FF/FL Numbers Explained: ASTM E1155 Floor Flatness Guide

What Are FF and FL Numbers?

FF and FL numbers are dimensionless statistical values that quantify how flat and how level a concrete slab actually is, based on measured elevation data rather than a visual inspection or a level held against the surface. FF measures floor flatness (localized bumpiness), and FL measures floor levelness (how much the slab tilts or drifts from a true horizontal plane). Higher numbers mean a flatter, more level floor.

The terms get used loosely on job sites, sometimes interchangeably, which causes real problems when a spec calls for one number and a contractor delivers the other. FF and FL are not the same measurement answering the same question. FF is asking: does this slab have ripples, dips, or trowel marks that create sudden elevation changes over a short run? FL is asking a completely different question: is this slab sloping or tilting over a longer distance, the kind of drift that would matter if you’re setting up a 40-foot-tall pallet racking run or an automated guided vehicle path?

The American Concrete Institute, in its publication “The Floor Flatness Report,” frames FF as an indicator of a slab’s bumpiness and FL as an indicator of how level the slab is, tied to slope measured over a distance of 10 feet (3 meters) and how well the concrete forms were originally set. That distinction, short-range bumpiness versus long-range slope, is the entire conceptual foundation of the F-Number system.

The Standard Behind the Numbers: ASTM E1155

ASTM E1155, “Standard Test Method for Determining FF Floor Flatness and FL Floor Levelness Numbers,” is the governing document for F-Number testing in the United States, published by ASTM International. It defines the quantitative procedure used to measure floor surface profiles and convert that raw elevation data into the characteristic F-Numbers referenced in project specs.

ASTM International publishes both inch-pound and metric (SI) editions, and combined editions such as E1155/E1155M have integrated both unit systems into a single document. That matters for cross-border projects: a distribution center built to Canadian requirements will often reference CSA A23.1, the Canadian Standards Association’s counterpart, alongside or instead of ASTM E1155, but the underlying F-Number logic is the same system either way. Somero’s “F-Numbers Handbook for Floor Flatness” confirms the F-Number system is the standard referenced by both ACI 117 (American Concrete Institute) in the U.S. and CSA A23.1 in Canada.

What ASTM E1155 does is give everyone on a project, owner, GC, concrete subcontractor, flooring contractor, a common, repeatable language for slab quality that doesn’t depend on who’s holding the straightedge that day. What it doesn’t do is certify structural adequacy, moisture condition, or long-term durability of the slab. It measures surface profile, full stop.

How FF and FL Are Actually Measured

FF/FL testing is done with a precision measuring device, typically a rolling or leveling instrument, that records elevation at fixed points along straight lines across a sample area, usually within 72 hours of the slab being placed and before it’s covered by another trade. The data collected feeds directly into the statistical formulas that produce the final F-Numbers.

LATICRETE’s Technical Datasheet TDS 233 describes the ASTM E1155 procedure as measuring points spaced 12 inches (300 mm) apart along straight lines within each defined sample area. Somero’s F-Numbers Handbook adds the mechanical detail that matters most for understanding what FF and FL are actually built from: elevation is recorded at one-foot intervals, and then two different comparisons are run on that same dataset. Elevation differences measured over a two-foot span are used to calculate FF. Elevation differences measured over a ten-foot span are used to calculate FL.

DT Industrial Flooring’s technical guide “Warehouse Floor Flatness: Ff, Fl & F-min” summarizes it the same way: FF evaluates bumpiness over a 24-inch baseline, and FL evaluates tilt or elevation drift over a 10-foot span. That’s the practical takeaway for anyone reading a test report, not a formula, but a physical distance. FF is a short-range check. FL is a long-range check. Both come from the same elevation survey, just processed at different scales.

This is also why F-Number testing has to happen at defined intervals across the slab rather than at a handful of spot checks. A single low reading in one bay doesn’t automatically fail the whole floor, but it does get flagged, because ASTM E1155 reporting is built around minimum local values as well as an overall statistical result.

Understanding the Math: What the Formulas Tell You

You don’t need to run the calculation yourself to read an F-Number report correctly, but knowing the logic behind it helps you interpret what a number like FF 35/FL 25 actually represents. PDH Online’s continuing-education course “Concrete Slabs and the Use of the F-Number System” lays out the formulas used to convert raw elevation-difference data into the final scores.

In plain English: less variability in the elevation readings, and a smaller average deviation, produces a higher F-Number. A slab with wildly inconsistent bumps, even if some spots are perfectly flat, scores lower than a slab that’s uniformly close to true, because the formula punishes variability, not just peak defects. That’s a meaningful distinction for contractors: chasing a few isolated high spots with a grinder won’t fix a low FF score if the rest of the slab is inconsistent.

PDH Online also describes the “Specified Overall Value” convention used in most project specs, written as something like FF 25/FL 20. That paired number is the minimum overall statistical result the entire tested area needs to hit, distinct from the minimum local value that applies to any individual measurement within that area. Reports typically show both.

Typical FF/FL Ranges by Facility Type

Required FF/FL values scale directly with what’s going to sit on the slab and how it moves. A warehouse with pedestrian pallet jacks tolerates far more surface variation than a facility running wire-guided AGVs or narrow-aisle reach trucks at height. The ranges below reflect commonly cited industry guidance, not a single universal code requirement, and actual project specs should always be set through ACI and ASTM guidance combined with project-specific engineering.

These figures, drawn from DT Industrial Flooring’s published minimum-requirement table, line up closely with Gradelog’s F-Number Calculator resource, which notes conventional warehouses typically need at least FF 25/FL 20, while high-tolerance floors around FF 50/FL 35 are common for distribution centers and food processing facilities. The two sources aren’t identical because “typical” ranges in this industry are guidance, not statute, shaped by equipment tolerances, rack manufacturer specs, and the owner’s own risk tolerance.

For GCs and flooring contractors, there’s a coordination point worth flagging here. LATICRETE’s TDS 233 addresses how Division 3 structural concrete FF/FL values need to correlate with Division 9 finish flooring requirements. A slab that passes its structural concrete spec under Division 3 can still create problems for a Division 9 flooring installer if the finish system (thin poured toppings, certain resilient flooring) has tighter flatness tolerances than the base slab was tested to. Coordinating those two specs before the pour, not after, avoids finger-pointing later.

What Happens When a Slab Misses Spec

An out-of-tolerance slab doesn’t announce itself immediately. It shows up later, often after the concrete trade has already left the site, when the racking installer can’t get uprights plumb, when a forklift operator reports a rough ride in one aisle, or when an AMR fleet starts throwing navigation faults in a specific zone. By that point, the remedy options are more expensive and more disruptive than they would have been at the concrete stage.

The standard remedies for a slab that fails its FF/FL spec are grinding down high spots, shimming to compensate for low spots or drift, or, in the most severe cases, re-pouring sections of the slab entirely. Each of those options carries schedule and cost consequences that ripple through every trade scheduled after concrete. Grinding generates dust and downtime. Shimming under racking or equipment can introduce its own long-term stability questions. Re-pours mean re-sequencing everything that was supposed to follow.

There’s also the dispute question, and it’s usually the most expensive part of a missed spec, not the physical fix. Without an independent, ASTM E1155-based test report generated close to the time of the pour, it becomes very difficult to establish whether the slab was ever within tolerance, whether it degraded afterward, or whether the original spec was even measurable to begin with. IFTI’s own field experience, echoed in DT Industrial Flooring’s technical guidance, ties inadequate FF/FL directly to operational risk: rejected or misaligned racking, degraded forklift and material-handling performance, and in facilities running robotics or AMRs, real questions about equipment performance and safety in the affected zones.

The action item here is straightforward: test before the slab is covered by another trade, not after equipment starts failing. A same-week or next-week F-Number report, taken while the concrete crew is still mobilized, gives everyone a documented baseline and a much shorter path to a fix if one is needed.

FF/FL and Moisture Testing: Two Different Questions

FF/FL numbers and concrete moisture testing measure two unrelated properties of the same slab, and conflating them leads to gaps in project quality control. FF/FL, under ASTM E1155, addresses surface profile: flatness and levelness. Moisture testing addresses whether the slab has cured and dried enough to accept a flooring system without trapping vapor that later causes adhesive failure or mold.

Moisture condition is governed by its own separate standards: ASTM F2170, which uses in-situ relative humidity probes inserted into the slab, and ASTM F1869, the calcium chloride test method. A slab can score a perfect FF/FL result and still be too wet to receive a moisture-sensitive flooring installation, and the reverse is equally true. Neither test substitutes for the other, and a comprehensive slab quality program on a distribution center or manufacturing floor generally includes both, run at the appropriate stage of construction.

Why Independent Testing Matters

An F-Number is only as trustworthy as the party that measured it. When the entity reporting flatness data has a financial interest in the outcome, whether that’s a concrete contractor certifying its own pour or a flooring manufacturer validating a substrate before installing its own product, there’s an inherent incentive problem, even when nobody involved intends to cut corners. Independent testing removes that variable entirely.

IFTI doesn’t manufacture, sell, or install flooring, leveling compounds, coatings, or racking systems. The only product is the ASTM E1155-based data itself, collected and reported without a stake in what the numbers say. That’s the practical reason architects, GCs, and facility owners bring in a third party specifically for F-Number testing rather than relying on figures supplied by whichever trade poured or is about to cover the slab: an independent report is evidence everyone on the project can rely on, including if there’s ever a dispute about who’s responsible for an out-of-tolerance result.

An FF/FL report is not a pass/fail guarantee, and it isn’t a substitute for project-specific engineering judgment. What it is: a precise, repeatable, third-party record of where a slab actually stands, at a specific point in time, measured against a defined standard rather than an opinion. That record is what lets a GC catch a flatness problem while the concrete crew is still on-site, instead of finding out three months later when a racking installer can’t get uprights plumb.