FF (Flatness) and FL (Levelness) numbers are a quantitative way to describe how flat and how level a concrete slab is, and they matter because coated, polished, and self-leveling floor systems all depend on a predictable, measurable substrate. The numbers come from a standardized measurement method, not from a visual inspection, which is what makes them useful in a written specification or a contractor's scope of work. This guide is for the facility manager, property manager, or owner's representative who is reviewing a bid for polished concrete, a resinous coating, or a self-leveling overlay and wants to understand what FF/FL numbers mean, how they are tested, and where their usefulness ends. It covers the definitions, the test method, how out-of-tolerance slabs get corrected, and how to ask for flatness in a new-slab specification.
FF and FL are two separate numbers that describe different things about the same slab surface.
Both numbers are dimensionless — they are not a measurement in inches, mils, or percent, but an index derived from the statistical method defined in ASTM E1155, which covers a quantitative method of measuring floor surface profiles to obtain estimates of a floor's FF and FL numbers. Higher numbers indicate a flatter or more level floor; lower numbers indicate more waviness or slope.
Because FF and FL describe geometry only, they are reported together as a pair (for example, "FF 35 / FL 25") and should always be read that way — a flat floor is not necessarily level, and a level floor is not necessarily flat.
ASTM E1155 is the test method used to generate FF and FL numbers. It covers a quantitative method of measuring floor surface profiles, and its results are used primarily to establish whether a randomly trafficked floor meets a specified FF/FL tolerance, to evaluate how a construction method affected flatness and levelness, and to investigate curling or deflection of the surface.
ASTM E1155 defines the procedure for measuring the surface profile and computing FF and FL. A few points worth confirming with whoever is doing the testing:
One important limit on this method: ASTM E1155 states that its results should not be used to enforce flatness and levelness tolerances on floors primarily intended to support fixed-path vehicle systems, such as narrow-aisle warehouse floors, where a profile run along the actual travel path is suggested instead. If your facility uses fixed-path vehicle systems such as narrow-aisle warehouse equipment, this distinction belongs in your scope of work — see the discussion of flooring for warehouse and distribution center facilities for that building type.
A slab's FF/FL numbers describe the substrate that a coating, polish, or overlay system will sit on top of. Coating and polishing crews work with the surface profile and condition they are given.
Concrete surface preparation work includes profiling, grinding, patching, and joint repair. For polished concrete specifically, the existing slab becomes the finished floor; see the Colorado polished concrete contractor hub for how that process is scoped.
What your written scope should state:
When a slab doesn't meet the flatness or levelness called for in the project specification, correction should be priced and documented as a distinct scope of work from the coating or polishing itself.
Questions to ask a contractor about correction work:
It can help to compare scopes using a tool like the FloorSpec cost calculator, and to review the Concrete Surface Preparation Cost in Colorado and Polished Concrete Cost in Colorado guides when budgeting for both the prep and finish phases of a project.
For new construction, FF/FL values are written into the project specification and the as-built slab is then tested with ASTM E1155 to check compliance with them. ACI's "The Floor Flatness Report" (Concrete International, January 2011) reproduces a table, citing ACI 302.1R, of typical values; these were not re-checked against the current edition.
| Application | FF | FL |
|---|---|---|
| Mechanical rooms, nonpublic areas, thick-set tile, parking structures | 20 | 15 |
| Carpeted commercial office areas | 25 | 20 |
| Thin-set flooring or warehouse floor, moderate/heavy traffic | 35 | 25 |
| Warehouses with air-pallet use | 45 | 35 |
These are typical values cited for general applications, not requirements for any specific resinous or polished-concrete system, and they should not be read as a manufacturer's substrate tolerance. The same article notes that overall FF and FL are specified alongside local minimum values, often set at 60% of the overall numbers, and that testing is done within 72 hours of finishing the slab. ACI 302.1R itself, which covers construction of slab-on-ground and suspended-slab floors for industrial, commercial, and institutional buildings including flatness and levelness requirements and measurement, is the broader construction guide — confirm the current edition's actual table with your design professional rather than relying on a reproduced table.
What a new-slab specification should include:
FF and FL numbers describe surface geometry only. ASTM E1155's scope is limited to measuring floor surface profiles — it does not address moisture conditions, surface strength, slip resistance, or how a coating or flooring system will perform once installed. A slab can have excellent FF/FL numbers and still have moisture or surface-contaminant issues that affect a coating, or poor FF/FL numbers that have nothing to do with its moisture condition.
Flatness testing and moisture or surface assessment are separate steps during concrete surface preparation. Ask your contractor how flatness, moisture, and surface profile will each be assessed and documented, and ask to see each report separately rather than a single summary conclusion.
Also remember that ASTM E1155 itself does not set the required FF/FL values for your project — those come from the project specification, informed by guides such as ACI 302.1R. The standard tells you how the number is measured, not what number your floor needs to hit.
FF describes how flat or wavy the surface is, based on elevation changes measured over short distances across the slab. FL describes how level the slab is, based on slope measured over a longer span. A floor can score well on one and poorly on the other, so both numbers are normally reported together.
No. FF/FL numbers describe surface flatness and levelness only; the test method they come from does not address moisture, surface strength, or slip resistance. Ask your contractor how moisture and surface profile will be assessed separately from flatness.
That should be defined in your contract and written scope before work begins, including which party pays for correction and retesting. Ask whether the correction work and the retest will use the same method and measurement pattern as the original test.
ASTM E1155 specifically states its results should not be used to enforce flatness tolerances on floors primarily intended to support fixed-path vehicle systems, such as narrow-aisle warehouse floors. For those floors, a profile measurement run along the actual travel path is suggested instead, so this should be addressed directly with your contractor and specification writer.
Guidance in the ACI Concrete International article describes testing being done within 72 hours of finishing the slab. Ask your testing provider to confirm their timing and whether it matches what your project specification requires.
Not necessarily; the appropriate FF/FL target depends on how the floor will be used. Published guidance cites different typical values for different applications, such as carpeted office areas versus warehouse floors with heavy traffic, so the right target is specific to your facility's use rather than a single universal number.
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