F-numbers provide architects and contractors a method of determining the flatness and levelness of a concrete floor slab. They are calculated using the standards set forth in ASTM E1155, which is the Standard Test Method for Determining FF Floor Flatness and FL Floor Levelness Numbers. The American Concrete Institute indicates acceptable ranges for flatness and levelness in ACI 302.1, Guide for Concrete Floor and Slab Construction. Architectural specifications will identify the acceptable FF and FL numbers for the project so architects must understand the limitations of concrete slab installation.
History of Concrete Floor Flatness and Floor Levelness
Traditionally, concrete floors were specified to have a deviation of less than 1/8" over 10-feet. This was measured by laying a 10-foot straightedge on the finished floor and measuring the greatest gap below the straightedge. This method worked quite well for decades. However, the method also proves to be unreliable and is prone to errors since no two people will ever get the same measurement. In addition, 1/8" over 10-feet was rarely achieved with the equipment available in the past.
With the introduction of high-stacking, narrow-aisle warehouses in the 1970s it became more important to have concrete floors that were much flatter than in the past. More recent warehouse technologies like air-pallet jacks and new technologies developed for TV studios have since created the need for even flatter floors. As the technologies developed, very flat and super flat floors were needed.
In 1979, Allen Face developed the F-number system, officially called the Face Floor Profile Numbering System, which was later formalized into the ASTM E1155 and the ACI national standards. He later developed the Dipstick® Floor Profiler and the F-Meter®, the tools needed to take more accurate measurements than the straightedge method.
The new F-numbers are more accurate than the measurements taken with a straightedge since the profiling machines take a measurement every foot in multiple perpendicular directions when measuring floor flatness. Large slabs require hundreds of measurements to achieve FF and FL numbers — 34 measurements are taken for each 1,000 square feet of concrete slab. The gathered measurements are then entered into a mathematical formula to derive the overall F-numbers. While it is likely that two people will come up with different measurements when using the straightedge method, two people using modern measuring devices should end up with very similar F-numbers.
Floor Levelness (FL)
FL numbers provide information about the concrete floor's levelness. Levelness depicts how closely the finished floor matches the intended slope indicated in the design documents. Elevation differences are measured every 10-feet within 72 hours after the concrete is placed and those measurements are entered into a calculation to determine floor levelness (FL). Higher FL numbers indicate a more level floor and the numbers are linear so a FL of 50 is twice as level as an FL of 25.
It is important to note that FL numbers generally only apply to slabs that are placed on grade. Elevated concrete slabs are problematic since those slabs tend to have camber built into the design and the floor slabs will generally sag once the support staging is removed. Therefore FL numbers are only specified on elevated slabs when measurements are taken before shores and forms are removed and the slab has no camber. In the case of an elevated slab that has camber, the FL the structural slab must placed first (deflecting to its final shape) followed by a topping finish slab that gets measured for Floor Levelness.
Floor Flatness (FF)
FF numbers depict the Floor Flatness, or how close to planar the floor is. In other words, Floor Flatness is a statistical measurement of how wavy or bumpy a concrete floor is and takes into account the amplitude (height if the waves) and the wavelength (horizontal distance between waves). Elevation differences are taken every foot within 72 hours after the concrete is placed and a formula determines the FF measurement. As with FL, the measurements are linear and higher numbers represent a flatter floor. For example, a floor with an FFmeasurement of 60 is twice as flat as a floor with an FF of 30.
FF numbers can be taken for slabs on grade as well as elevated slabs. Floor Flatness tends to be very important for spaces where equipment must be set perfectly level — spaces like TV studios, warehouses that use air-pallet jacks, and some research laboratories require floors that are flatter than a typical concrete slab.
Since the old measurement method was to measure the greatest defect along a 10-foot straightedge, it is helpful to understand how straightedge numbers translate to FF. In addition, it is helpful for architects and owners to visualize how wavy or bumpy different FF values are. Interestingly, most slabs placed in the past 50 years fall in the range of FF 15 and FF 35. Keep in mind that there is no direct correlation between FF numbers and the old straightedge numbers, but these values provide a general estimate:
FF 25 will have a single 1/4" defect across 10-feet
FF 50 will have a single 1/8" defect across 10-feet
FF 100 will have a single 1/16" defect across 10-feet
Classifying Concrete Floors Based on Flatness and Levelness
Concrete professionals use specific terminology to classify floor flatness and levelness. According to ACI 117, random-traffic concrete floors are classified as follows. Keep in mind that levelness is only used for slab-on-grade placement.
| CLASSIFICATION | SPECIFIED OVERALL FLATNESS (SOFF) | SPECIFIED OVERALL LEVELNESS (SOFL) |
|---|---|---|
| Conventional | 20 | 15 |
| Moderately Flat | 25 | 20 |
| Flat | 35 | 25 |
| Very Flat | 45 | 35 |
| Super Flat | 60 | 40 |
Super flat floors require special skill and equipment to achieve and should only be used for the most critical of concrete floors in specialized programs like television studios. Super flat floors may also be specified to have FF 100 and FL 50, but these are are for defined-traffic (single direction of travel) installations such as narrow-aisle warehouses as opposed to a random-traffic floors that the ACI 117 standard covers.
Acceptable FF and FL Numbers Based on Use
The question that often comes up for architects is, "how flat should I specify the concrete floors to be in my project?" This is a very tough question to answer and architects can sometimes err too far on the side of caution and specify floors much flatter than necessary, which can add cost to a client's project. The good news is that procedures and equipment have greatly improved over the past 20 years and floors tend to be much flatter than they were without much added cost.
According to the American Concrete Institute publication ACI-302.1, the following FF and FL values are acceptable based on the uses listed. Keep in mind that FL values only apply to slabs on grade.
| USAGE | FLOOR FLATNESS (FF) | FLOOR LEVELNESS (FL) |
|---|---|---|
| Noncritical spaces, mechanical rooms, back-of-house, parking, areas to receive thick-set tile | FF 20 | FL 15 |
| General office, light industrial, carpeted spaces | FF 25 | FL 20 |
| General warehouse floors, areas to received thin-set tile, laboratories | FF 30-35 | FL 20-25 |
| Warehouses with air-pallet use, ice rinks | FF 45 | FL 35 |
| Movie and television studios | FF >50 | FL >50 |
How to Specify Concrete Floor Flatness
Concrete floor specifications include a section within Part 3 - Execution for Finishing. Part of that section includes acceptable tolerances for the concrete floors. The tolerances can also be broken out further by program type; for instance, a specification may identify one set of tolerances for a warehouse floor, a different set of tolerances for a television studio, and a third set of tolerances for general office space.
Within the program tolerances, two sets of values will be provided: Specified Overall Values (SOV) for FF and FL as well as Minimum Local Values (MLV) for FF and FL.
Specified Overall Values provide criteria for the entire project through average FF and FL values for all concrete floors on the project.
Minimum Local Values provide criteria for the minimum allowable FF and FL values for each section of concrete placed (or for each "pour") on a project. MLV readings can be recorded for an individual floor or multiple MLV readings can be taken on a large floor made up of multiple sections ("pours"). Minimum Local Value criteria will often be lower (less flat/level) than Specified Overall Values to allow for a margin of error during concrete placement.
The specifications will also dictate when and where measurements should be taken – this is often accomplished by referencing ASTM E1155, the Standard Test Method for Determining FF Floor Flatness and FLFloor Levelness Numbers. In general, measurements should be taken once the concrete is ready to accept foot traffic, but ASTM E1155 requires tests to be completed within 72 hours of placement. In addition, the tests are taken before shoring is removed from the slab.
Specifications may also dictate how out-of-spec floors should be fixed. In general, grinding a floor will only make things worse. Flashing or self-leveling compounds can help fix floors if the concrete will not be exposed. The worst-case scenario of removing an out-of-spec floor and replacing it is generally avoided due to schedule and financial implications. However, the owner may elect to receive financial compensation from the contractor if they decide to accept an out-of-spec floor. Architects should understand FF and FL values so they can advise their clients on the level of flatness required by the program and also so they can provide advice on acceptable as-built flatness and remediation methods, if necessary.
Further Reading
The Face Company has answers to The 40 Most Asked Questions about F-Numbers, which is written for contractors, but is also helpful to architects and specifiers if you'd like to go much more in depth about FF and FL numbers.
