Strange as it sounds now, back in the 1970s, getting a concrete floor into a “flat mode” was at times measured as “no more than an 1/8 inch difference in 10 feet,” or for the old timers, “throw a nickel on a wet floor, and if the water doesn’t bead over the top, it’s okay.”
Around that time, The Face Company is credited with the first understanding of how to accurately specify, control and measure flatness and levelness of concrete floors on grade. While at the time I was not sure what a superflat floor represented, I got to work with Sam Face on a distribution center in Denver, sorting through all the means and methods.
One may ask, “Why does the floor have to be so level?” The main reasons have to do with high storage order picking equipment, maintenance of floors in a retail environment and the installation of racks and fixtures. Most floors today are classified in “F” numbers extending from zero to infinity. The higher the F number the flatter the floor.
- The F-Number System is now the American Concrete Institute standard for the specification and measurement of concrete floor flatness and levelness.
- There are two F-Numbers: FF for flatness, and FL for levelness. “Flatness” relates to the bumpiness of the floor, while “levelness” describes the local tilt or pitch of the slab.
- The higher the F-Number, the better.
- 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: FF 25 / FL20. Remember, because of deflection, elevated slabs are usually specified using only FF.