Wet drywall: can it be dried or must it be replaced?

What water does to drywall

Drywall is a core of gypsum between two sheets of paper. Gypsum itself tolerates a short wetting. The trouble comes from three things:

• The paper is food for mold, and growth can begin in 24 to 48 hours.
• The core softens when saturated. It loses strength, and fasteners pull through.
• It wicks. Water at the floor climbs up the sheet, often several times higher inside the wall than the visible tide mark.

Drywall also hides what is behind it: wet insulation, wet studs, and the back face of the sheet, which usually grows mold before the painted front shows anything.

It can usually be dried when

• The water was clean, from a supply line or similar
• Drying started within about a day
• The sheet is still hard when pressed, flat, and firmly attached
• There is no insulation behind it, or the cavity can be opened to air
• There is no visible mold, staining through or bubbling

Interior partition walls, which have no insulation or vapour barrier, are the best candidates.

It should be replaced when

• The water was contaminated. Sewage, a drain backup, or water that came in from outdoors. No exceptions.
• It is soft, swollen, crumbling or sagging.
• It was wet for more than a day or two before drying began.
• There is mold on either face.
• There is wet insulation behind it. Outside walls and many basement walls.
• It is a ceiling that has sagged or held pooled water.
• Seams have opened or the tape has let go widely.
• It has been wet before.

The press test and beyond

Press firmly with a thumb near the bottom of the wall. Firm and unyielding is promising. Any softness or give means the core is saturated.

Feel is only a start. A moisture meter gives a reading that can be compared with a dry wall elsewhere in the house, and shows how far up and along the wetting extends. A thermal camera shows the pattern at a glance.

How drywall is dried in place

1. Remove the baseboard. It traps water against the wall and hides the worst area.
2. Open the cavity. Small holes are drilled through the drywall just above the floor, where the baseboard will cover them, between each pair of studs.
3. Move air. Air movers blow along the wall, or a manifold system pushes dry air through the holes into the cavity.
4. Dehumidify. A dehumidifier pulls the moisture out of the room air.
5. Measure daily until the drywall and the framing behind match the dry readings.

Painted drywall dries more slowly, because gloss and semi-gloss paints hold moisture in. Several coats of paint or vinyl wallpaper can trap it altogether, and wallpaper is usually stripped.

Cutting it out: the flood cut

Where the lower wall has to go, it is cut in a straight horizontal line above the wet area, and the drywall below is removed.

• The height is set by moisture readings, not by the stain. Cut above the highest wet reading.
• Common heights are about 60 centimetres or about 1.2 metres, which suit standard sheet sizes for the repair.
• A clean, level cut makes the patch easier to finish.
• Insulation behind is removed to the same height or higher.
• Studs and bottom plates are cleaned, dried and checked before closing.

The opened wall also lets the contractor see wiring, plumbing and the back of the adjoining room’s drywall.

Ceilings

Ceilings behave differently. Water pools on top and the sheet sags between the joists. Insulation above holds more water against it.

• A ceiling that has sagged, or is soft, is replaced. It will not recover its shape.
• A bulge holding water is dangerous. Keep out from under it. With the power to the room switched off, a small hole at the lowest point lets it drain.
• A ceiling with a stain but no softness or sag, from a brief clean leak, can sometimes be dried, particularly with the insulation above removed.
• Textured ceilings are hard to patch invisibly and, in older homes, may contain asbestos. Test before cutting.

Special types

Moisture-resistant board, often green or purple, has a treated core and facing. It resists humidity and splashes better. It is not waterproof and is judged by the same tests.

Paperless and fibreglass-faced board gives mold less to feed on.

Cement board behind tile tolerates water. The framing and whatever is behind it still need checking.

Plaster on lath is denser and more tolerant of short wetting than drywall, but dries slowly. Plaster that has bulged, cracked widely or come away from the lath is replaced. Wood lath behind it holds moisture. Old plaster may contain asbestos and may have lead paint on it.

Fire-rated assemblies, such as the wall between a house and an attached garage, or between units, must be rebuilt to the same rating.

Before cutting in an older house

Drywall joint compound and textures used before the mid-1980s can contain asbestos. A restoration contractor takes samples and gets rush results, or treats the material as asbestos-containing if it must come out first. Tell them the age of the house and of any renovations.

The repair

1. Confirm the framing is dry by meter reading
2. Replace insulation and restore the vapour barrier on outside walls
3. Fit new drywall
4. Tape, fill and sand the joints
5. Prime, using a stain-blocking primer over any remaining water marks
6. Paint the whole wall, corner to corner. Patches of new paint on an old wall show.
7. Refit or replace baseboards

Painting over a water stain without a stain-blocking primer does not work. It bleeds through.

Mistakes to avoid

• Leaving baseboards on
• Judging by the surface alone
• Closing up before readings are normal
• Painting over staining or mold
• Leaving wet insulation in an outside wall
• Trying to dry drywall touched by sewage

More: water damage restoration.