I'm thick. Can you explain the role of the membrane/coating? What difference does it make whether the thinset is on the membrane or the concrete? Won't water get thru the grout anyway if it is porous? What stops the tile from popping off the membrane/coating the same way it has popped off the concrete? I was under the impression the membranes were used to waterproof the floor itself. In this instance, it's a solid concrete floor that's been a shower for 75 years.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13