The short version
- Stop the source and start drying before deciding anything about the floor. The Environmental Protection Agency advises drying wet materials within 24 to 48 hours to prevent mold.
- Cupping, where board edges rise above the middle, often reverses as the floor dries. Sanding a cupped floor before it has dried is what turns a repair into a replacement.
- The decision is made on moisture readings in the boards and the subfloor, compared with a dry part of the same house.
- Boards that do not recover are replaced and woven in, and the room is sanded and finished as one surface.
- Of the 635 places in our data at /data/, 138 have 35 percent or more of their housing built before 1940, where floors often sit directly on joists with no subfloor to hold water.
The first two questions: is the source stopped, and how long was it wet
Nothing about the floor can be decided until the water has stopped arriving, and how long it sat is the single fact that predicts the outcome best.
A supply line that burst and was shut off within the hour is one kind of event. A slow drip behind a dishwasher or a refrigerator that ran for a season is another, even though it involved far less water. In the first case the water stayed mostly in the flooring. In the second it had time to reach the subfloor and sometimes the framing.
So the first call after a leak is to whoever stops it and dries the house: a plumber, and for anything substantial a water mitigation company with drying equipment. The Environmental Protection Agency's guidance is to dry water damaged areas and materials within 24 to 48 hours to prevent mold growth, and that clock starts when the leak does.
We repair floors, and that work begins after the water is out. Calling a flooring contractor first is understandable, and what we will tell you is to get the drying started and call us when it is.
Photograph everything before the drying crew moves it, including the source of the leak. You will want those pictures later.
Cupping, crowning and buckling are three different conditions
Cupped boards have raised edges and often recover, crowned boards are raised in the middle and usually do not, and buckled boards have lifted off the subfloor and need to come up.
Wood absorbs water where the water is. A leak wets the underside of a floor first, so the bottom of each board swells while the top has not, and the edges rise. That is cupping. As the floor dries and the moisture evens out, the boards flatten, frequently much more than the owner expects.
Crowning is the reverse shape, with the center of the board higher than the edges. It most often appears when a cupped floor was sanded flat while it was still wet. The raised edges were removed, and when the boards dried and returned to shape, the middle stood proud.
Buckling is the severe case. The boards have swollen so much that they have pulled loose from the subfloor and lifted, sometimes by inches. Those boards are removed, the subfloor is dried and checked, and new flooring goes in.
| What you see | What is happening | Usual outcome |
|---|---|---|
| Edges of boards higher than the middle | Cupping: the underside is wetter than the top | Often flattens as it dries. Wait and measure. |
| Middle of boards higher than the edges | Crowning, often after sanding too early | Usually needs a full sanding once dry, or new boards. |
| Boards lifted off the floor | Buckling: swollen boards have released from the subfloor | Those boards come out and the subfloor is checked. |
| Dark or black staining along the seams | Water sat in the joints for a long time | Stained boards are replaced. Sanding does not reach it. |
| Soft or spongy feel underfoot | The subfloor has absorbed water or delaminated | The floor is opened up. The subfloor is the repair. |
Why the floor has to dry before anyone sands it
A cupped floor sanded flat while still wet loses its board edges permanently, and when it finishes drying it crowns, which is harder to fix than the cupping was.
This is the most expensive mistake available after a leak, and it is made with good intentions. The floor looks bad, the owner wants it fixed, and a sander will make it flat by the end of the week. It will also remove wood that the boards need once they return to their dry shape.
Drying takes longer than it looks. A floor can feel dry to the hand and look dry to the eye while the underside and the subfloor are still well above normal. With drying equipment it can be days. Left to the house's own heating and air, it can be weeks.
Waiting is hard to hear when a room is torn up. It is still the step that most often saves the floor.
If someone offers to sand a wet floor flat this week, ask what the moisture reading is today and what it was in a dry room of the same house.
The readings that make the decision
The floor is ready to assess when the moisture content of the boards and of the subfloor has come back close to the reading in an unaffected part of the same house.
A moisture meter gives a percentage, and the number means little without a reference. So we take readings in a dry room first, then work across the damaged area, in the boards and, where there is access, in the subfloor underneath.
The industry guideline for installing wood flooring gives a sense of the tolerance involved. For solid strip under three inches wide, flooring and subfloor should be within four percentage points of each other. For plank three inches and wider, within two. A repair that puts new boards onto a subfloor wetter than that is built to fail.
We also meter outward from the visible damage until the readings are normal. Water travels along the subfloor and under the boards before it shows on top, and the wet area is usually larger than the stained one.
What gets replaced, and how the repair disappears
Boards that have stained, split or stayed deformed after drying are cut out, new boards are woven into the floor on a stagger, and the room is sanded and finished as one surface.
Replacing boards in a rectangle leaves a rectangle. The repair is laced in instead, with new boards running different distances into the old floor, so there is no straight line where one ends and the other begins.
The new wood is matched for species, width and grade. In postwar houses that is usually simple, because two and a quarter inch red oak is still a standard board. In older houses with narrow strip or heart pine, the right match may be reclaimed flooring of the same age.
Then the whole room is sanded and finished together. A patch finished on its own never matches the sheen and color of the floor around it, however good the wood match is. Where the leak was in a kitchen that opens to other rooms, the finish is taken to a natural break such as a doorway.
Engineered wood and vinyl plank fail differently
Engineered wood usually does not recover from soaking because its layers separate, and vinyl plank often survives while the subfloor under it does not.
An engineered board is a layer of real wood glued to a plywood core. Prolonged water swells the edges and can loosen the bond between layers. Once the top layer has lifted or the edges have swollen, the board does not return to shape, and those boards are replaced. Matching a discontinued product is the difficulty, so leftover boxes from the original installation are valuable.
Vinyl plank does not absorb water. That is its advantage and also the reason leaks under it go unnoticed: the planks look fine while water sits on the subfloor or slab beneath them. A floating vinyl floor can be lifted, the base dried and checked, and the planks relaid if they are undamaged.
Laminate has a fiberboard core that swells when wet and does not recover. Swollen laminate is replaced.
Why old houses behave differently after a leak
In the oldest housing the floor is often nailed straight to the joists, so water passes through to the ceiling below instead of sitting in a subfloor, and the floor frequently dries better than a newer one would.
The dataset we publish at /data/ covers 635 towns and Philadelphia neighborhoods. In 138 of them, 35 percent or more of the housing was built before 1940, and in Philadelphia 40.0 percent of the housing predates that year. Floors of that age are commonly solid strip laid directly over joists, with plaster on the ceiling underneath.
After a leak, the plaster ceiling below is usually the bigger casualty. The floor itself has air on both faces and no plywood layer holding water against it. Old growth oak and heart pine are also dense and slow to absorb. These floors cup, and a good share of them flatten.
The caution is the wear layer. A floor that has already been sanded several times may not have enough wood left to be sanded again after it dries. That is measured at the edge of a board before anyone promises a refinish.
Who does what, and in what order
The plumber stops the water, the mitigation company dries the structure, and the flooring contractor repairs the floor once the readings are normal.
Those are three different trades. A mitigation company owns drying equipment, monitors moisture daily and deals with anything that has started to grow. A flooring contractor replaces boards, sands and finishes. We do the second and not the first, and we say so on the phone.
If the loss is being claimed on a homeowner's policy, the adjuster will want the cause, the photographs and a written scope for the repair. We write that scope after seeing the floor dry, with the area to be replaced and the area to be refinished stated separately.
The visit is free. Text a photo of the floor first if you like, and we can usually tell you from the picture whether it is time for us or still time for the dryers.
