What is more water resistant: Thinset or Grout? [Archive] - Ceramic Tile Advice Forums - John Bridge Ceramic Tile


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04-13-2015, 03:56 PM
Was curious, what is more water resistant... Thinset or Grout?

The reason I'm asking is because I have a niche in my shower that has a marble shelf in it, and in one of the corners of the niche the caulking gets a bit moldy over a year or so (the niches were caulked not grouted). I removed the caulk and realized it is because when water gets past the caulking over time, it sits in a little valley of dried thinset that is present between the back end of the marble platform and the inside wall of the niche. It is a bit hard to explain, but since the the marble shelf is angled downwards for proper water flow, it creates a small gap between it and the wall of the niche because the marble shelf is angled down slightly while the inside wall of the niche is straight, so there is a tiny valley between the back of the marble shelf and the wall of the niche. The marble shelf was set in thinset mortar but the dried thinset is not flat (the shelf is angled properly however), so when water hits the caulking in the niche and gets past the caulking, it can then sit in that small valley of dried thinset and stays there (the niche itself was Redgarded so it is waterproof, so that water just sits there and doesn't go anywhere else, causing the caulking to get mildewy/moldy over time). Any water that hits the marble shelf rolls off since it is angled properly. My plan is just to fill in that valley with something, like some more thinset to level out the mini valley between the marble and the niche wall so if water gets past the caulking, there will be no valley for it to sit in and will be forced to roll off the properly angled shelf. Then I was thinking maybe I should just fill the mini valley with grout if grout is more water resistant, then recaulk the niche.

So which one is more water resistant... thinset or grout? And what would you do to fix this minor thinset "valley" issue?

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04-13-2015, 04:59 PM
While the surface of the niche may be sloped...the waterproofing beneath it should be as well. Neither grout NOR thinset are waterproof, and any moisture that gets beneath them (and it will) needs some slope for it to drain off rather than accumulate. Industry standards call for any changes of plane to be caulked verses grouted. Personally, I like to be able to use an engineered flexible expansion joint there, and then you won't have ANY caulk in the joint at all.

OF the two, grout may be denser than thinset on average, but unless you go with something like an epoxy or acrylic grout, none of the cement based materials are 100% waterproof.

04-14-2015, 03:06 AM
Thanks for the reply, much appreciated. I believe the surface under the niche shelf was sloped properly as well, so water should be able to drain properly. I think what is happening is that water will just sit for quite some time in the corner of the niche due to the valley created by the thinset used to set the marble shelf in and against the niche. I think if I just fill that tiny valley in, it will hopefully just redirect the flow of water and then I won't have any sitting water there. I'll try using grout since you had mentioned it may be denser than thinset. I'm thinking that should fix it. If my caulking doesn't get moldy for another year, then at least it's an improvement, lol. Thanks again.

P.S. Actually, I have some follow up questions if you don't mind. I also have read and agree that all changes of plane should be caulked if possible. But I have a friend that only believes in using grout for the changes of plane due to aesthetics and feels they rarely cause an issue. He said he sometimes just puts a layer of clear silicone over the grout as well. Is it common for workers to grout changes of plane even though the recommendation is to caulk? He has family members that are involved in bathroom remodels and said that this is what they do as well.

And you mentioned an engineered flexible expansion joint. Even though I'm just going to put some grout in my little valley, then caulk the rest, I wanted to know how an engineered flexible expansion joint would apply in a niche. I googled it and it came up with a bunch of weird devices, none of which looked like it would work in a niche. I'm just curious if you don't mind, but can you show me a link of what device would work in a niche?

Houston Remodeler
04-14-2015, 05:45 AM
Most grout manufacturers and our buds at www.ColorRiteInc.com sell 100% silicone in sanded, unsanded, shiny and matte in over 300 grout matching colors. So there is no need to grout the corners for cosmetic reasons anymore.

Some really nice guy wrote a caulking tutorial (http://www.johnbridge.com/vbulletin/showthread.php?t=85634) if one is tenuous about using silicone.