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Unread 02-07-2020, 06:56 PM   #1
Lou_MA
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Silicone caulking best practices

I didn’t see this addressed in Paul’s old silicone caulking tutorial.

1. Is backer rod acceptable to use in showers? Or will it get moldy over time with moisture that may get behind tiles, especially at the shower floor-to-wall joints where standing water may accumulate?

2. The first attached pic shows how I typically set shower floors, leaving it shy of the walls, but with a gap that will be covered by the combined thickness of the wall tile + thinset.

However, this can leave a fairly deep and tall void (2nd pic, void shown as red area).

Do I try to fill this entire void with silicone?

The joint is only about 1/8” high. Pending question 1, do I try to first shove in some backer rod? Do they even make backer rod that thin?

Is there a better option altogether?
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Unread 02-07-2020, 09:37 PM   #2
smifwal
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Why not run the floor tile under the wall tile and close up that void?
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Unread 02-07-2020, 09:45 PM   #3
Lou_MA
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The floor tile does go under the wall tile, but in some cases (due to floor tile layout for example, where I want to end on whole tiles) it’s just barely covered once the bottom row of wall tile is installed. So there’s both a deep and tall void behind what would look like a smallish 1/8” joint on the surface.
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Unread 02-07-2020, 11:01 PM   #4
smifwal
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I am sorry I thought that you were doing it specifically that way. I don't know if there is max thickness of the silicone that is produced for the showers. I will ask my mapei rep, or maybe Dan or Holden will see this thread and comment. I will say that I wouldn't be hesitant to "fill er up". As for the backer rod, we use it in the corners but the water runs down, where we are talking about there would be standing water in essence. I don't really have a solid answer, but I hope someone will chime in with a correct answer or at least best practice
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Unread 02-08-2020, 07:46 AM   #5
jerrymlr1
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Why not just grout it? It looks better and vapor can migrate out. What's the worst that can happen? Cracked grout joint down the road? FWIW Lou I'm not a fan at all of silicone in showers.
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Unread 02-08-2020, 08:05 AM   #6
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It seems to me that when it turns out that there is such a slight overlap of the wall tile to floor tile it really doesn't leave much of the floor tile surface for the silicone to adhere to. In addition to having the usual 1/4" to 3/8" of wall tile edge, having at least 1/8" of floor tile surface for the silicone to adhere to would be a good idea.

I'd be hesitant to fill the void completely. I think doing so could/will create a dam to any water draining down the back of the wall tile and mortar, possibly accumulating, and preventing it from getting into the mortar and or mud bed under the floor tile and migrating towards the drain. Finally, filling the void will make it all but impossible to remove all the silicon, and therefore any festering mold/gunk when it does come time to re-caulk.

I think the same would hold true of using foam backer rod if it were packed in tightly and consistently along the length against the wall, but at least you'd probably be able to get it out at re-caulking time.
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Unread 02-09-2020, 01:28 PM   #7
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I would try to have the floor tile go underneath the wall tile with a gap in between. You don't want to try to fill the entire gap back to the wall because the silicone loses its performance.

The way that it was explained to me is that if you were to glue a rubber band down to a table it will lose a good amount of its ability to stretch. It will still stretch some but not nearly as much as if it weren't glued down.

That's what it's like if silicone is allowed to adhere to the back of a channel. That's why you only want it stuck on two sides and not three.

That's one of the reasons for backer rod. I think the rule of thumb is that you want the depth of the joint half as deep as the width for maximum stretch. So that's how far down to put your backer rod.
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