Q&A Column


What are the advantages of using F-numbers for specifying floor flatness?


Older specifications commonly specified a flatness tolerance of 1/8 inch in ten feet. While this standard was typically found in contract and guide specifications, it was rarely tested or evaluated. The performance of the floor system installed with conventional concrete placement techniques generally met the requirements of the owner and architect for most commercial applications. However, the majority of floors rarely complied with this standard. Accordingly, when the flatness of a floor system was challenged, serious conflicts would likely occur.

The F-number system provides greater precision in defining floor finish requirements to match project needs. The system consists of two values defined by ACI 302 as follows.

  • FF (flatness) defines the relative degree to which a surface conforms to a plane.

  • FL (levelness) defines the relative degree to which a surface is horizontal.

Under normal conditions, FF and FL values generally fall in a range between 12 and 45. Statistical data have been collected using the procedure described by ASTM E1155 - Test Method for Determining Floor Flatness and Levelness Using the F-number System on floor that have demonstrated satisfactory performance for various occupancies. Typical FF/FL values are as follows.




offices with fixed partitions, school classrooms, retail 20 17
offices with precut movable partitions 30 25
gymnasiums with special floor systems 40 30
ice rinks, television studios, narrow aisle warehouses 100 50

The old standard of 1/8 inch in 10 feet is equivalent to an F-number of 100, used only for super-flat floors.

The specifications may require prebid and construction meetings when floor tolerance requirements can be discussed with contractors. The specifications may also prequalify contractors to ensure they have successful experience installing floors with the flatness and levelness required by the project. A mock-up may also be specified to verify ability and to establish standards for the installation. Specifications must clearly define testing procedures to verify performance; it is recommended that surface measurements be made within 24 hours after the slab is placed.

The F-number system provides a rational means for specifying floor flatness and levelness, and can be tailored to the tolerance required for the project. Using the F-number system will generally produce an economical floor system with realistic tolerances that can be met by the contractor and measured by the architect.

Answer submitted by Dan Murphy, CSI, P.E., Meyer, Borgman and Johnson