The F-Number System is the industry-adopted standard for the specification and measurement of concrete floor flatness and levelness. F-Numbers, as they are called, extend in range from zero to infinity and communicate the way a floor measures—the higher the F-Number, for example, the flatter the floor. This flatness/levelness is notated as Ff for flatness (a measurement representing the bumps or grooves in the floor) and Fl for levelness (a measurement relating to the tilt or pitch of the floor).

Why is this important? With passing time, joints and cracks in a concrete floor will curl, making the floor less flat. Floor flatness and levelness should be measured within 72 hours of placement, but preferably within 24 hours. If that window of time is missed, any Ff/Fl numbers acquired may not accurately represent the flatness of the floor.

Floor flatness (Ff) is written first, such as Ff/Fl. The Ff number represents the flatness of your concrete floor and the Fl number indicates the tilt or pitch of your floor. This levelness indicates how closely your finished floor matches the intended slope as described in the original design documents. To properly measure Ff/Fl, elevation differences need to be measured every 10 feet within a 72-hour timeframe after concrete has been placed. Fl numbers are linear, so an Fl of 50 represents a floor that is twice as level as a floor measuring an Fl of 25. Taking into consideration the amplitude and wavelength of a particular floor, Ff numbers illustrate how close to planar the floor actually is; it is a statistical representation of the waviness or bumpiness of a floor. Changes in elevation are measured every foot within 72 hours of the concrete being placed, and use of a standardized formula determines the resulting Ff measurement. As is the case with Fl, Ff measurements are linear—the higher the number, the flatter the floor. For example, a floor with an Ff measurement of 80 is twice as flat as a floor with an Ff of 40. Ff numbers can be attained on slabs on grade as well as elevated slabs. The floor flatness is critical for spaces where equipment needs to be set perfectly level—or for any intended use that requires a floor to be flatter than a typical concrete slab.  

When we refer to floor levelness (Fl), we are generally referring to slabs on grade. These particular slabs are often designed with camber, which later becomes a concern when support staging is removed, as the result is typically a sagging floor. Fl is only specified on an elevated slab in situations where measurements have been taken in the beginning—before shores and forms are removed—and so long as the slab contains no camber. When a slab has camber, the structural slab must be placed first (and allowed to deflect to its final shape) and followed with a topping finish slab that then gets measured. At this point, the resulting floor levelness (Fl) measurement—which measures the elevation difference along a sample line at 10-foot intervals—communicates the separation of the floor from the original slope or plane.

Ff/Fl numbers have the following floor flatness classifications:

It can be very useful for architects and owners to visualize the defects represented by Ff numbers. Many were/are accustomed to the straightedge method, which consisted of measuring the greatest defect along a 10-foot straightedge. These values provide a general estimate of defect level with correlation to Ff number:

One thing to understand is that just because a specified floor was signed off on and paid for doesn’t always mean it measures as flat as it is supposed to. The only way to be sure is to properly measure floor tolerances using state-of-the-art equipment.

With the rapid advancement in machinery, robotics, and complex logistical systems, superflat floors have become critical for successful and efficient operations. Achieving a superflat floor can determine what kind of racks and logistics systems will be utilized during operation and will make it possible to acquire the most advanced of those systems. It will also impact the amount of maintenance required on existing equipment and eliminate unnecessary spend on repairs. It provides new opportunities for industrial plant and facility owners to implement modern robotics and rapid picking systems, knowing they have the right floor to sustain those advancements. A superflat floor also: