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What are the minimum and the maximum grout lines that you can cover with 100% silicone?

With grout line I mean the space between two tiles, without any applied; I don't mean covering a grouted space with silicone.

If I have porcelain tiles sitting on a shower pan at 1/16 gap or even less (I can put a taping knife through that crack but nothing more) would that be too little to caulk with silicone 100%?

On the other end at the top of the wall where it joins the ceiling I have a 0.25 of an in gap. Would the silicone hold here or it needs a backer strip ?

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  • Wouldn't you normally use grout sealant to seal the grout lines, not silicone caulk?
    – FreeMan
    Jan 14, 2022 at 15:17
  • I still call grout line the space between two tiles in a inner corner (wall to wall or wall to floor) As far as I understand silicone or more generic, caulking is recommended here
    – MiniMe
    Jan 14, 2022 at 16:01
  • Gotcha. Sorry, that wasn't abundantly clear to me that's what you were talking about.
    – FreeMan
    Jan 14, 2022 at 16:06

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Even a film of say 6mil will seal and hold together, so squeezing it into a 1/16 gap is not a problem, especially since you will likely build up the inside corner a bit anyway (as you smear it out with your finger).

As for the larger gap, anything over 0.25in I'd backfill first mainly to save on caulking or to prevent it from falling into a void (which you don't have), so for the top it's a matter of preference. Larger seems over 0.5in also become unsightly, but they will seal.

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    Also caulk/silicone works best when it remains in an hourglass shape to allow for compression and elongation. So if there was a large gap putting some foam backer rod in there isn't a bad idea.
    – redlude97
    Jan 14, 2022 at 18:43

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