The world may not be flat but concrete floors are becoming flatter, to the point of becoming “super flat.”

The measurement of floor tolerances for “flatness” is being forced away from the era of the ruler and 10 foot straightedge into the era of the computer. The old way of determining floor flatness was accomplished by placing a 10 foot straightedge on the surface of the concrete and then measuring the gaps between the straightedge and the concrete with a ruler. The most common tolerance is “1/8 inch in 10 feet.”

Now there’s new technology that tosses the ruler and lets you measure floor flatness with speed, precision, and incredible accuracy.

Today's flat floors are found in more than ultra-sophisticated warehouses using unmanned forklifts, where the goods and parts may be stacked on shelves 20 feet or higher. Flatter and more level floors have been shown to be lower maintenance and provide an improved operational surface for many types of businesses. As a result, the demand for flatter floors is being seen in many different types of projects, from warehouses to retail store operations. This article provides you with important fundamentals about how flat floors are achieved and how they are measured.

To avoid the problems inherent with the old straightedge approach, a rigorous new statistically-based system was described by Allen Face in his 1985 Concrete International article “Specification and Control of Concrete Floor Flatness.” The birth of the F-number system was thus announced, and ACI adopted it in 1989.

Those mysterious F-numbers

The F-number system addresses both floor flatness and floor levelness. FF is the flatness number and defines the floor's characteristic curvature over 24 inches. FL is the levelness number and defines the floor's characteristic elevation difference over 10 feet. Both F-numbers are computed from successive point elevation measurements on 12 inch centers, and both numbers extend from zero to infinity: the larger the number, the more “flat” or “level” the floor. The system is perfectly linear so that a FF 30 / FL 24 floor is exactly twice as flat and twice as level as a FF 15 / FL 12 floor.

Don’t read this unless you’re a contractor

To avoid boundary problems, measurements are not taken within 2 feet of construction joints or other full-depth penetrations. To fairly gauge the installer's compliance with contract tolerances, measurements must be taken within 72 hours after finishing. If I were a concrete contractor, I would want the measurements taken the same day and before the curing compound is applied, since the floor will be its “flattest” right after it has been finished. With ongoing moisture loss and autogenous shrinkage, curling starts immediately and will continue to worsen for as long as 2 years, with the F-numbers decreasing accordingly.

Testing, Specifying, and Guides

Anyone specifying, installing or testing floors should be familiar with the following references:

  • ASTM E 1155: Standard test method for determining floor flatness and levelness using the F-number system;
  • ACI 117: Standard specification for tolerances for concrete construction and materials;
  • ACI 302-8.15: Guide for concrete floor and slab construction.

To guarantee proper specification preparation, a comprehensive sample F-number specification has been provided at

The sample elevation readings should be collected and analyzed in accordance with ASTM E 1155. Basically, straight measurement lines are placed uniformly across the entire test section and are best oriented at a 45o angle to the longest construction joint. Point elevations are then measured at 12 inch intervals along each line. A minimum number of readings must be collected according to the square footage being tested. For areas less than 1,600 square feet, the minimum number of 10-foot elevation difference readings must equal 2 times the square root of the area. For areas greater than 1600 square feet, the minimum number of 10-foot elevation difference readings must equal the area divided by 30. From these data, the floor flatness FF and floor levelness FL can be calculated.

In addition to authoring the system, Allen Face also designed the three instruments most commonly used for F-number testing: the Dipstick (1982), the F-Meter (1993), and the D-Meter (2004).

The F-number system requires the specification of two separate sets of F-numbers: Overall (OA) F-numbers and Minimum Local (ML) F-numbers. Overall F-numbers apply strictly to the entire project floor area and no sub-division thereof. Minimum Local F-numbers apply strictly to the small individual rectangles defined by the column and half-column lines, or the construction and control joints, whichever is smaller. The proper numeric relationship between these various specified values is fixed: OAFF/MLFF = OAFL/MLFL = OAFF/OAFL = MLFF/MLFL = 5/3. If the OAFF is specified at 50, for example, then the other values should be specified at MLFF 30, OAFL 30, and MLFL 18.

Skill and equipment make the difference

Resulting floor flatness and levelness are obviously determined by the skill of the concrete forming, placement and finishing crews, as well as the quality of their equipment. The FL is most affected by form quality and the method of strike-off. Forms should be straight and sturdy to resist bowing and warping. Placement is affected by the manner of screeding (leveling) selected. Wet screeding without rails, is typically capable of producing FL values no higher than 18. Screed rails and vibratory screeds can achieve FL's in the mid to high 20s. Laser screeds can reach the 40s or 50s.

In contrast, FF is most affected by the finishing operations. Pan floating can increase the FF by 5 to 10 points, and re-straightening the floor – especially after floating- with a long-handled straightedge can add 20 or more FF numbers.

Daily F-number testing is the most effective means available for improving results, motivating the crew, and giving the owner what he wants. In contrast, the failure to monitor floor flatness daily results inevitably results in no learning, repeated disappointment, and costly conflict.

More information online

For more information on F numbers and flat floor measurement, please refer to Alan Face's article “Effective F Number Specification” published in Concrete News, Winter 2005, It can be downloaded at, Concrete News archives.

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© 2008 L&M Construction Chemicals, Inc. | ConcreteNews September 2008.

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