your waterproof membrane and second mortar bed may be sufficient to isolate your tile installation from the anticipated movement in the concrete slabs. Operative word there is may. I don't think reinforcement in the top mortar bed would be particularly helpful, but it wouldn't hurt so long as you're careful not to puncture the waterproof liner.

You would not put RedGard or similar over the top mortar bed at all. You can use it to waterproof the walls if you want.

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Unread 03-20-2022, 10:11 AM   #8
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Thanks CX, that's just the info I was looking for. And thanks for the instruction in hardware cloth vs. welded wire mesh, I thought they were interchangeable.

Just one question for clarlity: by "Unbonded", do you mean that I should NOT use thinset under the pre-slope mortar bed? Should I use a cleavage membrane over the slab after I fill in the holes instead?
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Unread 03-20-2022, 10:35 AM   #9
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That would be correct. Cleavage membrane, minimum 1 1/4" mortar bed, welded wire mesh in the vertical center. In your case you'll almost certainly need to raise your existing clamping drain to allow for the minimum thickness of your mortar bed at the drain, sloping up from there to the perimeter.

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Unread 04-12-2022, 01:00 PM   #10
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Can you use thin-set instead of deck mud for pre-slope?

I was about to start the pre-slope using thin-set slurry on a concrete slab, followed by metal lath embedded in deck mud to fill some deep holes and an expansion joint (see Pre-slope DEEP trouble). But I threw my back out and was forced to bring a professional tiler in. He just started.

Here's my question: rather than the thin-set slurry/metal lath/deck mud sandwich I had planned for the pre-slope, he just spread Mapei thin-set mortar and called it done, ready for the liner. Do I have to worry about bonding or strength or cracking? (BTW he said the slab was old enough to not move at the joint, and I should put a Kerdi liner down UNDER the pan liner to handle any expansion joint movement.)

Thanks in advance,
Caryn
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Unread 04-12-2022, 01:53 PM   #11
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Caryn, it'll help if you'll keep all your project questions on one thread 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. Your link doesn't work for me, by the way.

I very strongly disagree with your tile guy about the likelihood of future movement of that construction joint. I'll be willing to wager that it moves at least twice a year.

Using thinset mortar to create a pre-slope? Very poor idea to my thinking, but there is really no specification for what must be used for a pre-slope as it is not technically part of the tile installation. It's part of the plumbing. So long as he achieves the required minimum flat slope of 1/4" per foot from the farthest corner from the drain and it is substantial enough to support the shower floor, it's legal. I honestly can't imagine trying to achieve that using thinset mortar.

And I would still recommend isolating that shower floor from the slab, even at the pre-slope level.

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Unread 04-12-2022, 02:00 PM   #12
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Thanks, CX. And sorry about the separate posting, I was trying to save people from having to go through the whole story.

So yeah, I sat aghast while watching him work... at this point, should I rip up what he did and start over, or can I still embed metal lath in pre-slope mortar over the set thin-set? Not that there's any room around the drain...
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Unread 04-12-2022, 05:33 PM   #13
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I don't see anyway you'll get the proper mud thickness and pitch without taking it up and starting over.
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Unread 04-14-2022, 01:08 PM   #14
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Picture of the shower floor "repaired" with thin-set, and a close-up of the drain assembly.

Tear out and start over? or cover with membrane, then liner, then deck mud?
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Unread 04-14-2022, 01:22 PM   #15
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Caryn, lay a straight-edge of some sort that will reach from the farthest corner from your drain to the drain. Put some sort of spacers under the end on the drain until a level laid on top of the straight-edge shows level. measure the height of the spacers you used.

That height should equal at least 1/4" for each foot of horizontal measurement from the drain to that far corner.

And the slope should be very flat.

See what you find there.
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