Allen Face invented the F-Number system years ago to solve the problems the floor industry faced due to the ubiquitous "gap-under=straightedge" (GUSE) test.
The F-Number has been codified by the American Concrete Institute (ACI 117) and Canadian Standards Association for the specification and measurement of floor flatness and levelness.
There are only two F-Numbers: FF for flatness, and FL for levelness (Fmin though sometimes lumped together with F-Numbers is for an entirely different type of application)
The Ff F-Number specifies “flatness” which controls the bumpiness or short wavelength "waviness" of the floor
The Fl F-Number specifies “levelness” which controls the"levelness" or long wavelength "waviness of the slab.
Continuing with the waviness motif, Ff is controlling for choppy seas, Fl is controlling swells.
F-Numbers are scored in such a way that higher is better, (or at least flatter or more level) F-Numbers are a linear system.
Slabs-on-grade (SOG) are usually specified with an FF number and an FL number (the FF is always listed first), such as FF 50 / FL30.
Slabs-on-deck (SOD) are usually specified using only FF because the potential deflection of the deck under load while placing concrete makes it hard to control the Fl number.
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