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Mortar bed thickness and zero entry showers
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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.
Above image courtesy of Marazzi
QUESTION TCNA Method F103B-21: Under the requirements section, it lists mortar bed thickness 1-1/4" minimum to 2" maximum. I recently read in an article that 2" was the maximum for a mortar bed also. I have a general contractor and architect asking me why the mortar bed cannot exceed 2" and I am looking for an answer to that.

ANSWER A member of the Methods and Standards Committee noted that if you look at the method, it does say that if you want to go thicker than 2" for a mud bed, it must be documented or designed and called out for that by the specifier. So, you can go thicker than 2" with a mud bed, however, it will take more packing, is more labor-intensive, and you have to speak to the tile contractor about that cost.
The Grout You Know and Love in a Pre-Mixed Formula
It could be that putting the parameters of 1-1/4" to 2" falls within safe weight parameters for most structures, hence the need for approval from an architect if you were to exceed that. Each 1/4" of mud depth adds an additional 3 pounds per square foot. That extra weight could add up quickly and exceed the load the structure was designed to bear.   The other points mentioned are very important: both the labor and material cost increases as the thickness of the mud bed increases. Having these clearly defined parameters would help getting an accurate estimate to figure cost on projects. Also consider the additional exposure to failure of these types of mortar beds delaminating from compacted lower layers (either from working on top of the mortar or wheeling additional loads of mortar on top of lower layers). And it becomes increasingly difficult to properly compact increased depths of deck mud with standard flat trowel or wood float. Some installers use different tools – such as tampers – but it's undecided if any of these other techniques have been tested for strength or not. – NTCA Technical Team QUESTION I need your help approaching this zero entry shower. Possibly a B422C-20. The shower pan has been made out of fiberglass resin in a similar manner to a traditional hot mop. I don’t think the pan is recessed enough and it doesn’t extend far enough out of the shower area. Should I create a high point by the entry door towards the shower to accommodate for splashed water, since there isn’t a secondary drain here?
ANSWER The height of the subfloor outside of the shower area may have to be built up to accommodate the mud and drain and slope to the drain, but judging from your pictures, building up the subfloor height outside of the shower is not going to be a simple task. It looks like this wasn’t a well-thought-out overall plan to accommodate a curbless shower system.
Other than adding a curb to the shower, there appears to be no simple solution. It is very difficult when others come up with a plan that really doesn’t work and you have to try to make something work, or something has to be significantly redesigned – or you have to say no to the job. It gets complicated very quickly.
– NTCA Technical Team