What Are F-Numbers?
The F-Number System is the new American Concrete Institute (# 117) and Canadian Standards Association (# A23.1) standard for the specification and measurement of concrete floor flatness and levelness. F-Numbers replace the familiar "1/8th inch in ten feet" type specs that had proven unreliable, unmeasureable and unrealistic.
The new standards include two F-Numbers: FF for flatness and FL for levelness
Flatness relates to the bumpiness of the floor, while levelness describes the tilt or pitch of the slab. The higher the F-Number, the better that characteristic of the floor.
F-Numbers are linear, so an FF 20 is twice as flat as an FF 10, but only half as flat as an FF 40.
Slabs-on-grade are usually specified with an FF number and an FL number (the FF is always listed first), such as :
FF25 / FL20
Because of deflection, elevated slabs are usually specified using only FF.
When a floor is described as an "F 25", it usually means "FF 25".
The ACI/CSA F-Number System applies to 99% of all floor slabs - all floors that support random traffic, be it vehicular or pedestrian traffic.
In the tiny percentage of floors that have defined traffic, where vehicles are restricted in their movement by wire or rail guidance, a different F-Number - Fmin - is used. This System is used in conjunction with consultation services provided by The Face Companies.
Most Superflat floors should use the Fmin System, since most of these slabs support defined traffic.