For the past couple of months I noticed a gap developing between the sealant (caulk) round the edge of the bath-shower (behind the taps) and the tiles. I'd been meaning to do something about it for a while. Finally got round to taking a look and spent a pleasant couple of hours cutting/digging out the old sealant, behind the taps was a nightmare.

Having removed all the current sealant, When facing the taps the bath seems to slope downwards slightly from left to right. The gap between the tiles and the bath rim on the left is 3-4mm increasing to 8-9mm on the right hand side.
There's a small bit of tile grout filling in the gap on the left but nothing on the right, though I think the bead of grout may have been rotten with mould and it came loose when I pulled the previous sealant out.
I don't think the mould has made it to the plasterboard wall behind the tiles/bath. It seems there's another layer of transparent mastic/sealant covering the plasterboard wall, on which this mould has grown.

So as I see it, I need to:
1) Kill the mould. Do I just need to bleach/disinfect the area?
Should I let the gap dry out for a few days before or after killing the mould?
2) Fill in the wider gap on the right of the bath... do I need to put some new grout in there underneath the row of tiles, then new mastic/sealant on top?
Or do I just fill the whole lot up with new mastic/sealant?

I'm just aware that I could end up sealing mould in there with sealant or creating an airpocket in which more mould could eventually develop. I realise though that if I do a good job with the sealant, then hopefully there should be no gaps for water to be able to get down.

The new mastic/sealant I've got is bathroom anti-mould stuff designed for showers/baths. Not the internal frame stuff I've seen for door frams etc.

It's a new build property (3 years old) and we previously had significant mould problems in this area due to damp caused by the bath waste pipe not being correctly tightened to the plug hole, so water was running under the floorboards/carpet and being absorbed in places by plasterboard reaching the concrete floor underneath. It was fixed by the developer under their guarantee but it seems they mainly just sanded the mould in the walls down and painted over it.
No sign of that problem returning but I wouldn't be surprised if they didn't properly fix any mouldy plasterboard behind the tiles round the bath.

  • A picture would be nice.
    – user48010
    Mar 9, 2017 at 1:36

2 Answers 2


If there is moisture still there, then mold and mildew will return. If the problem was not fixed, then it will return. Bathrooms are particularly bad in this respect, since moisture is a natural consequence of using water. I would expect to need to redo things like caulking every few years to fix this. It will not last forever.

  1. Scrape/pull off the old silicon.

  2. Apply a solution of bleach to kill the mold. Be careful when you apply it, as bleach can damage the clothes you are wearing. Keep the area ventilated well.

  3. Let it dry. If you are in a hurry, a fan blowing on the area will help. A hair dryer can also speed that process considerably.

  4. Apply new caulk in a smooth bead. I use my finger to spread it along the joint. Definitely use a high quality silicon caulk with a mold/mildew inhibitor. These are caulks that are designated for use in bathrooms.

  5. Allow it to fully dry before you get it wet again. Give it a day or so. It will skin over in an hour or so, but make sure it has properly cured before use of the shower.

Expect to redo this process in some number of years. The exact time it will last depends on how well the area is ventilated.

Of course, there may be mold in the wall itself. This is difficult to deal with unless you tear the wall down to replace it completely, and even then water tends to attract the stuff. Mold spores are everywhere.

  • Job done. Ended up getting the sealant pretty lumpy in places, kept getting clogged up on the end of my finger. Plus getting it round the tap fittings sticking out the top of the bath was a bit of a faff. But there doesn't appear to be any gaps... which I guess is the main aim. We'll see how it dries and if I need to redo it in the next few days. Cheers!
    – batfastad
    May 29, 2011 at 19:54
  • Yes. This is a task that takes some practice. If you cut the end of the tube just right, and extrude it carefully at a constant rate, you can avoid any need to do the finger bit. I've gotten decent at that skill over the years. But in the end, I usually end up using my finger in at least a few spots. Keep a paper towel at hand for your fingers.
    – user558
    May 30, 2011 at 11:47
  • They can stink, but always use silicone-based caulking, not the latex stuff. I can't figure for the life of me the point of latex caulking - it seems to be inferior in every way to silicone. Jun 6, 2011 at 11:14
  • +1 Have also heard recommendations to fill the bath before beginning to seal - since the weight of the water "moves" the bath and pulls it down. Better to seal over a full bath and leave bath full while sealant dries? Jun 18, 2011 at 11:55
  • If the tub will move that much between full and empty, then when you later empty the tub out, it will also move! By this logic, one might choose to put the sealant on with the tub half full, so incurring the least movement overall in either direction.
    – user558
    Jun 28, 2011 at 9:37

Sorry for your Woo's! It is a shame to have this kind of problem in a new home. It sure sounds like you are doing all the right things so far. The important thing is to remove all the bad or compromised caulking and mold. Assuming the mold has not penetrated the wall board behind the tiles you should be ok treating the area with bleach and letting it dry throughly before proceeding. Using a fan can help speed the process. Rather than trying to replace small amounts of grout,I'd be fine with using a good anti-mold silicone caulk. You are right in all your assumptions, fill the voids completely, try not to leave air pockets. The only thing you may want to change is to use plumber's putty instead of caulking behind the bezel trims of your faucet handles. Put a "rope" of plumber's putty in the trim 270 degrees, leaving the very bottom of the trim free of putty. press the trims back on and remove the excess putty that squirts out from compression. The little void at the bottom is intentional so any water that may get behind the trim will drain out the bottom and not through the hole in the wall down into the wall. Keep and eye on the flooring under the tub from underneath if possible (from basement?) to assure mold has not spread or is continuing to grow from the previous leak. Fixing that mold, if it becomes an issue, is a completely different conversation. Hope I answered some of your questions. Follow up with more if we don't address all your concerns.

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