How long does it take for a floor to rot from water damage?
Real rot is slow. Wood rots when it stays wet for weeks to months, which is why it usually comes from a slow leak under a toilet, a tub or a dishwasher rather than from one spill. The damage that comes sooner is different: particle board and OSB can swell within a day of soaking, and mold can start on wet materials within 24 to 48 hours.
Why rot takes time
Rot is caused by fungi that break down wood, and they need the wood to stay wet. A floor that gets soaked once and is dried properly does not rot.
A floor that is wetted again and again, by a wax ring that has failed, a leaking supply line or a shower pan, is the one that rots. That kind of leak can run for a long time before anyone sees it from above.
What happens much sooner
Particle board and some OSB swell when they soak up water, and the swelling does not go back down. Laminate does the same. That can happen within a day.
Mold can start on wet subfloor, carpet pad and the backs of baseboards within 24 to 48 hours, which is why the EPA advises drying wet materials in that window.
Signs a floor has started to rot
A spongy or soft spot, especially around a toilet, a tub or under the kitchen sink. Tile that has cracked or come loose in one area. A musty smell. Dark, crumbly wood if you can see the subfloor from a basement below.
A soft floor is a structural question, not a cosmetic one. The subfloor, and sometimes the joists under it, have to be opened and checked.
How we handle a wet or soft floor
We find and stop the source with the plumber, extract and dry, meter the subfloor, then cut out and replace whatever is rotted or swollen before the new floor goes down. Laying a new floor over a wet or weak subfloor is how a second floor fails.
We install LVP, hardwood, laminate, tile and carpet, so the floor is replaced by the same crew that repaired what is under it.