A floor finish is a vapour barrier laid on top of the thing that needs to dry. That single fact explains almost everything about water under floorboards: the water gets in quickly through joints and edges, and then it cannot get out, because the surface that let it in is much better at stopping vapour than liquid. The result is an assembly that stays wet for weeks under a floor that looks and feels dry.
The signs, in the order they appear
Cupping first, where each board is lower in the middle than at its edges because the underside is wetter than the top. Then movement at the joints, squeaking, and boards lifting or peaking against each other. A tide line on the skirting and swollen baseboards. A musty smell that is strongest at floor level. Later, crowning, where a floor that was dried too fast on top ends up higher in the middle of each board. Moisture under hardwood floors reads high on a meter through the board long after the surface reads normal.
Signs of water damage under tile
Tile hides water better than anything else in a building, which is why it is so often found late. Look for grout that is darker along a line rather than generally, hollow sounding tiles where the adhesive bond has failed, cracked grout in a straight run, movement or a springy feel underfoot, efflorescence at the edges, and a stain on the ceiling below. Mold behind shower tile is the related case and usually means the waterproofing or the grout has failed and the wall cavity is being wetted every time the shower runs.
Why moisture under hardwood floors usually means opening the floor
Because air cannot reach the assembly. Air movers over a finished floor dry the finish. To dry what is underneath, either the floor is lifted, or a specialist mat or panel system is used that pulls air through the assembly from above, or the ceiling below is opened so the cavity can be dried from the other side. Which of the three is right depends on the floor's value, the subfloor material and whether the ceiling below is easier to make good than the floor above.
Water damaged baseboards: what comes out, and what to do first
Particle board or chipboard subfloor that swelled is finished and is replaced. Plywood usually dries if it is reached in time. Engineered board whose layers have delaminated does not come back. Solid hardwood is frequently recoverable if it is dried slowly and evenly, and is frequently ruined by being dried fast on one side only, which is what causes crowning. On the day, get a meter reading through the board, photograph the cupping, and do not sand or refinish anything until the assembly reads dry.
Questions people ask about water under floorboards
What do I do about water under floorboards?
Get a moisture reading through the board and the subfloor rather than judging by feel, then dry the assembly by lifting the floor, using a mat system, or opening the ceiling below. Air over the surface dries the finish only.
What are the signs of water damage on hardwood floors?
Cupping, squeaking, movement at the joints, a tide line on the skirting, a musty smell at floor level, and high meter readings through the board after the surface reads normal.
Can a cupped wood floor recover?
Often, if it is dried slowly and evenly from both sides and not refinished too early. Drying fast from the top alone is what turns cupping into permanent crowning.
How do I tell if there is water under tile?
Darker grout along a line, hollow sounding tiles, cracked grout in a straight run, a springy feel underfoot, efflorescence at the edges, and a stain on the ceiling below.