QUESTIONDoes bluestone and or other clefted/unguaged stone have a different allowance or classification when it pertains to lippage allowance?
ANSWERANSWER: According to the NSI-MIA Dimension Stone Design Manual VIII, the maximum allowable lippage (the amount of vertical difference between two adjacent tile edges) for a calibrated smooth finished natural stone is 1/32".
Although the standard also states "lippage will not be attainable in flamed, cleft, or otherwise textured finishes. In those installations, joint width should be increased to limit perceived lippage."
Lippage is the result of the method of installation, the condition of the stone, the condition of the substrate, and the skill level of the installer.
Ungauged stone should be installed in a wet-set mortar method that can help compensate for the variation in thickness of the stone. A cleft finished stone can't be fully compensated for its irregular surface, but it can be controlled by the qualified installer.
It is best to install a mock-up of the tile assembly or only install a small section of the tile, and then get the owner/architect to inspect and approve the mock-up. This will show them what you can do considering all of the factors, and give them an opportunity to take a different approach if they are unhappy with the result.