The ABCs of floor flatness and levelness

A forklift travels much more smoothly over a flat floor. High stacks of goods in a warehouse magnify any deviations from flatness. Shutterstock image.

What do we mean by floor flatness and levelness? ACI 117.1R-14 defines flatness as “deviation of a surface from a plane” and levelness as “deviation of a line or surface from a horizontal line or surface.”

Flat and level floors matter in such applications as manufacturing facilities, big box stores, and warehouses. Flat floors make it easier to maintain tolerances in manufactured products such as airplane wings. Robots and forklifts travel more smoothly–and safely–over flat floors.

Levelness matters when stacking pallets of goods in a warehouse or big box store: the higher the stack, the more it tilts if the floor isn’t level.

How to specify floor flatness and levelness

We’ve provided our favorite tips for achieving floor flatness (Ff) and levelness (Fl) in a previous post. It’s important to make sure you’ve specified appropriate values for both. On the one hand, you want your floor to be sufficiently flat and level to suit the purpose. On the other hand, you don’t want to pay for more than you need–or cause undue delays if the floor doesn’t meet the specifications.

ACI 117 lists five categories of flatness, with the range of measurements for each. A conventional floor might be used in a mechanical room, where neither functional nor aesthetic requirements mandate more than ordinary flatness. A moderately flat floor would be appropriate in a carpeted office. Where you’re going to install thin-set tile, you’ll want a flat floor. In a warehouse with high speed forklift traffic, you’ll want a very flat floor. And if you’re designing a warehouse floor for random traffic, you’ll want a super flat floor.

For the purposes of meeting the contract requirements, floor flatness and levelness are measured within 72 hours of placement. However, the concrete will continue to cure, dry, curl, warp, and shrink after measuring. How do you make sure the slab still meets your needs in the long term? You can specify extended curing to minimize shrinkage, cracking, and curling. You can design the concrete to shrink less. Or you can design a seamless floor so forklifts don’t have to contend with joints at all.

Achieving flatness and levelness

Design the concrete for dimensional stability (low shrinkage) using the aggregate suspension method. Consider steel fibers instead of reinforcing bars. Fibers do a better job of controlling cracking; you may even be able to do away with joints altogether. Also, with fibers you don’t have to worry about supporting the rebar so it stays at the right elevation to do its job.

It’s a lot easier to make a flat slab on ground than a suspended slab. Suspended slabs continue to deflect between the supports over time. A low-shrinkage concrete will creep less, but less is not none at all. If you must have a flat suspended slab, know that you’ll need to go back later to grind and/or top the slab to make it flat. You can either have it flat or of uniform thickness–not both.

Expect the concrete to crack. Low-shrinkage concrete will crack less. Steel fibers will keep the cracks narrow and tight so they’re less problematic.

Make sure your contractor has the necessary experience, equipment, and expertise to do the job. You don’t want your project to be too much of a learning experience for them!