Caulking and Sealing Tile Grout Tips
When installing new ceramic tile, absolutely make sure that the horizontal seam between the bottom row of tile and the tub and/or shower is not grouted! This seam must be caulked with the best quality 100 percent silicone caulk you can buy. These caulks are available in various colors, should you be using a colored grout. Grout will shrink after installation and a very tiny crack will exist, trust me.
The best way to caulk this crack, believe it or not, is to do it in your bathing or birthday suit. Why, might you ask? The reason is simple. The tub should be filled up to the overflow with water when caulking this joint. The added weight of 30 to 50 gallons of water plus your body weight can cause the floor below the tub to deflect. Leave the water in the tub as long as possible to allow the caulk to cure. Don't leave water in the tub like this, however, if you have small children at home! Left unattended, they may think the tub is a swimming pool. Drownings have been known to occur.
After the grout is dry (three to five days), seal the grout with a silicone grout sealer. These products are available at your tile supplier. Be careful not to get these products on the tub or shower floor, as they are very slippery. The grout needs to be sealed, as it can and will absorb water. Remember, grout is nothing more than fine cement. The sealers also help to keep the tile grout clean. Reseal the grout every six months, after thoroughly cleaning the grout.
Existing Construction
Do all of the above, however, clean, clean, clean before caulking or sealing. These caulking and sealing products will not stick to soap scum very well!

I just found this online. Disregard the part about the tub. It's pretty specific about what king of caulk you should use and that you should never grout that part. Don't know if any of this applies to you.

Francy: I'm not Sis, but this info really comes in handy for me, too. Many thanks for taking the time to post it! Lynn

Truthfully, I have the same issue. I have silicone caulk in the 'change of plane' area between the wall and the shower base and I have to redo it every year or two. Is IS a colossal pain! I had asked about it on the bathroom forum and there were those who insisted that if I had silicone caulk there was no way that mildew (or mold) could form. Well come and visit...there's always some to look at. I had my bathroom redone about 8 years ago and I think I'm up to the 4th time to replace the caulk.

Yes, you can replace just the missing sections of grout. I like to scrape out a little bit more around the area, mix up my grout and then add the rest needed.

Helene

I am not an expert by any means, but if I saw mold behind silicone caulk, I would be concerned that the mold was growing behind the tile. I can't imagine how fully cured silicone could get moisture behind it unless it was coming from inside out rather than outside in. Again, I'm a complete DIY novice, so perhaps there's a perfectly good explanation.

Takes a quick spray of Scrub Free with bleach. We have caulk not grout...and it will get a tinge of black on the surface...nothing behind it. We live in the humid South so that may be part of it but still. This fixes it perfectly..has for 6 years. There is no reason to tear it out or replace. Spray / wait 15 min or so/rinse. That is it. I only do it once every 2 weeks or so and it has kept it under control. There is nothing going on behind the walls or odors or anything in the crawl space that would indicate leaks etc. Hope this relieves your problem. It sure has fixed mine. c

Count me as another with black mold coming from BEHIND the caulk. It's in our shower stall that is tiled but it's where the tile and the threshold you step over meet, not around the base of the shower. I want the entire stall demolished. Then the million dollar question is what to replace it with.

For cleaning glass shower doors - my sister on a whim started using cheap shampoo and an old washcloth. A realtor asked her what she cleaned her glass walls and doors with because they sparkled.

Thank you all so much. My mold appears to be behind the caulk as well. I have taken the caulk out before, sprayed it with bleach mixed with water and let it air out for DAYS, recaulked and had it just come back. I am definitely going to try the scrub free with bleach.
Any more suggestions are welcome.

Uh oh!There may be a good chance you will have to gut that whole area; replace the moldy drywall with a water resistant / mold resistant product. I hope that the studs are dry and not rotted or moldy also. I think you need a professional to come in if that mold has been coming back all the time. Good luck.

EG3d--I hope that's not the case. We have a basement underneath and there is no signs of water leaking in that area.

If anyone else has this problem, have you had yours checked out? If so, I'd like to know what you were told. Thanks again.

It would be difficult to have mold on the drywall itself in my case. My shower pan has a flange which goes up behind the tile about an inch or so. There's a gap above that to the drywall. When I remove the damaged silicone, it appears to be clean in the area.

I've used plenty of soft scrubs, bleach and other bathroom cleansers frequently. They help at first, but the mold/mildew moves quickly. For me, the only solution is to remove it and start over.

Helene

It sounds like the problem is in the construction of the shower behind the tile and grout. Tile and grout are not waterproof. Water does pass through them, and if the shower was not properly waterproofed before tile you could have issues. Also the shower pan may not have been sloped properly causing the pan to hold water.

If there is no noticeable leaking that we can see in our basement under the shower area, how would we know if there is a problem behind the tile and grout. The problem that we're having is the caulk between the tiled wall and tile floor of the shower. Would love to know how this could be checked without tearing out my tile.