I am a a first time DIYer working on an old bathroom. The tub seems like porcelain-enameled steel. I have removed moldy caulk and cracked grout from around the tub. The most puzzling issue is documented in my picture. There is a 1/8 to 1/4 inch vertical gap between the side of the tub and the wall tile. The tile was cut so that the gap is rounded to the top of the tub. This gap is deep. The tile is strongly attached but there is no surface for filler to attach to in this space. I’ve read about backer rods. Is this the best approach here? I worry that if there’s a small crack or gap, over time the rod will act like a sponge, and I’ll end up with a moldy nightmare.

Also, at the place where the wall tile, the floor, and the tub meet there is some unattractive (but hard as nails) filler that seems different from either caulk or grout. It was nasty so I scraped the dark buildup off. It looks better now than before but it’s still far from attractive. I began to remove it but stopped. First, it will be hell to remove and I don’t want to break tile (there’s already a small crack on right side of tile). Second, perhaps this was a problem spot and the previous owner plugged it purposely this way.

Specifically, my questions are:

  1. Is backer rod the best solution to this vertical space problem?
  2. What would be best filler material for vertical gap? (Grout, caulk, or other)
  3. What is best material for the gap at top of tub (this will get soaked during showers)
  4. If the two materials are different, how do I make transition from one to other look best?
  5. What should I do about this bottom joint that seems important but so ugly?
  6. Out of curiosity, any idea what this harder-than-nails filler is at bottom joint? gap with no backing and mysterious substance at bottom joint

3 Answers 3


No need to worry. The tub has a flange on the top edge that the tile lips over. Water has a difficult time getting behind the tile there and what does can't go anywhere to do significant harm. The sides are a different matter. No flange there, but they aren't as exposed to water. Still all gaps need to be filled.

To answer your questions in order:

1-No backer rod is needed.

2,3 and 4 - The most common practice is to grout both the vertical and horizontal gaps between the tub and tile. An unsanded grout pushed into the gaps with a sponge grout float would fill the gaps. Even where there is a gap wider than 1/8 inch I would use unsanded grout. After it was cured there may be a few cracks. Then I would go over those areas with more unsanded. (Sanded is too coarse to fill the smaller spaces well)

Another option is to look for a Grout/caulk. Most home stores have these in a tube just like caulk. They are applied the same way as caulk and have a bit more flexibility than grout. They are the best bet if you can find the color that matches your existing grout. Minimal color choice is the biggest drawback. Use grout or grout/caulk for both V and H and there is no need to worry about a match.

5- The hard stuff at the bottom?....well you can try to get a bit of paint to match the grout, or work hard and fill with grout or grout/caulk.

6-What is it? A guess would be an enamel filler?

  • Thank you for a very helpful answer.
    – SKD
    May 29, 2023 at 6:25
  • YW, upvoting an answer is the method of thanks here.
    – RMDman
    May 29, 2023 at 12:24

Backer rod is non-absorbent; in fact, less absorbent than grout, unless something like epoxy-based grout was used. Backer rod is especially important to use where there is significant joint movement, as occurs with plastic tub surrounds and/or plastic tubs. I don't see anything in your photo that couldn't be handled by the usual sorts of bath caulking alone, provided the joint is clean and dry.

One can only guess what the hard filler material might be. There are many quite hard fillers available, such as Durham's "Rock Hard" Water Putty.

  • Appreciate your answer. Thanks!
    – SKD
    May 29, 2023 at 6:39

For the filler at the floor I'd be tempted to just use a chisel and gently tap away at the stuff.

As to the gaps, do what you gotta. If you can find a backer rod that will fit the gap, use it. If you can't, that probably means the gap doesn't need backer rod.

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