ASK THE EXPERTS QUESTION I will be tiling a shower/bathroom house at a campground this summer. The floor substrate will be concrete and the walls will be masonry block. The architect did not specify any waterproof membrane for the entire job. I am wondering if this will be okay since it’s all concrete. Should I suggest some kind of waterproof- ing/crack-isolation membrane? If so, could you refer me to the place in the TCNA Handbook that explains this? ANSWER Thanks for contacting the NTCA. We encourage everyone to use industry-recognized methods found in the Tile Council of North America Handbook for Ceramic, Glass and Stone Tile Installation, and ANSI. Thereareavarietyofshowermethods that would work with the installation type you described (See methods B421 and B422 TCNA Handbook). All incorporate waterproofing that meets ANSI A118.10. standards. Having the appropriate slope and drain connection is crucial in every shower design. Water can pass through grout, stone and many tiles, so installations need waterproofing beneath the tile. Although the sub- surface is block and concrete that may not be harmed by water, water can pass through these surfaces and get into areas where it is not desired. When waterproofing is not used and water soaks the block or concrete, those areas can continually remain wet, discoloring stone tiles and grout. A wet subsurface also creates a source of moisture that can encourage mold growth when organic materials like soaps are left behind. – Robb Roderick, NTCA Technical Trainer QUESTION I am looking for the relative humid- ity percentage in a newly-poured concrete slab to be able to set tile. ANSWER Unless installing an UN-bonded mortar bed, all TCNA Handbook methods for installing on concrete call for the slab to be well cured, dimensionally stable, and free of cracks, waxy or oily films, and curing compounds. Some manufacturers of uncoupling membranes may have instructions allowing for the use of their mem- brane over newly poured concrete since the membrane may act as a conduit for the concrete to prop- erly cure during the early stages of Ask the Experts Q&As are culled from member inqui- ries to NTCA’s Technical Support staff. To become a member and make use of personal, targeted answers from Technical Support staff to your installation ques- tions, contact Jim Olson at jim@tile-assn.com. Sponsored by 14 TileLetter | 2019