Concrete Floor Flatness

Traditionally, requirements for concrete slab flatness were expressed as the maximum allowable gap under a 10-foot straightedge. For example, variation in an architectural concrete slab might be limited to 1/8 inch in 10 feet. Slabs to receive resilient flooring materials might be limited to 3/16 inch in 10 feet, and utilitarian slabs might be limited to 1/4 inch in 10 feet. While this standard may still be used on small projects, flatness requirements for most concrete slab work today are specified using the newer ASTM E1155 Standard Test Method for Determining FF Floor Flatness and FL Floor Levelness Numbers.

ASTM E115, commonly called the F-number system, uses more precisely defined methods to establish requirements for both "flatness" and "levelness" of concrete floors. Flatness refers to the slabs waviness. Levelness refers to the slabs deviation from horizontality.

While the F-number system is more precise and more consistent in its results, it is also less intuitive. For example, requirements for a concrete slab to receive resilient flooring might be expressed as follows (higher number indicate closer tolerances, that is, a flatter, more level floor):

Flatness: Specified Overall Value (SOV): 35; Minimum Local Value (MLV): 24
Levelness: SOV:  35, MLV: 24

The specified overall value (SOV) is a value representing the floors overall flatness or levelness. The minimum local value (MLV) reflects deviations in flatness or levelness within any limited area. Though there is no direct correlation between straightedge measurements and F-number system values, the MLV for flatness is the closest measure comparable to the traditional straightedge measurement. According to ACI 117, F-number flatness (Ff) values correlate approximately as follows to the traditional straightedge measures:

Ff12:	1/2 inch in 10 feet
Ff20:	5/16 inch in 10 feet
Ff25:	1/4 inch in 10 feet
Ff32:	3/16 inch in 10 feet
Ff50:	1/8 in 10 feet

However, the above comparisons should be approached with caution. According to ACI, slabs specified with the traditional straightedge method generally do not meet the specified requirements. Therefore applying, for example, an F-number flatness of 50 where previously 1/8 in 10 feet was required may result in an unrealistically strict requirement.

From the perspective of an architect/specifier, working with the F-number system presents a number of issues:

• For most of us, F-numbers are more difficult to interpret than the traditional straightedge standard.
• F-number results, while more accurate, are more difficult to obtain. Testing a slab according to the F-number system requires specialized equipment and trained operators.
• According to ASTM E1155, F-numbers are measured within 72 hours of concrete placement. Therefore, they do not necessarily represent the final condition of the slab after more complete curing.

As one example of how these issues can affect a project, the American Society of Concrete Contractors, ASCC Position Statement #6, makes the point that F-number requirements established in the concrete sections of the project specification do not ensure that such slabs will necessarily meet the requirements of floor finish materials specified in other sections and installed later in the construction phase. This is true because floor finish manufacturers do not publish flatness requirements using the F-number system, and as noted earlier, F-numbers are measured early in the concrete curing process and do not necessarily reflect the final state of the slab after more complete curing.

In short, proceed with caution when attempting to set establish appropriate tolerances for concrete slab construction.

Further Reading:
Concrete Slab/Floor Covering Issues, on this site, discusses an inter-industry report on concrete floor issues, including flatness requirements.
The textbook discusses concrete slab construction beginning on page 505. Slab flatness is discussed on page 508.
The American Concrete Institute publishes technical papers on the use of the F-number system.
ASTM E1155 can be purchased from ASTM International.
At the time of this writing, The Face Companies - The 40 Most Asked Questions about F-Numbers is a good FAQ on the F-number system.