A mottled bond coat pattern may become visible through translucent or transparent glass tiles when they are installed over a waterproofing membrane.
A mottled bond coat pattern may become visible through translucent or transparent glass tiles when they are installed over a waterproofing membrane.

A.Sean Gerolimatos, technical services manager for Schluter Systems, responds: Glass tile has some special characteristics that make installing it over Kerdi and other types of waterproofing membranes problematic. One concern is ghosting, where a mottled — rather than uniform — bond coat appears through the translucent or transparent glass (see photo). To avoid ghosting, some glass-tile manufacturers recommend skipping bonded waterproof membranes altogether and installing tiles over a substrate that can wick water away from the bond coat, such as cement backerboard or a traditional mortar bed.

Another issue is bond strength. For a variety of reasons, glass tiles are generally more difficult to bond to the substrate than ceramic or stone tiles. As a consequence, most glass-tile and setting-materials suppliers recommend that glass tile be installed with polymer-modified thinset mortar, which generally is stronger than unmodified thinset...

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