What level of flatness ...?

Received an email with an interesting technical question:

What level of flatness do we need in a concrete slab for a ceramic tile installation – using the F-number system?

Note:

Measuring concrete levelness and flatness using the F-number system for ceramic tile & stone installations may be over and above what is needed. Typically measuring FF/FL tolerances would be for projects such as warehouses where forklifts or machinery requires tight concrete levelness and flatness tolerances.

And our Laticrete Technical Support reply:

Please note that the American Concrete Institute's (ACI) Floor Flatness (FF) / Floor Levelness (FL) are typically used by concrete specifiers / masons to pour their floors to specific tolerances.

Typically, the following are the FF #'s which roughly correlate to the respective floor tolerances:

FF 25 is roughly equivalent to a single ± 1/4" defect in 10'
FF 50 is roughly equivalent to a single ± 1/8" defect in 10'
FF 100 is roughly equivalent to a single ± 1/16" defect in 10'

Tile Council of North America / ANSI Standards typically require substrates receiving thin set applications to be within 1/4" in 10' (and 1/16” in 1’-0”). For large format tiles and running bond patterns, a tighter tolerance of 1/8" in 10' may be necessary.