I discovered a small gap in the caulking between the floor and the bathtub. The gap is an inch or two long -- but it's right where the water sprays at the edge, when someone takes a shower, and a very small puddle always forms there.

I propose to fill it with some silicone caulk. Do I need to pull up some of the floor first to make sure the subflooring is not damp?

Should I run a space heater and a fan for 24 hours before caulking?

Is there some way of detecting whether there's a moisture problem underneath the floor?

  • What type of floor? Is this shower on first or second floor?
    – JACK
    Jan 9, 2020 at 22:42
  • @JACK - It's a ground floor bathroom. There is a semi-finished basement underneath. The flooring appears to be what they call luxury vinyl tile -- it fits together sort of tongue and groove. Jan 10, 2020 at 2:05
  • Are there any signs of moisture accumulating on the ceiling of the basement under the bathroom?
    – JACK
    Jan 10, 2020 at 2:25
  • @JACK - Great idea. I see discoloration around the toilet, and I see discoloration and a bulge with cracking at the target spot where that gap in the caulking is, next to the tub. However, I see no mold and nothing feels damp. The bathroom floor looks fairly new, and I'm thinking maybe the damage was done a while back, prior to the new floor being put in. But it's a mystery why there's a gap in the caulking right where the puddle occurs. Note, the water in the puddle doesn't seem to soak in -- it just evaporates slowly if I forget to dry it off with a towel. Quantity usually: ~1-2 tbsp. Jan 10, 2020 at 3:42
  • Can you provide a few pictures of these areas?
    – JACK
    Jan 10, 2020 at 12:47

2 Answers 2


OK, you had me at "a bulge with cracking". This is a little scary. Even though you don't feel any moisture doesn't mean it's not pooling on the top and rotting the wood. Tubs are heavy and tubs filled with water are a lot heavier. I really think you need to check into this a whole lot more unless you want a tub in the basement. You mentioned LVT flooring in the bath. You might want to consider removing a few planks and examine the subfloor. Hopefully some planks were leftover from when the floor was put down. If you determine all is good, a good silicone caulk applied after everything is dry and scraped clean is all you need. As mentioned above, try to eliminate the water running down the side or at least throw a towel down there before showering. Good luck.

  • Very nice. I like this idea of lifting a few planks to check. That actually answers my question. // Just to reassure you -- the bulging and cracking is in a small, very localized, area. The tub itself is in no danger of falling or sinking. But it does point to the need to investigate further and see what's really going on. Jan 10, 2020 at 17:05

Caulking and mold; Caulking that lasts needs more prep time than the actual application of caulking. My experience with caulking repairs is that mold typically forms in voids * where caulking is missing, * or has delaminated from the surface, * ---OR if someone didn't kill and remove the old mold first before simply just gushing on new caulking! The old caulking should be removed. ----And, running scotchbrite over the surface will show where old caulking bits still resides, even when you didn't previously see it with "naked" eye.
I've see mold magically grow BEHIND caulking.
So, I have had success first; tsp-ing the mold, lightly rinsing, -thoroughly - drying .. assisted by hairdryer and time, THEN the comparitively easier part of applying new caulking.

Also, inhibit the water spilling over. There are simple over-the counter plastic-to-tile wedges in the soft form of a triangle to put on the corner of where the shower wall meets the tub or shower pan such that water would be lesslikely to drip off to the floor. If you're going to be caulking, it would be very simple to caulk that in too. Sub-floor and framing repair costs are massive. This wedge costs $5.

  • I guess the big question is, how do I figure out IF there is mold or moisture in the underneath part that I can't see? I have not detected any mold yet in what is visible. The house was empty for two years before we moved in. Jan 10, 2020 at 2:04

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