F-numbers are used in specifications for floors. The FF number is an indicator of floor flatness or smoothness, while the FL number indicates floor levelness. The higher these numbers, the flatter or more level the floor. The ASTM E 1155, Standard Test Method for Determining Floor Flatness and Levelness Using the F-Number System, describes the system and measurement method. The system uses floor surface curvature over a 24-inch distance as a measure of flatness, and floor slope over a distance of 10 feet as a measure of levelness.
On random traffic floors, a two-tier F-number system works best. The upper tier, called Specified Overall F-numbers, represents the minimum FF and FL values when all the individual floor sections measured are lumped together. Slab levelness, indicated by FL values, depends mostly on the contractor's accuracy in setting forms. Flatness, the FF value, is primarily affected by finishing operations after striking off. To meet many flatness requirements, finishers must restraighten or cut and fill the top surface of the concrete while it's still plastic. The best tool for this is a 10-foot or 12-foot highway straightedge used repeatedly during the finishing process.