Under ASTM E1155, FF (floor flatness) rates how smooth a floor is over short distances and FL (floor levelness) how closely it follows design elevation over longer runs; higher is better for both. Readings are taken within 72 hours of placement, before shrinkage and curling move the slab. A general slab might be specified around FF 25 and FL 20, while warehouses with tall racking push FF to 50 and beyond.
Spec calls for FF 45 FL 30, so the laser screed goes first and the finishers pan it twice, no shortcuts.
Also heard as
- FF/FL
- FF
- FL
- floor flatness numbers
Related terms
Screed
To strike off fresh concrete to the right height, or the straightedge, board or machine that does it.
Pan
A round steel float disc clipped onto a power trowel's blades for the first, flattening machine passes.
Curling
Upward warping of slab edges and corners, caused by the top drying and shrinking faster than the bottom.
Bird bath
A low spot in a finished slab or pavement where water collects and sits after rain or washing.