04/09/2026
Explaining flatness and its accuracy
Q. I keep hearing about flatness but isn’t that the same as level?
A. No it’s not. Level is the same as slope. So if you were to place a level on a floor and the bubble doesn’t remain in the center, there is a slope in the floor. Flatness on the other hand is a measurement of two points. So if using a 10’ screed point A would start with the screed touching the surface and point b, which would be at 120” would touch the surface as well. Anything under that screed not touching the surface would then be the flatness measurement. That’s in a perfect world. Often along the way of that 10’ the screed may touch the surface which if it does the span has just been decreased and can no longer be reported as x” in 10’ but rather where it touches. So if it touches in the middle and the gap between screed and floor is 1/8” then it’s reported as 1/8” in 5’. When measuring all of the humps, valleys and crowns in a floor count and any coming into contact reduces the span.
Q. If span is reduced by crowns in a floor, can’t we just measure the crown?
A. Absolutely but then you are no longer measuring flatness but for a crown. To be fairly accurate you need to place a level, preferably a digital 4’ or 6’ level centered on the crown so that the gap at the ends of the level are equal in distance from the floor. If there is only one gap at the end where the level is hanging in mid air while the other end is touching the level you are then technically measuring slope in the area.
Q. Is using a screed or straightedge on top of a concrete slab an accurate way of determining flatness?
A. According to the concrete industry it’s an approximation, but normally all that’s required of installers and inspectors as it’s still recognized by the industry. But the most accurate way of determining flatness is the F number system and using a device called a dipstick which is very expensive equipment. It’s found under ASTM test method E1155. In my personal experience it has its applicability 48/72 hours after placement on the slab but I would argue it’s practicality in the flooring business especially after the concrete has cured for personal reasons I won’t get into here. It essentially uses a grid system of measurement in which it takes measurements every 2’ at which the center point 1’ a measurement is obtained and converted into an Ff number. There is a chart that is produced by the concrete industry that provides an approximate conversion into a fraction of an inch in 10’. So an Ff 50 corresponds approximately to a flatness of 1/8” in 10’ which would be a very flat floor. Most specs for flooring call for a Ff of 35. There is another method using an F meter but I won’t get into that here. Just know there are many options for determining flatness of a slab.
Q. Can an inspector use a laser level to test for flatness?
A. An inspector can do just about whatever he wants on a jobsite but the question is not what he uses but how he interprets the results. If I take a laser which I’ve done and point it at an object, and at the end of the object I produce a result, what is the result? If done correctly I obtained a slope measurement. A slope measurement is the same as level according to the concrete industry. With only some minor exceptions that’s not what the flooring industry cares about they want to know the flatness of the surface. Now I’m not saying an inspector cannot obtain flatness with laser but two things would be in question, one the accuracy of the laser as most have a tolerance listed in their technical data such as 1/16” or 1/8” and the second is determining two points which are exactly at 0”. There must be a point A and a point B within a span that both are exactly at 0” or no deviation. And somewhere would be a deviation between the laser and floor in which to capture a measurement between said two points to create a span. Sounds challenging, it is. I would put my string against any laser any time.
Q. Can an inspector measure for flatness on top of an existing floor covering?
A. Many inspectors do this including me. It’s not so much about obtaining the measurement as it is about interpretation of the results. I one time received a call from a sales rep asking me to come to a jobsite as his customer was tearing up and replacing a floor because the inspector wrote in his conclusion that flatness was off by 7/16” in 6’ and the claims department denied the claim and blamed the installers. I went there and did all my normal tests then let the installers take up the floor. What did I find? A rigid core floor that was floated over a concrete slab that was floated with patch/leveling compound. Not one area was 7/16” in 6’, in fact the vast majority was 1/16” in 6’ with the exception of one area was 1/8” in 6’. So how did the inspector get 7/16” in 6’ then? I cannot say for sure as I wasn’t there. I only know that no flooring was taken up during his inspection so he had to be measuring on top of the floor. A cupped floor (we refer to as curling). Surely the inspector would have surmised a measurement on top of a cupped floor would produce a measurement at the center of each plank and that would essentially create a “cupping measurement” but he didn’t write that in his report. He wrote that the subfloor was out of flat and he was disproven. The floor was torn up for the wrong reason as they had a subfloor moisture issue causing the curl.
