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Unread 09-17-2022, 09:20 AM   #1
md9918
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Vertical hairline cracks in tiled window box

I built a shower in May which included tiling a window box inside the shower. The box is framed in 1x6s which are nailed to the rough opening in the cinder block exterior wall. I didn't do much substrate prep except to apply Kerdi. This morning, I noticed vertical hairline cracks in the tile on both sides of the window inside the window box. I am completely confounded as to what could have caused it. We have had no extreme weather since the tile was installed-- Temps have ranged from about 40 degrees to upper 90s during that time.

This window is on the second floor of my home. I tiled a window box on the first floor in an identical fashion and it is almost directly below this window and its tiles seem fine (although they are slightly beefier-- 3x12 vs. the cracked ones which are 2x8).

Any idea what could have caused these cracks? Pics below (I've been meaning to get in there and clean up the grout lines!)

My best guess is moisture from outside causing the wood underneath to swell-- but it's crazy to me that it happened in exactly the same place on both sides of the window, and that it's not happening in the first floor window. Also, the top of the window box does not have any cracks.
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Unread 09-17-2022, 09:43 AM   #2
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Welcome back, Mike.

I'm confused about what we're seeing in the photos. Well, first I'm confused about what you're calling a "window box."

How 'bout a photo showing some perspective of the cracked areas you're showing us. I'm not seeing any gray tiles in any of the other photos.
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Unread 09-17-2022, 10:59 AM   #3
md9918
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Ah-- I can see the reason for the confusion. Those are progressive pictures. The "grey tiles" are the exterior wall-- cinderblock interlaced with brick. (They built these houses tough in the 50s!) That photo was taken while I was building the bathroom and I included it to show the framing situation.

I've attached a couple "zoomed out" pics of the hairline cracks for context (I had been trying not to show too many messy grout joints!). You can't really make out the cracks in these pics, but I circled their location.
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Unread 09-17-2022, 11:14 AM   #4
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Almost without exception, cracks like this stem from differential movement of substrate and/or tile assembly. A couple of questions.

1. From last pic I gather that some sort of jamb extension was added to what appears to be greater depth of finished jamb. How was it accomplished?

2. From your text it sounds as if you applied Kerdi membrane to wood substrate. Is that correct?
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Unread 09-17-2022, 12:03 PM   #5
md9918
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Ahh- you're right. I had to build the wall out 2.25" to make the tub drain clear the floor joist. That's probably right about where the studs I installed meet the original 1x6 box. I suppose something like Ditra would have stopped this from happening.

Is there any way of making it less noticeable from a cosmetic standpoint short of a tear out? (Would probably end up being the whole shower, right?)
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Unread 09-17-2022, 01:58 PM   #6
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I really can't think of any way to disguise the cracking other than covering it up. It will likely get worse over time.

Ditra is designed for floors as a decoupling membrane. I've never used it as you describe. Bear in mind you would become the engineer and your shower the test subject for such an installation.

Were it me, I would make that jamb a single front to back piece from something more stable than dimensional lumber and I'd line jamb with Kerdi Board. I imagine the Kerdi Board may provide some isolation by itself, but again, you'd be working outside of recommended use.

I think it helps to think of the tile base assembly as monolithic structure prior to tile installation.

And FYI, framing lumber is often much higher moisture content than, say, a pine 1x6. I'd venture the added 2x6 is shrinking. It will reach equilibrium at some point but may never sync with the 1x6 entirely.
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Unread 09-17-2022, 03:18 PM   #7
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I agree, installing Kerdi over dimensional lumber is likely the problem.
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Unread 09-19-2022, 05:46 AM   #8
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It would be pretty simple to trim out that area with either PVC or synthetic marble. Do a tidy job and nobody will ever notice, the thinks it’s just part of the window trim.

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Unread 09-19-2022, 03:19 PM   #9
md9918
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Great idea, thanks. I will live with it for now and keep an eye on it, but may go that route if it gets worse.
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