Maxx Minutes Blog

Floor Flatness vs. Floor Leveling

An important consideration during a project’s design phase is what the end-use will be and what finished floor goods will be used. Though these factors may seem arbitrary, they actually have a lot to do with floor  assembly’s installation, specifically where specific floor flatness and levelness requirements need to be met.

What is the difference between floor flatness and floor levelness?
When a building is constructed, no floor is perfectly flat. There will be undulations in the floor that occur for a variety of reasons, including building joints, shifts and settling in the building, etc. Also when concrete is installed, it will curl to some degree, inevitably creating slight waves in the floor.  Floor flatness refers to this bumpiness or waviness. Floor Flatness (FF) is important when considering what finished floor goods will be installed, because hard surface finished floor goods will telegraph floor undulations.   Floor Levelness (FL) describes the tilt/pitch of the floor in a 10’ average area and is an important factor where drainage needs to be accounted for or in commercial facilities where heavy machinery or high shelving will be placed. 

How are floor flatness and levelness determined?
Floor flatness and levelness are determined in one of three ways.

  • F Numbers – F Numbers were adopted by the American Concrete Institute in 1990 because of their ability to effectively identify and measure the undulations of a concrete floor. F-Numbers are derived from a statistical analysis of the floor's elevation measured at one-foot intervals.  The elevation differences over two feet are used to determine FF (Flatness), while the differences over ten feet are used to determine FL (Levelness).  While this method is essential for concrete floors with flatness criteria, it is not necessary for the finished floor goods industry.
  • Surface Waviness Index – Developed primarily to measure floor surface wavelengths from 2 to 10 feet because they are the undulations that most impact a forklift’s ridability at typical speeds on floors designed for random vehicular traffic. A digital flushness gauge is slid in a planar fashion over a surface. Depth readings are taken at regular intervals along a scaled axis and then computed to determine the degree of waviness.
  • Straightedge Approach – The straightedge method is performed by laying a 10’ straightedge at a random point and then measure the gap under the straight edge. This method is preferred by the floor covering industry for its simplicity and its ability to measure levelness across construction joints and near floor penetrations.

There is no direct equivalent between the three systems of measurement; however some rough correlations can be made. (Figure 1)

Figure 1: Flatness Measurement Systems Loosely Compared

Floor Surface Classifications

F-Number Specified Overall Flatness (FF)

Surface Waviness Index (SWI2-10)

Straightedge Approach





Moderately Flat








Very Flat




Super Flat




When determining if specific flatness criteria should be specified and if so what measurement system should be used, it is helpful to know the typical floor criteria used throughout the flooring industry. (Figure 2)

Figure 2: Typical Floor Industry Flatness and Levelness Tolerances


Flooring Type

Floor Finish




American Concrete Institute

Thick-set tile


FF 20

FL 15

All concrete slabs




FF 25

FL 20

All concrete slabs


Thin-set flooring


FF 35

FL 25

Slabs on grade


Thin-set flooring


FF 20

FL 15

Elevated slabs

American Concrete Institute

All flooring types

Troweled finish

FF 20
5/16” in 10’

FL 15

For slabs specified as troweled finish

American Society for Testing & Materials

Resilient flooring


3/16” in 10’


Requires no defects that telegraph through

Tile Council of America

Thin-set tile

Hard trowel/broom

¼” in 10’;
1/16” per 1’



National Terrazzo & Mosaic Association


Broom finish

¼” in 10’




Sand cushion

Float finish

¼” in 10’




Epoxy terrazzo

Light steel trowel

FF 30/15

FL 20/10


Resilient Floor Covering Institute

Resilient flooring

Hard trowel/smooth

5/16” in 10’



Carpet and Rug Institute





Has no requirements

Maple Flooring Manufacturers Association

Gym floors

Troweled smooth

1/8” in 10’



Wood Flooring Manufacturers Association

Gym floors

Good float finish

¼” in 10’