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Drywall wicks. Water touching the bottom edge travels upward through the gypsum core well above the visible line, which is why the damage is always higher than the water was and why a flood cut is made above where you think it should be.
How far up the water goes
Capillary action pulls moisture upward through the paper facing and the gypsum core. How far depends on how long the sheet stood in water, how saturated the space was, and what is behind it.
Insulation in the cavity makes it worse rather than better. Wet batt insulation holds water against the back of the drywall and against the framing, so the sheet is being wetted from behind long after the standing water is gone.
This is why a surface that feels dry at knee height can read wet on a meter. Judging by touch consistently underestimates the line.
What a flood cut is
A flood cut removes drywall in a clean horizontal line above the highest measured moisture, typically at a convenient height for reinstallation. Common practice cuts to a level that makes the replacement piece a standard dimension.
The cut serves two purposes. It removes unsalvageable material, and it opens the cavity so insulation can be pulled, framing can be inspected, and drying equipment can reach inside the wall. A wall dried only from the outside is not dried.
Cutting too low is the frequent error. It leaves wet gypsum above the cut, which continues to hold moisture against new material installed below it.
When drywall can stay
Occasionally, with clean water, brief contact, no insulation in the cavity, and immediate drying, drywall survives. It is uncommon in basements because most basement walls are insulated and because response is rarely that fast.
The test is a meter reading, taken at multiple heights and compared with unaffected drywall of the same type elsewhere in the building. Not appearance, and not whether it feels dry.
When it always comes out
- Any contact with sewage or floor drain water.
- Any contact with outdoor floodwater.
- Water that stood more than a day or so.
- Any sheet showing softness, crumbling, or paper separation.
- Anywhere the cavity behind contains wet batt insulation.
- Any visible mold growth on the surface or the back.
What is behind the wall
Once open, inspect what the cut reveals. Batt insulation that has been wet comes out — it loses R-value and holds water against framing. Rigid foam is sometimes salvageable. Framing is usually salvageable if dried promptly, and readings should be taken directly on the studs and bottom plate.
The bottom plate sitting on the slab is the last thing to dry and the thing most often missed. It is also where mold appears first if drying stops early.
Before closing it back up
Framing and the cavity must read dry against a comparable unaffected area. Antimicrobial treatment, if used, goes on cleaned and dried material — not as a substitute for drying.
Reinstalling too early is the single most common cause of a repair that fails months later. There is no way to see it happening, which is why documented readings matter.
Related pages
For flooring below the same wall, see wet basement carpet and pad. For mold considerations, see mold after a basement flood. For the drying process, see removing standing water.