I suggested to the client that he might like another type of floor material to avoid any other chance of this happening again due to the softness of the marble. I under- stand everyone’s reluctance to do this, but I just wanted to increase our chances of having no further complications. The tile installer feels pretty con- fident the uncoupling membrane will do the trick. My stone guy examined our tile and he said the product was as sound as you are going to get with Carrara marble. What do you think? ANSWER Good to hear from you. As you are aware, the NTCA always encourages people to follow indus- try guidelines set by both the Tile Council of North America (TCNA), and American National Standards Institute (ANSI). I was able to read the correspon- dence about this stone tile failure. The questions raised are covered in our industry standards. The assembly that failed was a backer board installation over ply- wood. Attached is the diagram from the TCNA Handbook method F250, which is the closest method to what was described. If you notice two lay- ers of plywood are required under- neath the backer board install. For a similar installation using ceramic only one layer of plywood would be required. That detail is found in method F144 on page 162 of the 2017 TCNA Handbook. There are two standards for allowable deflection in tile installation. For ceramic and porcelain tile, the stan- dard is L/360. For stone tile installation the stan- dard is L/720. The L in this equation is the length of unsupported span under the installation in inches. This means that only half of the move- ment which would be allowable with a ceramic install would be allowable with a stone installation. The question about adhesive under the plywood against the floor joist is addressed in ANSI A108.11 it states “…a 1/4” bead of construction adhesive should be applied to the center of the top of the joist and the plywood fastened to joist with 6d ring shank nails. There should be a 1/8” gap between the subfloor sheets.” There was a mention of using an uncoupling membrane for the second installation. A second layer of plywood is required when using uncoupling membrane. Check with the uncoupling membrane manufacturer for complete instruc- tions on this type of installation. I hope this information helps. – Robb Roderick, NTCA presenter/trainer ASK THE EXPERTS –––––––––––––––––––––––––––––––––––––––––––––––––––– 24 TileLetter | February 2018