Water under a wood floor.
Why boards cup, when they flatten on their own, when they don't, and how readings over time tell the difference.
Wood floors cup because the underside got wetter than the top. If they're dried soon enough, they usually flatten on their own. Nobody should sand or replace anything until the readings through the board have stayed level for a month. A floor that's dry and flat is a floor you keep.
You spent months choosing the species, the width and the pattern with your designer. When water gets under it, the question isn't how fast it can be replaced. It's whether it has to be.
Drying what can be saved is most of the job. The rest is finding out, with instruments rather than opinions, whether the wood has changed for good.
Why floors cup
One face of the board gets wetter than the other.
Wood takes on moisture and swells across its width. When the underside of a board is wetter than the top, the edges rise and the floor cups. The flooring manufacturers' association puts it plainly: hardwood "will cup for one reason and one only," a moisture imbalance between the two faces. [18] The federal preservation guidance says the same thing: cupping "is caused by one side of the board gaining or losing moisture faster than the other side." [19]
A surface finish slows drying from the top, so moisture that got in from below can stay there a long time. [19]
Cupping is the floor telling you where the water is. The raised edges mark the wet area better than any stain does, and the readings underneath them are where the map starts.
When it recovers
"If cupped boards are dried soon enough, they usually return to a flat position." [18]
That is the whole case for drying first. The same guidance warns that boards left cupped for a long time can change inside so that drying no longer flattens them. [18] Time matters, and so does drying the right way: the wet face has to dry without the dry face being pushed further.
FEMA says a wet area should dry completely, usually two to three days, before any rebuilding. [3] A wood floor with a finish on it is often slower than that. Drying can take weeks. [19] It is finished when the readings say so, not when the surface feels dry.
The floor and, where it can be reached, the subfloor are read at marked points, and an unaffected area of the same floor sets the standard to dry to. The same points, read again over the job, show the wet edge moving inward until it's gone.
Why nobody should sand yet
A cupped floor sanded flat before it dries will crown.
Sanding cuts the raised edges down. When the boards then finish drying and flatten, those edges end up thinner than the centers, and the floor crowns the other way. [18] Both the manufacturers' guidance and the federal procedure give the same rule: no repairs until the readings have stayed balanced between the face and the back of the board for 30 days. [18] [19]
That month of patience is cheap next to a floor that has to be done twice.
Drying ends when the floor reads dry and the record shows it. Sanding, refinishing and the decision to replace belong to your flooring contractor, with the readings in hand. A mitigation company that doesn't do that work has no reason to call a floor lost that isn't.
When it's permanent
Some changes don't dry out.
A floor that has lifted off the subfloor, boards warped so far they can't be worked back into place, and boards that stayed cupped long after they read dry are past what drying can fix. The federal procedure reserves replacement for boards "severely warped or buckled" that cannot be worked back into place, and calls for a full resand and refinish if a floor is still cupped once it's fully dry. [19]
Nobody should tell you a floor is saved before the readings say so, or that it's lost before they do either. What a good record tells you, at every visit, is what was measured.
A new floor that's starting to cup, a hallway that feels damp, tile just installed over a slab you aren't sure about: readings answer it, guesswork doesn't. How investigation works.
Get it up, then get help.
Towels and a wet vac for standing water, rugs off the floor, nothing heavy set back down on wet boards. Then the first-hour steps.