ASK THE EXPERTS QUESTION I have a floor installation with a relative humidity reading of over 90%. Can you advise me of steps I might take to prevent a failure in this installation? ANSWER This is a very high reading and beyond the capabilities of most setting materials. In general, most setting products perform well with readings of 3 lbs/1,000 sq. ft./24 hours in calcium chloride test, or readings less than 75% using a relative humidity test. The effect of moisture on floor covering is a huge problem across the United States. Not addressing this issue with a moisture mitiga- tion system will affect the lon- gevity and performance of this installation and probably lead to some type of failure. Some manu- facturers have moisture mitigation systems that include waterproof- ing membrane, and specific thin- set mortars that are warrantable up to 12 lbs/1,000 sq. ft. and above 90% relative humidity. My advice to you is to use a mois- ture mitigation system and the appropriate setting materials that can handle that level of moisture. There are thousands of setting products that are affected by dif- fering amounts of moisture in a variety of ways. Using the informa- tion you have received from a rela- tive humidity test in concert with technical data from your setting material provider is paramount for a successful installation. – Robb Roderick, NTCA technical trainer/presenter QUESTION I was helping a friend with a concrete shower base install and ran into a problem. We poured the concrete base with the required portland cement/sand mixture at the correct slope. About a month after the tile install, the grout start- ed chipping out. After regrouting it continued to fall apart. We pulled up the tile and it came up pretty easy. It’s been about five days now and the concrete still looks wet. I cleared out the area around the drain to make sure the weep holes were clear and they were. Why is the base still wet? I’m hesitant to install new tile on top until it’s dry. Help please! Sponsored by 20 TileLetter | October 2017