Mold at bottom corner of my shower [Archive] - Ceramic Tile Advice Forums - John Bridge Ceramic Tile


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07-20-2019, 09:32 AM
Dear Folks,
About a year ago I finished a small bath Reno/expansion. It was a tiny half-bath, now it is a tiny full bath (with toilet, sink, and shower rather than bathtub). I did the poured shower pan myself, but no vinyl liner underneath because I was planning to use Redguard for the waterproof layer. I put up the CBU on the walls, then two coats of Redguard on showerpan and 3/4 way up the walls. Then tiled showerpan with 2” (inch) square tiles, and walls with 1’ (ft) Sq tiles. At corners between walls and between walls and floor I used sanded caulk color-matched to the grout. Everything came out pretty well, considering it was my first time tiling., or so I thought. Now I have mold growing at the bottom back seam between the wall and the floor. Obviously, I have some water retention (“and painful bloating”) somewhere. What is my best course of action here? I would prefer not to redo the whole shower - that would be a painful learning experience. From what I had read I thought I did it right. I did not seal the grout, as my understanding was that the grout is porous, the porcelain tiles are porous, a little water seeps through and is channeled by the Redguard membrane down to the drain. Obviously, that is not happening as it should somewhere. Suggestions please?
Puzzled in NC,

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07-20-2019, 10:15 AM
Welcome to the forum, Alvin. :)

When you applied the Redgard, how was it applied, how many coats, and to what thickness? Did you use a wet film thickness gauge?

Did you use any reinforcement fabric or mesh on any of the seams?

07-20-2019, 10:16 AM
Also, regarding the caulk in the corners, specifically what did you use?

07-20-2019, 10:18 AM
Welcome, Alvin. :)

1. What drain did you use in the shower and how did you attach the RedGard to that drain?

2. How did you attach the wall tiles and with what adhesive? Did you use a spot-bonding method?

3. What caulk, very specifically (brand name, make and model) did you use at the changes of plane?

Photos are always helpful. Use the paper-clip icon above the Reply dialog box to attach photos from storage on your computer.

[Edit] Young Kevin is quick this morning, eh? :)

Tool Guy - Kg
07-20-2019, 12:39 PM
Welcome, Alvin. :wave:

Lotta good questions from these guys. Assuming the shower was built without problems that the answers to the previous questions would could just clean the caulk from time to time.

I personally don't like acrylic caulk (which is what sanded caulk is 99% of the time). It's nowhere near as flexible as silicone and when the walls move a tiny bit from themal and moisture changes, it separates from one side of the wall and allows moisture to harbor for a relatively long period of time. But you don't necessarily need water retention from a crack or from behind a tile assembly to get acrylic caulk to form mold. There's plenty of moisture and food (bar soap and dead skin flakes) for it to grow. For me, I'd swap out the acrylic caulk for 100% silicone (most silicones have a certain amount of mildewcide in them) and try to slightly improve the ventilation within the shower/bathroom (to reduce moisture, hence less mildew/mold).


07-20-2019, 02:40 PM
And if the shower has been properly constructed, the best defense against mold is to have a properly sized and used exhaust fan in the bathroom.

My opinion; worth price charged.

07-20-2019, 04:00 PM
If the seam ends up being very deep, rather than filling it all up with caulk, it usually ends up working better to use a foam backer rod stuffed in there with caulk on top. The backer rod must be sized properly. That way, the caulk ends up shaped more like an hour-glass versus a rectangular block which allows it to stretch in the narrow, middle area, and is therefore less likely to tear away from the surfaces. It also ends up using less caulk. The backer rod tends to be cheap in comparison to caulk.

07-21-2019, 06:40 AM
Like the others said, a perfectly built shower can harbor mold if there's not enough ventilation. When not in use, keep the door open at least part way.

07-31-2019, 11:32 AM
Dear Folks,
Thanks for all the replies. I am sorry to be so long in getting back, I had some other things going on at the end of the summer term (I am a college teacher) and it took me some time to find the necessary info.

I will try to answer these questions in the order they came in, and I have taken some pictures as well.
1) Kman - I painted the Redguard on with a brush, I think, though I may have used a roller as well. I don't remember for sure, I think just a brush. If nothing else, it is expensive, and I felt I would lose a lot on a roller, but again, I am not sure. I did two coats, one all over the shower walls and floor, and one up about 3/4 of the way on the walls and floor (running out of Redguard). I did not use a wet film gauge, I did not know there was such a thing. I did have the reinforcement mesh on the seams of the cbu's on the wall.
2) Kman - For the caulk in the corners, I used sanded caulk to match the sanded grout used on the walls. I will include a picture of the label, it is made by TEC Invision. It does not say anything about acrylic on the label, but it does say mold and mildew resistant.
3) cx - The shower drain was just a pvc shower drain purchased at Lowes, with a square chromed insert that screws in. I grabbed a shot of it from their website. As I remember, you screw it into the wooden base of the shower, build the slopeing cement floor around it, and then tile the floor, and then put in the top insert. The Redguard was painted across the cement and into the drain, though when the insert was finally screwed into the base, it peeled much of that Redguard back out.
I attached the tiles with mortar, MAPEI porcelain tile mortar, polymer enriched thin-set. I put mortar on the cbu on the wall and also on the tile, using the trowel with the 3/8 inch teeth.
The caulk was mentioned above, made by TEC Invision. It was sold there with the grout with the same color designations.
4) Tool Guy - Kg - Just cleaning the caulk is not an option. That is exactly the problem. My wife washes the shower about every time she showers (she keeps a bottle of spray cleaner in there, and just wipes/scrubs it down before she gets out of the shower), and the mold is now in caulk seams in the bottom back corners and can't be scrubbed out. I like your idea of replacing the caulk with a silicone caulk, that would be a great fix if that works.
5) cx - There is an exhaust fan in the bathroom, though I confess I often do not use it. I don't like the noise of it. Someone suggested a switch with a cut-off timer, and that might be worth the investment, to let it run for 30min after a shower.
6) jadnashua - thanks for your comment about backer rod. I often see that mentioned but can never find that at the store. Regardless, I don't think the caulk is really thick in the seams in question, but I will look into it when I am pulling out the old stuff.
7) Davy - yes, the door is usually left open when not in use, it is a bathroom off the bedroom.

07-31-2019, 01:15 PM
Hi Alvin. Fellow college teacher and amateur tiler here! :wave:

From your description on how you tied in the Redgard with the drain, I have concerns about the longevity of your assembly. You mention that you peeled Redgard off when you screwed in the drain. A water-tight seal between the Redgard and the drain flange is essential. Usually, there are much more reliable methods to provide a seal between drain and floor waterproofing. With a PVC liner, you get the clamping action of the drain plus sealant. With a proprietary bonding membrane in conjunction with a drain to match it, you get a factory-designed system that works together.

Yours wouldn't be my preferred method of waterproofing the shower floor. It could be fine but it could also cause leaks in the future. If you have access from below, you might want to check for leaks over time.

If you have water accumulation behind the tile and that is indeed causing your mold issues, then it's possible that you had insufficient thinset coverage when installing your tiles, in particular the bottom row. Do you remember how much coverage you got? Did you check during installation? If there are cavities behind your tile, that would support your suspicion of water accumulation.

Panasonic makes a line of bathroom fans (Whispergreen) that are awesome. DC motors that are very quiet and efficient. Well worth the money. I love mine.

It seems that your shower floor has some dirt, too. Is that mold as well, growing from the grout joints? Also, what did you mean by "bloating"? Is something swelling or shifting? if so, that would be a bad sign. The slope of your floor also looks a bit uneven. Do you get puddles of standing water that won't drain after taking a shower?

07-31-2019, 02:25 PM
The op said The shower drain was just a pvc shower drain purchased at Lowes, with a square chromed insert that screws in. I grabbed a shot of it from their website. As I remember, you screw it into the wooden base of the shower, build the slopeing cement floor around it, and then tile the floor, and then put in the top insert. The Redguard was painted across the cement and into the drain, though when the insert was finally screwed into the base, it peeled much of that Redguard back out.

the instructions I have seen show using fiberglass cloth to embed the redgard into over the drain. You then clamp onto that. It gives it more bite.

try to ignore the horrendous looking speed bump!

07-31-2019, 04:12 PM
Mike, please add some attribution to that quote so it's not confused with something CX posted, eh?

07-31-2019, 09:51 PM
To makethatkerdistick,
Thanks for your reply. What do you teach? I teach chemistry. I didn’t mention anything about bloating or swelling. And I don’t think I have voids behind the tiles. I was pretty careful when applying the thin set to the cbu walls and to the tile itself to be sure I had complete coverage. However, that is a good thought; voids behind the tiles, boy, would that ever be a mold issue!!
I do have access from below, from a crawl space. So far no leaks, but something to watch for. Yes, I was not happy about the attachment of the Redguard to the drain, especially when the insert caused it to peel up. I considered breaking up the floor (not yet tiled) and starting over but I could not bring myself to do it. The method shown in the video right after your post is obviously much better.
I don’t think there are mold on the floor issues. It is uneven, I freehanded the slope, but I don’t think there are puddles there once the water is turned off. I like those plastic guides in the video but was trying to save money. Any dark spots on the floor are just color variations in the tile.

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08-01-2019, 07:33 AM
If you didn't build your shower pan with liquid membrane something like this (, it's probably leaking. Have you tried doing a flood test?

08-01-2019, 10:17 AM
That is the "divot" method. just another way to do it. Just because he did not do that does not mean it is leaking....

He can always pop off the tiles and clean the thinset off around the drain and redgard with some fiberglass around the perimeter. yeah, hard to perfectly tie in the old redgard to the new, but overlapping the areas where you scape with new redgard should do it, no?

08-01-2019, 10:35 AM
The divot method is really the only way to effectively tie a liquid membrane into a clamping drain. Sure, it may not be leaking, but the mortar bed is almost certainly saturated, hence the mold growth. Trying to apply a band aid in this situation will likely be ineffective, IMHO. We do need more info about the construction though.

08-01-2019, 11:01 AM
well, ensure that the slope does indeed pitch away from the wall. Pour water right on the floor in the area of the mold. see how well it flows away.

mold will grow on silicone caulk. especially if glycerin soaps are used or other natural conditioners.....

08-01-2019, 12:02 PM
mold will grow on silicone caulk. especially if glycerin soaps are used or other natural conditioners.....

Yup, I can attest to that!

08-01-2019, 07:05 PM
If I understood correctly, his mud bed is under the Redgard. He stuck the tiles to the Redgard and apparently filled in around the drain with thinset. If that's so, the weep holes are full of thinset.

Is that the steps you took, Alvin?

08-02-2019, 07:40 AM
oh, I thought he had a preslope with redgard on it....

08-12-2019, 09:03 AM
Yes, I did not redgard under the drainpan slope but on top of it. The weep holes WERE probably filled with redgard or with mortar, but (in theory) there should be no water beyond the redgard, right? I have never done a flood test.

Also, I could not quite tell from the pictures what was involved with the divot method. Could someone tell me a little more about it?

I think my immediate solution is going to be to replace the caulk with a pure silicon caulk and see if that helps. Obviously I would prefer NOT to redo tile, but if necessary I will do that. I did like the idea someone mentioned of peeling up the tile in the area around the drain, and then re-applying the redgard with a mesh liner to make sure it was all connected well. Someone mentioned the mortar bed being soaked with water. I can see that would generate lots of mold, but no water should be getting to the mortar bed assuming the redgard layer is intact.

Again, thanks for all the input and ideas. You guys are great!

08-12-2019, 09:13 AM
I looked again at a previous comment about the mortar bed being saturated, and that causing mold. In my previous reply I was thinking you mentioned the actual drain pan being saturated. I didn't see how that could happen, though if the mortar under the floor tiles was saturated, and staying that way, I could see that would lead to mold as well. However, the mortar is on top of the redgard membrane, so shouldn't the water flow out of the mortar, hit the redgard, and go on down to the drain? That was my theory when I put it together. The only way water in the mortar (between the tiles and the redgard) would stay there is if it ran down to the drain but somehow could not get into the drain. The upper piece of the drain, the metal grate piece, screws into the lower piece (plastic). The plastic is tied to the redgard. I didn't think the connection between the metal piece and the plastic is watertight - it just threads in, and how far it threads in is more dependent on your height of the tile layer, not on getting a good snug fit or seal.
Thanks again to everyone for your input.

08-13-2019, 09:07 AM
when you tightened on the drain, you mentioned it tore into the redgard. If the joint from the shower pan mortar to the drain has ANY cracks in the redgard, water will bypass the drain and soak the mortar pan.

08-13-2019, 05:23 PM
Alvin, it doesn't sound like you have any type of membrane under the mudbed so if water gets under the Redgard, the mud won't be saturated. Instead, it would leak.

In post 14, you can see how Greg painted the membrane over the bottom section of the drain flange and then he bolted the ring onto the bottom flange sandwiching the membrane between the two parts. Is this how you did it or did you just paint the membrane against the top screw on grate?

Here's another pic of the Divot method with the ring bolted to the bottom flange. The top of the ring shouldn't be covered with membrane, doesn't need to be.

You said that you tore the membrane when screwing the drain on. That's the puzzling part to us.:scratch:

08-13-2019, 05:28 PM
Here's that pic. Same thing as Greg's, just with the ring bolted on.