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Unread 04-29-2022, 06:21 PM   #1
johnnycom
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Mold between tile and slab?

Hi Everybody!

I’m a long-time reader who has benefitted from the information on this fantastic resource on a number projects. Thanks to all who contribute! I’m in the middle of a full bathroom remodel now with a question for which I’m not finding any clear answers on my search.

The bathroom is built on a slab on grade that may at one time have been a patio, later converted into living space. I suspect there may not be any vapor barrier underneath the slab.

The situation:

The existing quarry tiles are almost 30 years old and were set directly to the slab with thinset mortar. The tiles were surprising easy to remove from the slab — popping off with almost no effort, leaving all the mortar attached to the tile backs. Both the surface of the slab and mortar on the tile backs have a thin black film. This film is extremely thin and is not sticky, actually there is nothing to scrape away, so probably isn’t cutback adhesive. After removing the tiles, a moisture meter revealed high moisture in the slab.

My questions:

1. Is the black film black mold? We’ve noticed a pervasive moldy smell in the house and I’m wondering if the tile in the bathroom and kitchen on this slab could be the cause. The tile backs appear to smell moldy to me but my smell isn’t the best after a couple days of ripping up the bathroom.
2. What is the most likely cause of the separation of the mortar from the concrete? I wouldn't have thought moisture would affect cementitious materials.
3. If the black film is indeed mold, where do I go from here? I’m considering scrubbing the floor with bleach, perhaps scoring it with a diamond wheel, and putting down ditra before tiling. Would this be enough to provide a vapor barrier for our living space? Anything I need to do to make sure the Ditra remains adhered? Would a modified mortar be a good choice in this situation?
4. How would I adhere a moisture barrier to the part of the slab under the tub?

One more thing - although the slab is low to the ground, there is decent drainage outside away from the slab, but I’m in a rainy-winter part of Northern California, so not much possibility to reduce moisture externally. Thanks for any input. I’m sure to follow up with additional questions!

I'm attaching a photo showing, left to right, backsides of removed tiles, the damp black slab immediately after removing tiles, and the slab dried out from which tiles were removed yesterday.

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Unread 04-29-2022, 11:24 PM   #2
jadnashua
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Mold requires organic material to grow (along with moisture but the spores are everywhere). I suppose there could be some organics in the ground water, but normally not an issue. The slab may not have been very clean when the tile was installed.

Cement based thinset, once cured, shouldn't be affected by the moisture, but if it was there during the initial install, yes, it could. If the tile were installed with a mastic, that's organic. If the slab's hydraulic pressure is low enough when you try to replace the tile, you might look at Ditra. That waterproof layer has air channels in it and once the vapor pressure reaches a certain point, it becomes stabilized, and doesn't let any more in. If the slab had any treatment on it, that can become a bond breaker. You also need to 'burn' or 'key' the thinset into the pores of the slab before you comb on the setting bed. Failing to do that can lead to a poor bond. If the thinset was a modified thinset (older modifiers tended to contain latex, and that's organic), that may be the food for any mold. There are more modern modifiers that are often based on acrylics or other materials that aren't as susceptible to mold growth.

When working around what might be mold, you really should be wearing a good HEPA mask.
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Unread 04-30-2022, 02:48 AM   #3
Davy
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That looks like cutback to me and only certain thinsets will stick to it. Cutback acts as a bond breaker itself. I have removed it with a grinder/cupwheel setup but asbestos could be a problem. Keeping it wet and scraping up ( razor blade scraper) what you can might be the best and safest way to remove some of it. Scraping won't remove it all like grinding will. It would have to be tested for asbestos before grinding to be on the safe side. If you're going to tile over it, use a thinset that's recommended for that.
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Unread 04-30-2022, 09:32 PM   #4
johnnycom
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Thanks for the info. I pulled the tub today and found a couple of remnants of what’s probably asbestos vinyl tile clearly stuck to the slab with cutback - so good call Davy. Will send in to lab for test. I do believe that mold is growing under the tile in the space where the thinset separated from the slab because the mold smell is so strong. With two two messy boys growing up using this bathroom, one can only guess what mold food may have soaked down through the grout…

As the rest of the floor has the cutback almost completely scraped away so nothing is left but a “stain,” seems like using a cutback-compatible thinset may be the way to go. Is there any advantage to doing the grinding instead (assuming no asbestos in the cutback)?

And for sure I always wear a high quality P95 respirator on these projects.
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Unread 04-30-2022, 11:04 PM   #5
Tool Guy - Kg
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Grinding will open the concrete pores, producing a better bond than just using a thinset mortar rated to go over a cutback stained slab.

Glad you’re using a good N95 masks for the job. But if you decide to grind and it does contain asbestos, you’d need to upgrade to a P100 or N100 respirator, along with all the other PPE.

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Unread 05-01-2022, 08:23 AM   #6
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What Bubba said. Thinset sticks much better to raw concrete than to cutback.
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Unread 06-16-2022, 03:56 PM   #7
johnnycom
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Cut tile edge vs factory edge along schluter Jolly

Well, the asbestos possibility turned out to be a non-issue so I happily ground away the cutback and am ready to start tiling my project.

I'll be tiling the tub surround to the ceiling, then additionally tiling the entire wall beyond the tub/shower controls to a height of 45 inches, which will back the Toto wall-hung toilet and form a backsplash to the vanity. I'm planning to trim the top of the tile with Schluter Jolly trim. I'm looking for guidance as to whether it matters much if the tile edge adjacent to the Jolly is a factory edge or a cut edge so I can plan the layout of the tiling pattern that will continue throughout the tub surround. My tile is 3/8" and I'm planning to use LFT mortar with a 1/2 inch by 1/2 inch trowel to adhere it to sheetrock. I purchased the 1/2 inch Jolly.

Thanks for any guidance.
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Unread 06-17-2022, 05:37 AM   #8
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Typically cut edge smoothed with a stone.
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Unread 06-17-2022, 08:02 AM   #9
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With a wainscot or backsplash like this, you want a full tile at the top of the wainscot. That full tile will carry though the tub surround.

From there, you will have to play around with the height to see how it affects the cut around the bathtub, the cut at the ceiling, and the cut at the vanity top. May as well check the cut at the floor too but I would put that area as lowest on the totem pole.

If you need to adjust it higher or lower then you move the entire layout higher or lower.
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Unread 06-17-2022, 09:03 AM   #10
johnnycom
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Thanks James, that’s kind of what I figured. That layout actually works for me, surprisingly given there’s a window and niche involved as well. Thanks also PC for the reminder of the rub stone for elsewhere I might need to terminate at a cut edge.
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Unread 06-17-2022, 11:55 PM   #11
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Hi John,

Your 1/2” Jolly is probably too big. The leg of the profile that runs under the tile typically is touching or nearly touching the back of the tile. That means you probably need something shorter than what you bought. The general guide is to purchase the profile at the same height as the tile. Some folks like it slightly taller if a lot of mortar is going under the tile. With all this in mind, I also will consider if a factory edge (slightly thinner) or a cut edge (slightly thicker) is abutting the profile.

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Unread 06-18-2022, 05:28 AM   #12
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James/Tiger Mountain is right on. I completely missed the top of wainscoting application and assumed standard vertical.
Regardless of orientation, it important to consider the size of the last row or column when planning your layout.
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Unread 06-18-2022, 09:35 PM   #13
johnnycom
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Thanks guys for the heads up. I was actually test laying things out last night before reading your responses about the Jolly width and found that out for myself! But could have been a lifesaver. It’s great how everybody here looks after everybody!
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Unread 06-18-2022, 10:46 PM   #14
jadnashua
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In a class at Schluter, they recommended not pressing the profile into thinset unless it was used on a floor where you might be walking...spread thinset over the flange and press the tile down until the tile is properly aligned with the profile which should never be higher, but can be up to 1/16" or so below the edge of the tile.
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