Jadnashua
Retired Defense Industry Engineer xxx
And, as you know, if you really want to use a modified, Schluter will allow you to with prior approval and IF you use a rapid set mortar. A rapid set mortar does not use latex type modifiers that need to dry, so in that case, cures in a known, reliable timeframe.The ones which require ONLY NON MODIFIED.
There are valid technical reasons to avoid a modified thinset over any waterproof surface bonded membrane - drying time, as recognized in the TCNA handbook says, and I quote:
When latex/polymer modified Portland cement mortar is used to install ceramic, glass, and natural stone tile in an area that my not thoroughly dry out in use (e.g., swimming pools and gang shower, etc.) or where initial drying is inhibited (between tile and an impervious substrates), it is recommended that installation be allowed to dry out thoroughly before exposure to water. This drying period can range from 14 to more than 60-days depending upon the temperature and humidity and other climatic conditions, and whether the installation is interior or exterior...
Schluter, being conservative, specs an unmodified, that they know will reach maximum strength in a known timeframe. When I lived in Jordan, I was able to tour some ruins from BC Roman structures that had sections of floor installed with some glass that were still intact...no modified thinset around then...between a waterproof membrane and an impervious tile is the best possible bond for those types of tile and a dryset mortar. Now, if you can afford to wait, and want to follow the guidelines, do you really want to tell the customer - hey, the shower is finished, but you shouldn't use it for two months!
Will a failure occur, probably not. Will it be as strong as it should be? Not if you don't let the modifiers dry out first. Will you have a failure, probably not. The stuff is just that much overkill. Might you have problems trying the same thing over something like cbu? Yes - modified has its place, and bonding high density tile to a rigid surface or wood is the best use of the stuff.
So, the guideline is not a Schluter thing...it is a TCNA guidelines thing and experience over lots of years and research of the product - a dryset is a known entity...a modified throws you a curve. It's not that the other membranes are really different, it is the intolerance for potential failure and knowing your product and standing up for it in the face of opposition.
Just how long do you think a monster tile like a 48x48 on an accessible shower with a linear shower drain over a waterproof membrane will take to dry out the modifiers? probably a lot longer than the time listed above. And, some of them are barely 1/8" thick...with soft mortar underneath for several months...
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