Dimensional Tolerances in Construction and for Surface Accessibility

F - number system
The F - number system, ASTM E 1155, Standard Test Method for Determining FF Floor Flatness and FL Floor Levelness Numbers, was developed primarily to aid in the construction of superflat industrial floors. When tolerances are specified using the F - number system both the overall levelness and the flatness can be defined. The flatness number also gives an indication of the "bumpiness" of the surface.

The F - number system develops two number ratings, the FF and the FL. The FF defines the maximum floor curvature ed over a 24 - inch (600 mm) length computed on the basis of successive 12 - inch (300 mm) elevation differentials. The FL defines the relative conformity of the floor surface to a horizontal plane as measured over a 10 - foot (3.05 m) distance. Statistical sampling procedures are used to determine a floor’s [sic] F - numbers. F - numbers are reported as two numbers such as FF30/FL24. The higher the number, the flatter and more level the floor.

There are several methods given in ASTM E 1155 that can be used to measure a floor and develop the F - numbers. However, in practical terms, sophisticated electronic measuring devices developed specifically for this purpose are used. They are expensive and require some amount of training or a testing agency can be employed.

Although direct equivalents are not appropriate, an FF of 25 approximately correlates to a ¼ inch variation under an unleveled 10 - ft straightedge. An FF of 50 approximately correlates to a 1/8 - inch variation under a 10 - ft straightedge. Other approximate correlations are shown in the following table.

Concerning issues of vibration and rollability, the F - number system is probably a better measure than the straightedge because it takes into account local variations of flatness. Although it was developed to measure level floors, it can be used measure sloped floors.

For slabs - on - grade the F - numbering system works well. However, to determine the F - number for levelness of suspended slabs, measurements must be taken within 72 hours of floor installation and before shoring and forms are removed. For elevated slabs under current standards, the specified levelness and flatness of a floor may be compromised when the floor deflects when the shoring is removed and loads applied. However, local variations that could affect vibration and rollability would probably not be affected to any significant degree by slab deflection.

Additional limitations with the F - number system are that the measurements do not cross construction joints and only come within two feet from penetrations. Construction joints as well as other types of joints can affect vibration and rollability. The F - number system is optional as part of ACI 117 - 06.