Concrete FF flatness scores and polished floor requirements

A polished concrete floor needs an FF flatness score of 50 or better to take a high gloss. Most commercial pours in Oklahoma come in at FF 25 to 30. That gap decides whether your slab takes a shine or fights it. No grinder or densifier can add flatness back once the truck has left. So the number goes in the contract before the pour, not after.

Flatness and smoothness are two different things, and the difference matters. Flatness is the best start you can hand a polishing crew. When that crew shows up, the slab either has the flatness it needs or it doesn’t. Nothing in the grind-and-polish process can create flatness that was never poured in. Fixing a low FF score means grinding the whole floor down to one even plane. That costs real money, changes how the floor looks, and may not work at all on a thin slab.

What does FF flatness score actually measure?

FF stands for Floor Flatness. It’s one of two numbers in the F-number system. That system comes from ACI 117, the American Concrete Institute’s rules for how close concrete work has to come to plan. The other number is FL, for Floor Levelness.

The two numbers measure different things. Mixing them up is one of the most common mistakes on polished concrete jobs.

FF (Floor Flatness) measures the small bumps and dips across short spans, from 12 inches to 10 feet. The higher the number, the flatter the floor. At FF 20, the floor has real high and low spots over those short spans. At FF 60, the height changes by only a few millimeters over the same spans.

FL (Floor Levelness) measures the slow tilt across the whole slab. A floor built to slope toward a floor drain has a low FL on purpose. A dead-level floor has a high FL. For polished concrete, FL matters for drainage and for running equipment. It has nothing to do with how well the floor polishes.

So a floor can be dead level, with a high FL, and still score FF 22 because it’s full of small bumps and ridges. That slab won’t take a high gloss.

Smoothness is a third idea, and it trips people up too. A troweled slab can feel smooth under your hand, with no rough patches or stones poking up. That’s texture. Flatness is the shape of the surface, its highs and lows. A slab can feel smooth to your palm and still have a 4-millimeter ridge every 3 feet, and that ridge shows up as a low FF score. The polishing grinder is 12 to 18 inches wide. It rides over the ridge, hits the high points, and skips the low ones. What you see later is bands of shine and dull across the finished floor.

Why do standard commercial pours only hit FF 25 to 30?

ASTM E1155, the standard test for F-numbers, doesn’t set a minimum FF for everyday flatwork. ACI 117 suggests FF 20 for formed floors and FF 25 for slabs on the ground with no special needs. General contractors spec to the code minimum. The concrete crew delivers FF 25 to 30, because that’s the number the inspector signs off on.

Nobody in that chain is thinking about polish. They’re thinking about strength, drainage, and not getting a call back about cracks. FF 25 passes every normal inspection for a slab on the ground. If you plan to polish that slab, FF 25 is a problem, and nobody upstream will flag it for you. The pour crew did exactly what the spec asked. The spec just never asked for a polish number.

The tools matter too. Hand screeding means dragging a straightedge across wet concrete to level it, so it rides on the crew’s skill and pace. On a big pour of 2,000 square feet or more, a good hand-screed crew might hit FF 30 to 40 on a good day. In an Oklahoma summer, the working window can shrink to 90 minutes at 95 degrees and up. Crews rush the flatness work to beat the set, and the slab often comes in at FF 22 to 28. Nobody did anything wrong there. The clock just ran out.

A laser screed works another way. A spinning laser sets one fixed height across the pour. The screed head moves up and down on its own to strip off extra concrete, to within 3 millimeters of that height. Laser screed often turns out FF 45 to 60 or better on the same size pour where hand screed gets FF 25 to 40. The machine costs more to rent and needs an operator, but that difference runs in the hundreds of dollars, not thousands. Fixing a low-FF slab before polish runs in the thousands.

What are the minimum FF flatness scores for polished concrete?

The finer the grit the polish stops at, the flatter the slab has to be:

When we pour a slab that will be polished, we spec FF 50 minimum. Full stop. That’s the target for “Even,” the E in SPEC, inside our Pour-4-Polish framework. It’s what the grinder needs to give an even shine across a whole floor with no fix-up grinding.

We didn’t pull those numbers off a spec sheet. They come from grinding and polishing hundreds of slabs across Oklahoma since 2004. We’ve watched the same grinder head skip the same low spots on the same kind of slab enough times to know where the line sits. Below FF 35, you’ll see bands where the grinder hit unevenly, and nothing stops them. Below FF 45, those bands show up any time light comes in low across the floor. Below FF 50, you won’t get a true high gloss without heavy fix-up grinding, and that grinding changes how the floor looks.

Writing flatness into a concrete contract

“Flat concrete” in a contract is nothing you can enforce. It’s a phrase two lawyers can argue about after the slab has cured. A flatness spec that’s ready for polish needs these five parts:

1. Name the minimum FF. Write it out: “Slab shall achieve a minimum FF 50 as measured per ASTM E1155 typically within 72 hours of final troweling.” Don’t leave it at “F-numbers per ACI 117,” which falls back to much lower numbers.

2. Name the test method. ASTM E1155 is the standard. A profileograph or F-meter moves across the floor and takes readings at set spacing. Require the reading before the polishing contractor shows up, not after.

3. Say who fixes a miss. “Any areas measuring below FF 45 shall be corrected by the concrete contractor at no additional cost before polishing work commences.” Without that line, you’re haggling after the fact.

4. Require laser screed on pours over 1,000 square feet. You can name the machine in the spec. Hand screed is cheaper and faster on small pours. On a big slab you plan to polish, laser screed is the right tool.

5. Add a reject line for added water. “Addition of water to concrete at the job site beyond the specified slump shall constitute a non-conforming pour.” Slump is how wet and runny the mix is. Water added on site softens the concrete. That hurts density, and it hurts flatness too, because a soft mix is harder to screed true.

What happens if the FF score is too low?

If the slab comes in at FF 28 instead of FF 50, you have three choices. None of them are cheap.

Choice 1: Fix-up grinding before polish. The polishing crew grinds the whole floor down to the low points, taking off enough concrete to cut away the high spots. At FF 28, that means 2 to 4 millimeters off the full floor. A slab that starts closer to FF 30 needs less, about 1 to 3 millimeters. The grind goes deeper into the stone in the mix, so the color and look of the floor change. On a slab thinner than 4 inches, that lost depth can cut into the strength of the slab. It costs $1.50 to $3.00 a square foot in extra labor and tooling, on top of the polish price.

Choice 2: Grind the worst spots and settle for less shine. The crew knocks down the obvious high spots and takes a semi-gloss finish instead of high gloss. The floor looks fine, but it isn’t what you specced or budgeted for. You’re living with the miss, and the floor shows it.

Choice 3: A self-leveling overlay. A cement-based or polymer self-leveling mix goes over the old slab to make a new top surface at the target FF. That adds $3 to $6 a square foot in materials and labor. The new surface needs its own cure time and a compatibility check before polishing starts. It also adds height, which can clash with door clearances and the transitions to other floors.

All three cost far more than asking for FF 50 up front. The concrete itself costs no more at FF 50 than at FF 28. The added cost is the laser screed rental and a skilled crew, which runs $0.10 to $0.25 a square foot on a large pour. Fix-up grinding runs $1.50 to $3.00 a square foot. Put those two numbers side by side, and the choice makes itself.

That’s why the concrete finish is only as good as the pour. The polishing crew can sharpen what the pour crew hands them, but it can’t create flatness that wasn’t there on pour day.

How do we test concrete flatness?

Our flatness process follows “Even,” the E in the SPEC framework. On any slab specced for polish, we take F-number readings with a digital profileograph, per ASTM E1155, 48 to 72 hours after final troweling. You get the reading report before the polish work begins.

On any slab over 800 square feet that will be polished, we spec laser screed. Home garage slabs and smaller commercial pours are a different story. There, a skilled hand-screed crew can hit FF 45 to 55 consistently, working with a 10-foot straightedge and checking with a digital level.

Slab Boss Polish, our polish line, sets its flatness floor by how the floor is built:

• Base build: FF 45 minimum. It suits utility and commercial spaces where high gloss isn’t the goal.
• Fully optioned build (Grout and Cutback, and Double-Pass Densification): FF 55 minimum. This is the build for living spaces and any floor where you want a deep, high-gloss shine, and it’s open to any slab that clears the flatness bar. A true mirror or showroom finish needs FF 60 or better. Not every slab can take that step up, so we look at the slab before we quote the fuller build.

The fully optioned build hits FF 55 most reliably on a slab poured through our Pour-4-Polish model. We set FF 55 before the pour, because we pour the slab and we polish it. One spec covers both jobs, so there is no handoff between a concrete crew and a finish crew. That’s also why field and edge polishing run 30 percent less than the standalone polish price when a SPEC pour covers the same polish footage on the same quote. A general contractor with separate concrete and finish subs has to settle flatness after the slab is poured, when it’s too late to change.

For projects in Oklahoma City, Edmond, Tulsa, or anywhere along the I-44 corridor, we talk flatness when we quote, not after the slab is poured. The polished concrete service page walks through the full process, from checking the slab to the final shine.

The number that sets the ceiling

An FF score below 50 puts a ceiling on what any polishing process can do. The pour that missed the mark wasn’t bad by normal building standards. It may have passed inspection, looked fine, and met the general contractor’s spec. But it won’t take a high-gloss polish without fix-up work that costs more than getting the number right on pour day.

Write “FF 50 minimum, measured per ASTM E1155 typically within 72 hours” into the contract before the truck ever shows up. It’s one sentence, and it does more work than every choice made after the slab sets.

Get your price now for a polished concrete floor from a crew that pours to polish spec, not just structural spec. Or call us at (888) 852-3528 to talk through your project.