I have 20" porcelain tiles on hand-mixed thinset over cleaned bare slab (no barrier). Total job was just over 1100 ft2, so roughly 1/2 the downstairs area. Within weeks we see the white efflorescent salts. We knew this was possible as we had taken up a vinyl tile floor from the same area that had been installed over a thin set float over a TiteBond moisture barrier then latex glue about three years earlier in a partial downstairs 'update'. That floor lasted two years but was failing within one year. We had the porcelain tile installer jack hammer out the float (and thus the moisture barrier / residual glue) before letting him set the new tile directly on the slab. We have had two civil engineers (both PE's) to the house since to advise on the issue and investigate drainage (seems to be the natural starting point). Nada issues, though I have spent the last year reducing dirt levels around our slab and adding some additional drains. And I have had a plumbing inspection service both video our sewer / drain system and do a static pressure test. Again, no issues. All pressure potable piping is above ground in our area. No water lines under the slab. I have pressure checked on sprinkler system lines that are near the slab. Nada issues. We live on the upper Texas Gulf Coast on land developed 45 years ago from raw pasture land. Typical clay layers starting some 18" below grade. The house was built in a period (1978) when plastic sheeting was spread for a vapor barrier; its status today cannot be determined. House as no major slab movement but one partial crack across the back middle of the house. No observed water staining near that crack when the original carpet / pad were removed for the vinyl floor. The main salt shows are in the den (where the partial slab crack exists) and over into the breakfast room. No shows in kitchen or laundry area. And the slat shows we do have are always in the same place. We wipe them up every two weeks or so, and within a week there is enough to be noticeable. As a reference, when the water comes up to the grout top, it pools as clear water in a puddle ranging from barely visible to maybe an inch by three inches. When it dries you see the salt show. We went through 30+" of rain with Harvey, and there was at the time and has been since no difference in salt shows - volume, how quick they appear after cleaning, etc. Currently just living with the salt shows. No tile failures. Theoretically the installer offers a Lifetime Installation Warranty, so if a tile pops, then I can to them and seek some relief. Interested in the back-butter step mentioned in a post above. Certainly that was never discussed or done on this floor. Installation company is a large chain store, at least in our area. If anyone has further action steps or advice I am open to suggestions. Thanks for taking the time.