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Unread 03-22-2021, 09:47 PM   #1
hski7
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SLC issue, Ditra heat going on top

Hi all, I've been lurking for a while and now a reason to post. I'm re-doing our 5' x 13' second floor bathroom and on checking the wood subfloor, I found that the floor was out of level outside of the (to be curbless) shower area. The middle of the floor was the highest point, and the floor against the worst offending wall was about 3/4" lower (just because that part of the house is old, I think). So, a few YouTube videos later and I'm pouring down Sika 125 SLC. Seemed easy enough I thought...

Well, I put down two bags after priming the 5' x 10' non-shower area and then I re-measured using my laser level and I'm still off, this time by about 3/8" in the worst area.

And this is where I likely made some bad decisions -- I was thinking, John Bridge says I need the floor height within 1/4" over 10 ft. Also, I am prepping a curbless shower, where I've lowered the subfloor into the joists (using the Ebbe kit, which was great btw) and thus I don't want to put down too much SLC and raise the entire floor up too much, or else the Ditra heat won't line up well with the curbless shower. Basically I didn't want to add more SLC to the highest area of the floor so I drew a topographical map of sorts on the floor and planned to selectively pour a third bag of the Sika 125 SLC over the two lower areas that still needed it.

At this point, an error prepping the SLC. The bag ripped, an unknow quantity of water that had been perfectly weighed flew out of my bucket and so the SLC was likely a bit thicker than it should have been. A "new" highest floor area was born! I didn't want that area to win so I grinded it down with my angle grinder and a concrete disc (which was a horrible job obviously). I succeeded in bringing it down to the same height as the original highest floor point and now, I am left with a floor that is only 1/4" off in the 10 ft, BUT there are numerous 1-2 mm low areas given the uneven pouring of bag #3 of SLC.

I have attached some pictures. At the end of the day, Ditra heat is going down and the floor will then be covered in 6" hexagons that are 3/8" thick.

My options I think at this point are:

1. Lay the Ditra and then tile as usual, as though I've got a perfect substrate. If this doesn't turn out great though, my wife will be angry and I'll likely never hear the end of it. She's already less than impressed with the delays caused by the SLC issues.

2. Scrape up all the SLC and start again. I really dread this option. If I do this, I will have to re-prime and use all three (or more? I don't want to use too much as I need my floor + Ditra to line up with the curbless shower) bags at once. Maybe I should draw my map again on the wood subfloor and buy those little sticky floor levelling pegs/rulers and pour just on both sides of the highest floor area, to help prevent unnecessary build up??


I'll abide by the expert advice provided here. Thanks.
Mark
Toronto, Canada
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Unread 03-23-2021, 09:57 AM   #2
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Hi Mark,

I'll tell you what I would have done.

I would have removed the subflooring and sistered the joists in a manner that would have rendered the subfloor level. SLC can work, but its name is misleading. It doesn't automatically level the floor.

I would remove the SLC you've installed, but hang on. There may be a better answer from someone else.
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Unread 03-23-2021, 11:20 AM   #3
hski7
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Thanks John. I hadn't thought of that idea though my joists are i-joists and I'm not sure I can handle sistering joists myself. Thanks for the input and I'll keep my fingers crossed here for some more ideas! The tiles I ripped up were rendered flat with what looked like a mud pack / wire mesh combo, maybe I should have considered doing that instead of mixing the SLC. Cheers.
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Unread 03-23-2021, 02:22 PM   #4
cx
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Welcome, Mark.

Unusual to have a seriously un-level floor built with engineered joists, but I guess you're demonstrating that it can happen. Were it mine I'd be very curious as to how it got that way.

Keep in mind that your tiles don't care a whit about your floor being level, they care only about flat. The larger the tiles, the more they care. If surfaces had to be level to tile them we'd not be able to tile the walls at all, eh?

I'm very curious, too, about lowering the subfloor into the joist structure with those I-style engineered joists. I'm not familiar with the Ebby product you referenced. Can you provide a link to that for us?

As to repairing your current situation, I can't think of anything other than removing what you've got unless you're willing to continue building up the floor 'till it's flat.

My opinion; worth price charged.
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Unread 03-23-2021, 06:05 PM   #5
jadnashua
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I’ve only used SLC once (and it worked out well for me), but I can tell you that, at least with the stuff I used, I had some drips out of the room where I mixed it, and once those cured, I could NOT chip them off of the sub flooring (and those drips did not have the primer that makes the bond even stronger!). I ended up grinding them off, which worked for that small area.

IOW, it might be a major bear to remove what you have.

As mentioned, SLC doesn’t really self-level. It needs some help. Think about what happens when you pour pancake batter into a frying pan...it doesn’t spread to fill the whole thing, it ends up with a meniscus (a beaded edge like a water drop on a freshly waxed car). You need to spread it around and then let gravity do its thing. It’s MUCH easier to get things flat and level when you are making the pour thicker. Note, read the instructions carefully. Some of them want at least 1/2” above the highest point...some can be used to a feather edge over a wooden subfloor, but most cannot. FWIW, my experience was with Arden products. Their liquid backer board product is one of the few that can be used to feather edges on a wooden subfloor.
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Unread 03-23-2021, 07:20 PM   #6
hski7
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Thanks CX. Sounds like the consistent advice is to remove the SLC and then investigate the joists and reason for the uneven floor. I assume I would do that by removing the subfloor and taking a look at the joists?

I gather that implicit in the recommendation is that the Ditra overtop of the floor as it is currently, won't work out well. I figured that was the case but was praying the All Set + Ditra + mortar + tile could cover some of these bumps and the 1/4" out of level.

Jadnashua, grinding out 150 lbs of SLC would be horrible. I really hope I can scrape them off!!


Here is a link to the Ebbe subfloor lowering kit. $80USD. It worked well. Some pics too.

ebbe-america.com/product/e116-3-4-floor-recess-kit/
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Last edited by cx; 03-23-2021 at 07:51 PM. Reason: Light up link
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Unread 03-23-2021, 10:59 PM   #7
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It looks like beads of adhesive on the top of the joists, but seems you were able to get the ply off from an area already, so it may not be all that hard to pry up the rest. Prying up the old sub flooring, fixing what’s needed to put new sub flooring down so that it is flat AND level (might mean planing high points, or sistering the existing ones with cleats, and then add new subflooring, and you’d then, not be adding any height, have a level AND flat surface.

While the price of lumber has gone way up in the last six months or so, it’s still cheaper than SLC!
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Unread 03-24-2021, 06:00 AM   #8
hski7
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Thanks Jadnashua. Yes, I was able to get the subfloor off the joists to recess the subfloor for the Kerdi shower pan, to make the shower curbless. But I'd have to scrape off the SCC first in the 5' x 10' area, then proceed as you've mentioned. Which I think I will do... I' m not happy with the floor at the moment and don't want it any higher.

I agree with CX, in thinking about it; not totally sure why my i-joists aren't perfectly flat the whole way along. Though -- this area of the house is a 14-year-old addition to an original structure, so I assume that has something to do with it. And where the bathroom ends, there's a break in the joists for an opening between the first and second floor -- see pic. Before the reno, there was a mud pack and a wire mesh of varying depths on the old tiled floor, which had been there and stable with 12x24 tiles and no issues for 14 years.

So if I scrape up the SLC and get back to the original subfloor, can I just mud pack it with wire mesh to get it flat underneath my Ditra heat? Instead of going the SLC route. I don't know if that option will work given the high point of the floor but I'd be interested in proceeding that way to make things progress faster. Will Ditra overtop a mud bed adhere to a mudpack?? I think so.

Thanks all.
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Unread 03-24-2021, 07:15 PM   #9
jadnashua
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Industry standards call for a mud bed over a wooden subfloor to be WAY thicker than you want! Most call for at least 2”, but many get away with less.

A carbide blade designed for removal of stuff could be set to the right depth to go through the SLC and almost through the subflooring, and then you pry it up in chunks. Those blades will cut through screws and nails, and SLC easily...wear a good dust mask, eye protection, and gloves.
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Unread 03-24-2021, 07:26 PM   #10
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I'm gonna hafta disagree with Jim on that. Per industry standards, the minimum thickness of a mortar bed over a cleavage membrane and metal lath over a wood framed subfloor is 3/4-inch. A reinforced mortar bed requires a minimum of 1 1/4-inch thickness.

The maximum for the mortar bed over lath is 1 1/2 inches; the maximum for the reinforced bed is 2 inches unless thicker sections are specified by an architect.

My opinion; worth price charged.
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Unread 03-25-2021, 08:40 AM   #11
hski7
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Thanks Jim and CX. It looks like I'll be removing the SLC and pulling up the subfloor to figure out what the issue is. Hopefully it'll chip off!

I called Schluter too and they do not want the Ditra heat put over a drypack -- which sounds like it would have to be too thick anyways for my needs. Will update the thread at some point.

Cheers.
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Unread 03-25-2021, 09:00 AM   #12
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Quote:
Originally Posted by Mark
I called Schluter too and they do not want the Ditra heat put over a drypack...
I find that interesting. Did they offer any rationale for that?
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Unread 03-25-2021, 03:02 PM   #13
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Subscribing because I had been planning on the same, Ditra heat over deck mud...
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Unread 03-25-2021, 07:59 PM   #14
hski7
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Schluter said they were uncertain how well it would work with the drypack and that it "was not recommended. We prefer Ditra Heat over SLC." They said it had been done but that they couldn't vouch for the longevity and how the constant heating/cooling would affect the drypack and mesh.

It was also mentioned here that the drypack should be at least 3/4" thick which means it wouldn't work for me anyways.
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