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Medium bed terminology/ marble on shower floors
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Ask the Experts Q&As are culled from member inquiries to NTCA’s Technical Support staff. To become a member and make use of personal, targeted answers from Technical Support staff to your installation questions, contact Jim Olson.
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QUESTION What is the status of medium bed terminology in the tile industry?
ANSWER Generally speaking, it is not used. 
Mortar manufacturers once used it (and labeled their products) to refer to their dry-set, thin-set mortars that could support the weight of large-and-heavy tiles without collapse as it cured and/or function at a bond coat thickness greater than the usual nominal 3/32".

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“Medium bed” may still be found on some dry-set, thin-set mortar packaging and literature, but it should be read or viewed as a product manufacturer marketing term and not an industry-recognized product descriptor. “Large and heavy” is now the common descriptor for this type of product. ANSI A118.15H identifies large-and-heavy-tile mortars and their properties and criteria. “Medium bed” is not a recognized installation method. The recognized installation methods are “wet set” or “dry set using a thin-set mortar.” Please see ANSI A108.1A and ANSI A108.1B for these methods.  Some users or specifiers of mortar labeled “medium bed” may have made the assumption that it could be used in place of proper methods or materials to flatten substrates. This misconception has led to installation failures where the mortar has been used incorrectly or pushed beyond its ability, usually exceeding its recommended bond coat thickness, and creating a variety of symptoms that can lead to system failure. Always check with the manufacturer of any mortar, especially those that still may be labeled or marketed as “medium bed,” to understand the recommended application and proper use of their product. – NTCA Technical Team QUESTION I am trying to find where TCNA speaks specifically to “not” using marble on shower floors. Can you tell me where to find that in the 2021 manual? I am looking for something “official” to show a customer who is insistent on having an elaborate Carrara or Calacatta marble mosaic on her shower floor, and other stores in town are not cautioning her against this. Also, in the process of doing a Google search to try to find this information (no luck), I did come across a TileLetter article from 2020 that basically argues against the idea that marble material is not the ideal product for a shower floor; that it goes back to installation products and process used. I’d appreciate any input you might share with me! ANSWER There are several industry-recognized installations that incorporate stone in a shower assembly found in the TCNA Handbook. The materials section of each Handbook method directs you to the natural stone selection guide found at the beginning of the Handbook. There is not a method for stone tile use in steam showers found in the TCNA Handbook. In the Natural Stone Institute Design Manual there is a section called “Wet Areas” that says not to use moisture-sensitive stone or flawed stone in wet areas. Stone tiles have a classification system of A, B, C, and D. These classifications are based on the amount of flaws and fillers that are found in the stone. The Design Manual cautions against the use of classification C and D stones in some wet areas. In addition, page 264 of the current NTCA Reference Manual addresses this situation in “Stone in Wet Areas.” The TileLetter article you are referencing is in the Tile & Stone section of the July 2020 issue. I hope this information helps. – NTCA Technical Team