Beyond the Build

Continuing Education

F-Numbers and Concrete Flatness

We recently we had a project where very tight tolerances for the floor finishes were important. I thought I would share what this measuring system is all about, and give a quick summary on how to get there.

Until the 1980s, concrete flatness was measured as “in 1/8th inch in ten feet,”  the gold standard up until that time. When I first got in this business, old-timers would throw a coin on the slab and then wet the area. If the water did not cover the coin, the slab was flat enough. It seems funny now, but that was the way it was.

These days, a laser screed is used to insure the flatness. In this process, a laser beam is rotated around the building, setting a standard level elevation. Receivers read the beam and confirm that the screed is at the correct elevation for the floor flatness requirement specified.

Why do we care so much about flatness? A flat floor is important to the installation of fixtures and maintenance of tiled floors, demountable partitions and high rack order picking equipment. There are checks and balances to ensure a floor is flat enough for the long-term health of the structure.


The F-Number System from the American Concrete Institute (ACI) is now the standard for measuring floor flatness and levelness of concrete slabs. Floor Flatness, abbreviated FF, is a measure of how bumpy the floor’s finished surface may be, and thus the magnitude of slope changes. The Floor Levelness (FL) number is a measure of the inclination of the floor compared to its design inclination, which may not be horizontal.

Floor Flatness and Levelness will depend on:

  • Effectiveness of the specific placement and finishing procedures.
  • Initial strike-off phases of the installation and subsequent finishing operations like floating, straightening and troweling.
  • Workability, finishability and setting times of concrete to be used.
  • Sun, wind, rain, temperature and other exposure conditions and their effects on personnel and concrete.
  • Timeliness of concrete and consistency of concrete.
  • Curing from the top and bottom of the slab.

Our first work in this area of flat floors was with The Face Companies in the latter part of the 1980s. With their initiatives, easy and accurate systems of measurement have been developed.




Merrill Stewart Jr.

Merrill Stewart is Founder and CEO of The Stewart/Perry Company, a commercial building contractor based in Birmingham.