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Unread 02-17-2020, 09:18 PM   #1
maxwell_smart007
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SLC under kerdi pan - questions

I was just about to install the kerdi pan, but upon mockup, I realized that it slopes away from the drain in one corner. I checked the floor, and it's indeed out of level in that corner.

I have a new plywood floor (extremely well supported from below and verified with the calculator) with the drain hole pre-cut, and the hole matched to the kerdi pan. The pan was cut down along the edges (taking from the outside evenly to ensure the slope stays true. The shower is roughly 3' square

I checked the pan on a known level surface, the the pan has the correct slope, so it's the floor that's the only issue. I estimate that it has to come up about 3/8 -1/2" on one side. Since thinset shouldn't be laid that thickly, and I don't want to end up with any pockets or weak spots due to 'smoothing issues', I want to use SLC to ensure a dead-flat floor.

With that in mind, I installed the curb (after cutting it very carefully to make sure it's level, despite the floor slope) with a very, very slight tilt toward the shower, using Laticrete 253, and re-tested the kerdi pan to make sure it still fits after curb install.

Next step - I want to dam off the cutout hole using foam sill sealer reinforced by cardboard (and perhaps caulking?) and then pour a very thin layer of SLC.

My question - since the pan is only out of level on one side, do I need to SLC the entire floor? I.e. it slopes toward the drain and then continues to slope beyond the drain. If I only bring up the one side, I'm thinking that I'd run into other issues - not the least of which being that I'd have to use modified on half and unmodiifed on the other - which I'm not going to do. Is SLC strong enough to be reliable under a foam pan? (the foam gives me pause - I've already slightly dented it kneeling on it to check the corner - but I assume it'll be more sturdy after install).

The walls are already meticulously covered in kerdi, so a mud pan isn't an option.

So the plan - dam off the hole (should I smooth a bead of kerdi fix to the curb/perimeter?) then get a quality SLC, prime the floor, pour the slc on the whole floor, wait a few days, and then use laticrete 317 to install the pan and then finish the membranes as usual.

Any concerns with this plan?
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Unread 02-17-2020, 09:38 PM   #2
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Andrew, the mud bed is the perfect fix for you problem. The fact that your wall Kerdi is already installed does not change that in the least.

You solve at least three problems with the mud bed. First, you no longer have any concern for the out-of-level condition of your subfloor.

Second, you get a shower floor with a level perimenter, which you currently do not have with your foam tray.

Third, you get a shower floor with the required minimum slope of 1/4" per foot from the farthest corner from the drain, which you do not have with your foam tray.

Not to mention that you get a more solid shower floor for your tiles.

My opinion; worth price charged.
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Unread 02-17-2020, 09:46 PM   #3
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When you say “the whole floor” do you mean the whole floor of the room, or the whole floor of the shower? A picture might be helpful.
I used a Schluter foam pan in my shower and had some floor slope to deal with as well. I used Henry Liquid Backerboard as a leveler as it can go down to a feather finish (I didn’t go that thin, probably a bit under 1/2” at the thinest).
The whole process was pretty straightforward and has been in daily use for 2 years with no problems.
Since you have the walls Kerdied already, you have to band the walls to the tray membrane. Make sure it fits well.
As they say - your mileage may vary.
Here’s a link to something similar to what I’m interpreting what you want to do: https://www.johnbridge.com/vbulletin...9&postcount=78
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Last edited by Gozo; 02-17-2020 at 09:53 PM. Reason: Added link
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Unread 02-18-2020, 07:05 AM   #4
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If it's out of level only in that one corner, Andrew, Jeff's got you covered with the feather finish product. Keep in mind that those self levelers don't really self level all that great so you'll likely have to tool it a bit, which will likely leave ridges and maybe dips, both of which you'll have to address once the leveler is cured.

You'll want to fill any dents in the foam prior to tiling, thinset mortar will be fine as long as the dents aren't deep. And no, the pan won't be more sturdy after it's installed so use something to distribute the load from your knees and or hands. Thick foam 2X2 interlocking play mats, available at HD and probably Lowes, will do the trick (but still be careful) and you can cut them smaller if ya want.

How's Hymie?
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Unread 02-18-2020, 07:24 AM   #5
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While a drypack mortar pan would work, you've already got and have cut, so can't take back, the foam pan, so, I'd use it. Sounds like you've cut it symmetrically, but even if not, at 1/4"/foot or so, unless extreme, the vertical thickness on the edges won't be very much.

Many floor patching/leveling materials can't go to a feather edge, so to minimize the height buildup and materials cost, I think Henry's Liquid Backerboard is probably your best choice.

When setting the tray, unmodified will be fine all over. Wipe the surface down with a wet sponge...the surface should have no liquid water, but just wetted, then spread your thinset and set the pan down.
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Unread 02-18-2020, 11:08 PM   #6
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Thanks for the advice everyone - I'll see what's available locally...I'm hesitant to get rid of the pan, and I have no issues taking my time and waiting for curing times, so I'd prefer to level the floor and then reevaluate.

Unfortunately, getting products is hard in the middle of nowhere; I think I saw Sika 211 and a mapei trowel-on (planipatch?) at the local lumberyard, and stone mason self leveling (which I don't trust) at the hardware store. I'm not a fan of any of these three options.

I might be able order Ardex products through a local store - I'll ask.

Otherwise, I may have to go to the city at some point.

(And I'm impressed that someone knows a Get Smart reference - would you believe that that's the second biggest news I've ever read!)
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Unread 02-18-2020, 11:33 PM   #7
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If you add your location to your User CP (linked in the dark blue bar above) we'd be able to help suggest materials and where to buy.


Maybe a rep will step up
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Unread 02-19-2020, 06:49 PM   #8
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I got the Henry LBB from HomeDepot. Wasn’t a stock store item; had it delivered to the house and the UPS guy put it in my garage. Didn’t have to schlep it to and from or anything. Was actually kind of convenient. I’ve ordered all sorts of crazy heavy stuff since.
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Unread 02-19-2020, 10:08 PM   #9
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One thing to be especially attentive to is the production date AND condition of the packaging for any SLC product (and anything cement based, actually)...you really want the stuff to be fresh, and, have the packaging entirely intact.

IF the stuff is approaching a year, or appears hard, or there's any damage to the packaging, don't buy it!

Cement really wants to cure, and moisture is the thing that starts it off. The moisture actually becomes part of the cured cement, it's not a catalyst, it's part of the chemical reaction.
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Unread 02-22-2020, 01:00 PM   #10
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I purchased the SLC, and then went in and cleaned the heck out the floor, sanded it to get all residue off, and checked it again. Turns out, it was likely residue on the floor that was messing with my measurements (and a crappy level). I don't think I need the SLC. The floor is actually pretty darned close to dead flat.

I checked the pan again after cleaning the heck out of the floor, and while it does slope, it seems rather negligible on that one side...perhaps because the drain is not in the exact centre of the floor and it's not a long distance to the wall on that side.

Is the kerdi pan designed to have enough slope to drain from all areas, even if it just seems to touch the line on the level? It slopes much more significantly when I check the other side of the pan. Can I add perhaps just add a minor preslope out of mortar in that one corner on the plywood, let it dry, and then install the pan? or feather in a bit more into that corner on top of the pan and under the kerdi? (or do I just trust the pan)?
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Unread 02-22-2020, 01:28 PM   #11
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...or...

Do I just throw the expensive/chintzy foam pan away and do a mortar base? I was thinking I'd have to do a preslope and install a liner, etc...but since kerdi will waterproof, I don't think that's the case. (which removes my biggest concerns).
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Unread 02-22-2020, 01:53 PM   #12
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Andrew,

Since your shower foot print is 3' ish square measure the thickness of the foam pan at the perimeter (it should be the same around the entire perimeter) and also at the drain opening. For instance, if the perimeter is 1 thick, and the opening is 1/2" thick the pan has 1/2" of slope to the drain over 1.5 feet, a comfortable bit more than the recommended 1/4" per foot.

Then use a level and tape measure to find out how far the subfloor slopes away from the drain.
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Unread 02-22-2020, 09:18 PM   #13
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Kerdi attaches to a mud bed fine as it does to the foam pan.

If it's a small area, and you don't need more than 1/4", I'd be inclined to maybe use some thinset, let it cure, then put the pan down. Something like LiquidBackerBoard is designed for this, and can be applied to a zero thickness which is probably what you need. Most generic SLC materials over a wooden subfloor want it applied thicker with lath, and a primer, so that adds to your costs. You do need to protect the foam pan more than a mud one, but once the tile is installed and the thinset cures, it's structurally fine.

A mud pan lets you get it the exact size you need regardless of the underlying subfloor's surface. But, if you can't take the pan back, I'd tend to use it. You do want a level surface to then install it, though.
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Unread 02-23-2020, 11:42 AM   #14
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Quote:
Originally Posted by Andrew
Do I just throw the expensive/chintzy foam pan away and do a mortar base?
Exactly what I'd recommend, Andrew.

And when you make your mortar base, be sure to measure from the farthest point of the shower footprint to the drain when calculating your slope. The example Dan gave in post #12 is a bit shy of the actual minimum slope requirement of 1/4" per horizontal foot as the measurement from your drain to the far corner of your shower is a bit over two feet.

And, recycling from my post #2, even if you level your existing subfloor perfectly, I think you'll find you don't meet that requirement with the foam tray you currently have.

My opinion; worth price charged.
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Unread 04-01-2020, 10:28 AM   #15
maxwell_smart007
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Kerdi flood test - thinset wicking

I just did the floodtest for my Kerdi mud, and after 18 hours, there's wicking up mud that's on the surface of the kerdiband.

Adhered with fresh Laticrete 316, good bond, cured for over a week before the flood test.

Is this a concern? It appears to be wetting the thinset that's on the surface of the band, rather than the areas that don't have thinset on the surface. I have some Kerdifix in the corners just to be safe.

I know it's not pretty - but it was methodical with lots of curing time between steps.

...is this normal?
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