There's a large gap under the part of the shower stall that touches the wall because of the way the bathroom floor is slanted. When we first moved in the caulk that was put there by the workers washed away. I removed all the original caulk and started over. I filled in the large gap completely with about a tube of caulk. That evening the toilet flooded the outside and washed away some caulk. By then I had realized there should have been some material stuffed into the gap for all that caulk to adhere to. I found some foam/plastic tubing that goes around mirrors and cut a couple pieces to fit the gap. I stuffed them in on the side where the caulk had come lose. Then I re-caulked that side. Now it's been a couple of days and while the outside of the caulk is stiff it still gives way when pressed. I guess it's still wet inside.

My question is: when will it be safe to use the shower?

shower caulk

  • A picture would really help here. – Daniel Griscom May 15 '16 at 23:49

safe to use the shower is a relative question. when will it be safe to use it so that its use doesn't compromise the caulking seal? never - it was already compromised the moment you did it that way.

caulking shrinks as it dries, and the speed at which it dries changes how it polymerizes. when you make a huge blob of caulking, you seal in much of the solvent and that changes how the sealant cures dramatically. essentially, the inside of the caulking is still wet with solvent, which prevents the material from properly linking together to make a waterproof plastic membrane. since that waterproof membrane has to adhere to the surrounding surfaces to keep water out, it can't. thats the key to success.

too much caulk means the membrane never fully forms. you get a layer on the outside just thick enough to hold in the solvent. now, when you wet it with water, the water also seals in the solvent. since there is flex in every joint, everytime you stand on it or near it, or swing the door, or fart strongly, the joint moves. however, instead of a strong, well adhered plastic membrane flexing and returning, all you are doing is tearing the thin outer membrane and letting in a little air to further cure a tiny bit inside the tear. this will continue for weeks until the boundary between the new sealant and the substrate is completely sheared through, and even then you may not have fully cured caulk. as the curing process continues, the caulk will shrink until that boundary is not only fully disconnected from the substrate, but also shrunk to the point where the boundary is a gap. then water gets in again.

to do it right, you have to follow these steps:

1) remove all old sealant mechanically. 2) using whatever solvents are necessary, clean off all remaining traces of the preexisting sealants (this is important, as you need new sealants to bond to the substrate, not the old sealant) 3) mechanically abrade (sand) the new joint areas to promote proper adhesion. 4) if the gap is bigger than 1/4", fill it with a compatible seam filling rope or extrusion that is compatible with your sealant and the type of movement you are going to have. 5) seal with a good quality urethane or silicone sealant. don't use latex sealants inside showers or bathtubs, no matter how much it says you can on the packaging.

and for the love of god, do your work carefully and with some attention to craftsmanship. it looks like you put that sealant in and smoothed it out with an old shoe.

  • I smoothed it with my finger. It was hard to make it perfectly smooth without creating any surface-tension holes. The gap is triangle-shaped. I stuffed some tubing in but it's hard to get an exact fit. I saw someone use insulation to fill a gap. I'm in China and it's hard to get the right materials. Everything was done very haphazardly and anything that I do myself is already a great improvement. Although it would be nice to have a solution that lasts for years, I'm satisfied with something that will improve on what we had before. I'll have to order filling rope or extrusion from the States. – Reed G. Law May 16 '16 at 2:06
  • +1 for the old shoe mental imagery. – Tashus Feb 13 at 16:48

Answer: there's no way to tell. Caulk isn't meant to fill large gaps (more than 1/2" is one spec I saw, and even that seems wide to me). Even if you waited long enough for the caulk to dry, it's quite possible that the caulk will fail due to the stress of stretching across that gap.

The best (and perhaps even quickest) solution would be to clean off all the caulk and start over. Fill the space with some sort of rigid, structural material (e.g. a tapered board), and then waterproof/caulk that.

  • I've already added a couple of pieces of tubing. Those were added later as mentioned above. Rather than start over now I'd rather use the shower first on the chance it does hold. Just wondering if the internal caulk will continue to dry now that the outside is hard. – Reed G. Law May 15 '16 at 23:55

Just to complete the answer above.

When you make a huge blob of caulking, you seal in much of the solvent and that changes how the sealant cures

It's true, but it's possible to solve that problem by mixing the silicone with an adjuvant (like water, or even sugar!*) that will help to polymerizes. So you could make a relatively large balls of silicone. So this technic which might work to fill some large gabs (I did not try, if somebody has, please confirm if it works).

(*) A guy experimented different adjuvants (to build custom insoles) on this YouTube video

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