The three things to look at first
Swelling is the easiest sign to see. Wet gypsum board puffs out, goes soft, or sags, and on a ceiling it can bulge between the framing. The EPA's table says drywall may be dried in place only if there is no obvious swelling and the seams are intact; if not, remove, discard and replace it.
Seams are the second check. Look along the taped joints and corners for paper tape that has lifted, bubbled or split. Separated seams fall on the replace side of the EPA rule even when the rest of the panel looks flat.
Time is the third. The 24 to 48 hours is a deadline for getting the material dry, not a waiting period before you call someone. The EPA notes that mold can grow even when things are dried within 48 hours, and that it won't always grow after 48 hours, so treat the number as a guideline. Write down when the leak likely started, when you found it and when the water was shut off. If you don't know when it started, say so to whoever assesses it rather than counting from the day you noticed.
When the drying rule doesn't apply
The EPA table is only for clean water, such as a supply-line leak or an overflowing sink. If the water came from a sewer backup, a floor drain, or floodwater, or you suspect it was contaminated, the EPA says protective gear and containment are needed and you should use a professional with experience in contaminated-water cleanup. It also says not to run fans until someone has confirmed the water was clean, since fans can spread contamination.
Drywall that stayed wet for a long time is also a different case. USG, a major drywall manufacturer, says it does not require removal of its panels just because they once got wet. But it adds that if conditions for mold exist and panels were wet continuously for more than 24 hours, or on and off for days or weeks, replacing the affected material is the best protection against mold. A slow leak behind a shower wall that wets the board again and again is that second pattern.
Why a dry-looking wall can still be a problem
USG points out that cleaning only deals with the surface of the board, and that moisture and mold can also be inside the wall cavity. So a painted surface that feels dry tells you little. A musty smell that won't go away is a better clue that something inside is still damp; our guide on a musty smell in a closet on an outside wall covers how to track that down.
Keep away from ceilings that are sagging with water and from wet outlets or light fixtures until someone qualified has looked at them. If you suspect mold, don't cut exploratory holes or disturb it yourself; leave that to the assessor.
Who to call and what to ask
Fix the source first: a plumber for a pipe or fixture leak, a roofer for water coming through the roof. For the wall itself, call a water-damage restoration company, and if mold is suspected, ask about their mold remediation experience and check references. A painter or handyman is the right call only after the drying question is settled.
Before work starts, ask for a written scope that says which sections will stay and which will be cut out, how they will check moisture inside the wall and in any insulation, what readings would make them switch from drying to removal, and who confirms the wall is dry before new board or paint goes on. The EPA's guide to mold and moisture says cleanup isn't finished until the water problem is fixed, visible mold and moldy odors are gone, and a follow-up visit shortly afterward shows no new water damage or mold. It also says not to paint or caulk over moldy surfaces, because the paint is likely to peel.
If new drywall has to match the rest of the room, our guide on what a drywall repair quote should say about matching texture lists what to get in writing.
Permits in Philadelphia
Philadelphia's building permit page lists gypsum wallboard installed over existing wall surfaces in one- or two-family homes among the interior finish jobs that often don't need a permit, along with repairs that are regular maintenance, as long as the property isn't on the Philadelphia Historic Registry. The same page says a permit is needed for work that changes the interior of a building or for major repairs that aren't regular maintenance. If the job grows to removing whole walls, framing or wiring, ask the contractor in writing whether a permit is needed and who will pull it. Our contractor hiring checklist covers the license and insurance questions to ask in Philadelphia and Bucks County.