ASK THE EXPERTS QUESTION An architect has requested my input relative to developing a labor and material specification for installing new porcelain floor tile over existing granite floor tiles in a high-traffic lobby in a commercial office building. Can you direct me to any relevant literature or information that addresses such applications? Thanks. ANSWER I suggest referring your architect to the 2016 TCNA Handbook methods TR611, TR711 and par- ticularly TR712. Please note that if the installation is not, or cannot be made acceptable for tiling over with a thin bed system, Method F111, or another method, may be required. As described in TR712, it is criti- cal that the existing installation be sound, well bonded and without structural cracks. It must be deter- mined if the existing installation will properly support the new installation. The existing tile and its bond to the substrate and the condition of the substrate will all reflect on the performance of the new installation. If there are exist- ing structural cracks, their cause will have to be explored before using the existing surface as a sub- strate. It is advisable to consider the need for a partial or full crack isolation membrane. Those meth- ods are F125-Partial and F125-Full in the TCNA Handbook. Any existing expansion in the substrate beneath the exist- ing installation must be honored in the new installation. TCNA Handbook Method EJ171 will be the reference to all expansion and other types of joints that must be honored and designed and installed into the new system. Note that EJ171 states the archi- tect shall specify the location of any expansion joints and other soft joints throughout the field and other locations such as the perim- eter and any change in plane. Have the architect specify in writ- ing (via drawings) where these are to go and which materials and EJ171 details should be used to construct them. Checking for the ability to bond to the existing tile is imperative. If there are sealers or oils or waxes, etc., on the existing sur- face, they must be removed. If Sponsored by 20 TileLetter | August 2017