Floor Flatness in Missouri: The Number Before You Buy
Every flooring warranty specifies a flatness tolerance, almost nobody measures it before buying, and it is the single cheapest thing you can find out about your own floor.
The numbers
- Plank — most manufacturers specify around 3/16 in over 10 ft (and often 1/8 in over 6 ft).
- Tile — tighter, nearer 1/8 in over 10 ft, and tighter still for large-format tile, because a big rigid plate cannot follow a curve.
- Natural stone — the same flatness demand plus a stiffness one: L/720 deflection against L/360 for ceramic.
These are not aspirations. They are the conditions under which the product is warranted.
How to measure it
Lay a ten-foot straightedge on the floor. Slide a tape or a set of feeler blocks under it and find the largest gap. Do it in several places and in two directions, and write down the worst number. That is your floor’s flatness, and it takes about ten minutes.
Level and flat are different things and only one of them matters here. A floor can slope noticeably across a room and still be perfectly flat; a floor can be dead level on average and still have a dip that fails the spec. Flooring cares about flat.
What to do if it fails
Self-leveling underlayment or a patching compound, applied before the floor goes down. On a timber subfloor this may mean sheet goods or additional decking first. It is a real line item and it belongs in the quote in writing, with a price, before anyone starts.
The alternative — laying anyway — produces the two failures people blame on the material: a floating floor that sounds hollow where it bridges a dip, and tile that cracks where the substrate flexes.
Who should measure it
The installer, as part of quoting. An installer who has laid a straightedge on your floor is quoting a job; one who has not is quoting a surface and will raise the difference later. It is the single most useful question for telling two quotes apart.
Common questions
My floor slopes. Is that a flatness failure?
Not necessarily. Slope is a level problem; the spec is about local deviation. A consistently sloping floor can pass a flatness check comfortably.
Does carpet care?
Far less than anything else — it follows the substrate. It is the most forgiving covering in a house that sits out of true.