PDB Debrief: Floor Flatness & Floor Levelness

March 2022

Floor Flatness (FF): The FF number essentially tells you how flat your concrete floor is. Bumps and grooves cause your Floor Flatness numbers to drop. Higher F-numbers equal flatter floors. Floor Level-ness (FL): The FL number indicates if you have any tilts or pitches in the slab. FF is directly impacted by the troweling operations and FL is directly impacted by the strike-off operations.

First and foremost, specifications should be reviewed during design. These numbers are important on all projects, but especially so in areas where floors are constantly subjected to heavy traffic, high weights, or the need to move items over the floor. A flatter floor is needed in a large warehouse more than in a standard office setting. Specifications may be written for much stricter floor flatness than is needed for the application of materials and use of the space.

Even though ACI recommends testing within 72 hours, our best practice is to test within 24-48 hours, and that is what is found in Division 03 specs. The longer we wait to test, the more the edges will curl.

However, our flooring subs will remeasure prior to installation of the floor as well (Division 09 specs). There may be discrepancies due to the elapsed time between these two measurements, and the methods used to make the measurements. It is also worth noting, even if we meet the specified FF/FL numbers, we may not meet the flooring manufacturer’s tolerances, This is a common gap and is often missed, which can cause the need to add a floor leveling system, adding time and cost. It is also worth noting that these manufacturer’s tolerances will be measured by the variance over 10’, rather than the FF number.

Floor levelness can affect installation of flooring, but also equipment, and can affect the ability of the end user to use their space functionally. If the client needs to be able to move furniture or fixtures around a space, the more level it is, the easier that task will be. An uneven floor could also cause unwanted sliding or gravitational force on items placed on the floor.