I caulked this gap but the caulk cracked, water soaked in and became moldy. I've removed most of the caulk and thinking about shoving some grout into gap at bottom then finishing off with a grout/caulk combo. Should I use some backer bead material first? Or just grout? Or any other suggestions? I really want to be done with this but don't want to replace tile! Thanks![enter image description here][1]

  • One trick of caulking/grouting a bathtub, is to fill it with water first.
    – crip659
    Jan 28 at 23:06
  • What kind of caulk did you use?
    – popham
    Jan 29 at 10:03

3 Answers 3


People who don't know any better will often use ALEX caulk for something like this, reasoning that "caulk is caulk." These caulk jobs fail spectacularly, where it can take less than a month.

For a ceramic tile bath enclosure, 100% silicone caulk is what you want. Technically there's a testing standard called ASTM C920 for qualifying the suitability of sealants for different levels of joint movement. You want a caulk that's "class 25" or better under ASTM C920 . The 25 is basically its stretchiness, where class 50 is twice as stretchy as class 25. The cheapest 100% silicone caulk that you can find at a corporate hardware store will probably be class 25. I've seen "supreme" 100% silicone listed as class 50. Be careful shopping for silicone caulk, though. There are all sorts of marketing tricks trying to get you to buy the wrong stuff. You don't want "siliconized" caulk. You don't want "better than silicone!" caulk. You want 100% silicone. It should stink like vinegar, although there is such a thing as "neutral cure silicone" that doesn't stink. I avoid it because it can behave poorly after it has expired.

Before applying the silicone caulk, you gotta get all of that old stuff outta there. A plastic putty knife is my favorite tool against a tub surface, where metal tools against a tub should be a last resort to avoid scratching the tub's finish. Ceramic tile isn't so delicate, so metal tools on the tile surfaces is fine.

The horizontal joint under the bath controls appears to need backer rod. The backer rod should have a diameter equal to the joint width plus 1/8". For setting the backer rod's depth, the depth of the caulk at its thinnest point should be half the joint's width. I mention it so you don't make the joint way too deep, not because missing the mark by 1/16" will cause a catastrophe. Making it too shallow so that the backer rod pokes through? That's a catastrophe.

The vertical joint in the corner looks constructed for a triangular bead of caulk with little to no filling of the gap beneath it (I like this style for horizontal joints too, but your horizontal joint is too wide). No need for backer rod here, but install it flush to the tile surface if you use it. I think it makes a more resilient joint, but I don't believe there's a chance of it debonding if you don't use the backer rod. Do be sure that the edge of the caulk bead overlaps the tile edge by at least 3/16" so that it adheres well.

I can't tell for the other joints with all of that stuff still in them.

The silicone is unpleasant to work with if you're used to something like ALEX caulk. You'll need a roll of paper towels when you're applying the stuff.

  • Is using silicone caulk at the top of the tub/bottom of the tile the right thing to do? Does anyone leave this uncaulked so that water under the tile can move down and out of the wall? Jan 30 at 13:11
  • @Jim, for out of level tub installs there's often a jagged edge of cut tiles at the bottom that needs hiding. For an old install like this with possibly no waterproofing behind it, that's an interesting question, though. I can't imagine gravity pulling dampness down to run out the bottom significantly. Without that, I'm skeptical that diffusion toward and evaporation from that little bit of wall material behind the tile can remove moisture in the long run versus the additional moisture from an occasional splash. That big of a gap, though? Maybe you're right. Could you tolerate looking at it?
    – popham
    Jan 30 at 18:03
  • appearance of a large gap is awful. I asked to see if anyone would say the gap is necessary, or even a benefit, for drying. Jan 30 at 18:19

Grout is not appropriate to use in gaps at changes of plane or material. Use good quality, flexible caulk. Clean and dry the gaps as much as possible, I don't know of any caulk that will stick to wet substrate or soap residue. If that gap at the tub is deep, use of thin backer rod (plastic foam usually) will greatly reduce chances of the caulk bead failing.


Caulk should be firm and not crack in seams and corners. If you had water infiltration and in the past the grout cracked, it is most likely that the sheathing behind the tile is compromised. If that is the case anything you do is an exercise in futility.

Press firmly on the walls not only near the corners but all over. If any part gives at all, your best bet is tearing out the walls and rebuilding with cement board and proper waterproofing.

I know this is not the answer you want to hear. However there is no value in providing a false cure that just masks the real problem.

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