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.
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.
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.