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Unread 12-04-2019, 03:41 PM   #23
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Join Date: Aug 2018
Location: Bay Area, CA
Posts: 135
It's not the grout.

area of top of 2x2 hex: 3.5 sq in
area of sides of 2x2 hex (assuming 3/8" tick tile, 1/2 of sides covered in thinset): 1.3 sq in

The tiles are soaked through, with the most exposure and ability to dry coming from the top/bottom surfaces. Also, the sealer you used isn't waterproof, so the top of the tile will absorb water just as readily as the untreated sides. Take some scrap tiles, soak it in a bucket, remove and let dry. They will dry very fast even after being fully submerged. You have moisture trapped in your tile and/or mud bed likely from one of the three reasons mentioned above by Davy (deck mud too dense, poor slope, clogged weep holes) all of which will be exacerbated by the floor tile choice. If you suspect the weep holes are clear, and are confident in the porosity of the mud job, you may just want to learn to live with the discoloration.

I built a shower similar to yours last year. In my research, I saw how widespread this same issue was with marble floor tiles of all shapes and sizes. Not wanting to risk it, I quickly ordered a grey through-body porcelain penny hex that closely matched my grout color and complemented the marble wall tiles. My shower pan is a surface applied membrane, the tile is maybe 1/4-3/16" thick, and it all dries out very very fast.
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