Floor flatness is a measure of how smooth a concrete surface is over short distances. It is often given as an FF number, along with an FL number for levelness.
Understanding Floor Flatness
Flatness and levelness are not the same. Flatness (FF) describes bumps and dips over a short span, and levelness (FL) describes how close the floor is to a true horizontal plane over a longer span. Higher FF and FL numbers mean a better floor. Warehouses with forklifts often require tight numbers, while a home garage is more forgiving.
How Floor Flatness Is Measured
Contractors measure flatness with a straightedge, a dipstick profiler, or a laser scanner. The common field check is a 10 foot straightedge laid on the slab, with a gap measured in the low spots. Standards like ASTM E1155 define the FF and FL method for new slabs.
Why Flatness Matters for Coatings
Glossy epoxy and metallic floors show every dip because the material flows into low spots and pools there. Low areas get thick and may cure unevenly, and high spots get a thin coat. On a polished floor, uneven grinding exposes aggregate in some places and not others.
Practical Tips for Handling Poor Flatness
Mark high and low spots with chalk after a straightedge check. Grind high spots down and fill low spots with a patch or a self-leveling epoxy or cement. Warn the customer before the job if the slab will show waves in the final finish, and get it in the estimate.
Also Called
FF/FL numbers, Floor levelness
