Wet Drywall, When You Can Dry It and When You Must Cut It

What Water Does to Drywall

Drywall is a core of gypsum between two layers of paper. The gypsum can take on water and dry out again with little harm if it happens quickly. The paper is the problem. Wet paper is food for mold, and the back side faces a dark, still wall cavity where mold grows unseen.

Drywall also loses strength when it gets wet. Soaked long enough, the gypsum turns soft and chalky, the paper peels, and screws pull through. A wet ceiling panel sags because the gypsum cannot hold its own weight plus the water.

Water wicks up drywall from the floor, often 6 to 12 inches above the actual water line and sometimes more. That means a wall can be wet far higher than the flood mark suggests. The only way to know is to measure.

Paint and wall coverings matter too. Flat latex paint lets a wall breathe and dry. Vinyl wallpaper, oil based paint and tile backer on bathroom walls trap moisture inside, so those walls dry much more slowly and often have to be opened.

When You Can Dry It in Place

Drywall can often be saved if all of these are true.

• The water was clean, from a supply line, faucet or similar source.
• You started drying within about 24 to 48 hours.
• The board is firm. Press it with your thumb. It should not feel soft or crumble.
• There is no bulge, sag, peeling paper or mold.
• The cavity behind it is not filled with soaked fiberglass insulation.
• The wall is painted drywall, not covered in vinyl wallpaper.
• Readings are dropping each day.

Most interior walls between rooms have no insulation, which is why they often dry well. Exterior walls usually do have insulation, and wet fiberglass batts hold water against the drywall for a long time. If a wet exterior wall is not drying after two or three days, the insulation is the likely reason.

Walls behind kitchen cabinets and vanities are tricky. Water pools under the cabinet base and the wall behind stays wet. Pull the toe kick panel at the bottom of the cabinet to let air in, and check the wall above the countertop and inside the sink base with a meter.

When It Has to Come Out

Cut out the wet drywall if any of these are true.

Gray water, such as washing machine drain water, is a judgment call. Many pros remove drywall that gray water touched in a home, especially if it sat more than a day. If in doubt, cut.

Cutting out a strip of wall is cheap next to a mold problem later. A 2 foot strip patches easily, and it lets you see and dry the framing.

Reading a Moisture Meter on Walls

A moisture meter tells you what your hands cannot. There are two types.

• Pinless meters read through the surface without holes. They are fast for scanning a wall to find the wet area.
• Pin meters push two pins into the material and give a more exact reading at that spot. They leave tiny holes.

Meters give numbers on different scales, and many read drywall on a relative scale rather than true moisture content. So do not chase a magic number. Instead, find a dry reference. Take readings on the same kind of wall in a room that never got wet. That is the target.

To map the wet area, start at the floor and take a reading every 12 inches going up, every 2 feet along the wall. Mark the highest wet spot with painter’s tape. Connect the marks and you have your wet line.

For wood framing, most pin meters read true wood moisture content. Wood is generally considered dry below about 15%, and above 19% it is wet. Mold can grow on wood that stays above about 20% for long periods.

Measure at the same spots every day and write the numbers down. Falling numbers mean drying is working.

How to Dry a Wall Without Removing It

If the wall passes the checks above, set it up to dry from inside the cavity as well as the surface.

1. Remove the baseboard carefully with a pry bar and a putty knife behind it. Number the pieces so they go back in place.
2. If you plan to replace the baseboard anyway, drill a few 1/2 inch holes through the drywall in each stud bay, about 2 inches above the floor, where the baseboard will cover them. This lets air into the cavity.
3. Pull the carpet back from the wall if the floor is carpeted.
4. Point air movers along the wall at a low angle, and run a dehumidifier in the closed room.
5. Keep the room around 70 to 80°F with humidity under 50%.

Take off switch plates and outlet covers only after the breaker is off, since these openings also let a little air into the cavity.

A clean water wall usually dries in 3 to 5 days. If readings stall above the dry reference by day 3, stop waiting and cut. The longer wet paper sits in a closed cavity, the more likely it is to grow mold.

How to Make a Clean Flood Cut

A flood cut is a straight horizontal cut that removes the bottom of the wall. Done well, it is easy to patch.

• Turn off the power to the room at the breaker and confirm outlets are dead with a voltage tester.
• Find your highest wet reading and add at least 12 inches. Many pros cut at 2 feet or 4 feet so the patch uses a half or full sheet of new drywall.
• Snap a chalk line or draw a level line at that height around the room.
• Score along the line with a utility knife, or cut with an oscillating tool set no deeper than the drywall, which is usually 1/2 or 5/8 inch.
• Cut carefully around outlet boxes. Do not cut near where wires run up from a box.
• Pull the strip off by hand, then pull or cut the screws or nails left in the studs.

Bag debris as you go and mist dusty material lightly to keep dust down. Wear an N95 respirator.

In a home built before about 1980, have the joint compound and any texture tested for asbestos first. In a home built before 1978, painted walls may carry lead paint. Do not cut or sand those walls until you know.

Insulation, Framing and Closing the Wall

With the wall open, pull out any wet insulation. Fiberglass batts that got wet sag, lose their insulating value and hold water against the framing. Bag them and replace them. Wet cellulose and foam board need to go as well unless a pro says otherwise.

Now dry the framing. Wood studs and the bottom plate usually dry in 2 to 5 days with fans and a dehumidifier once the wall is open. Do not close the wall until the wood reads dry, generally below about 15%, and the bottom plate matches studs in a dry part of the house.

If you see light mold on the studs, clean it with a detergent solution and a scrub brush, let it dry, then HEPA vacuum. Heavy growth that has stained deep into the wood, or growth over more than about 10 square feet, is a job for a mold remediator.

Do not rely on paint or a sealing primer to fix wet or moldy framing. Coatings go on after the wood is clean and dry, never in place of drying.

Take photos of the open wall, the dry readings and the framing before the new drywall goes up. That record helps with an insurance claim and when you sell the home.

Step by Step

1. Stop the Water and Kill the Power

   Make sure the leak is fixed. Switch off the breaker for the wet walls and confirm outlets are dead with a voltage tester.

2. Map the Wet Area

   Take meter readings every 12 inches up and every 2 feet along each wall. Mark the highest wet point with tape and take readings in a dry room for comparison.

3. Check the Save or Cut Signs

   Press the board for softness, look for bulging or mold, and think about the water source and how long it was wet. Any red flag means cut.

4. Remove Baseboards

   Pry baseboards off gently and number them. Pull carpet back from the wall if needed.

5. Dry or Cut

   For a savable wall, drill vent holes behind the baseboard line and set up fans and a dehumidifier. Otherwise snap a line at least 12 inches above the wet line and cut the drywall out.

6. Pull Wet Insulation

   Remove and bag any wet insulation from open cavities. Photograph it first.

7. Dry the Framing

   Run fans and a dehumidifier until studs and the bottom plate read below about 15% and match dry areas. This usually takes 2 to 5 days.

8. Document and Close Up

   Photograph the dry framing and your final readings. Then install insulation and new drywall.

Mistakes That Make It Worse

• Judging a wall by touch. Drywall can feel dry on the face while the back paper and cavity are still wet.
• Cutting exactly at the water line. Water wicks higher than it looks, so the cut needs to be above the highest wet reading.
• Cutting with a blade set too deep. It is easy to hit wires, pipes or a drain line behind the wall.
• Leaving wet insulation in place and closing the wall. It traps moisture against the framing for months.
• Painting over a water stain before the wall is dry. The stain bleeds back and mold can grow under the paint.