Mold on the ceiling: what causes it and how to fix it

From below or from above?

Ceiling mold has two possible water sources, and they look different:

• From the room below: humid air condensing on the ceiling. The growth is speckled and spread out, in corners, above showers, along outside walls and around vents. There is usually no water stain.
• From above: a roof leak, an overflowing attic air handler, a bathroom or laundry room on the floor above, or a pipe in the ceiling. The growth centers on a water stain or ring, and the drywall may be discolored, soft or sagging.

The first is a humidity and ventilation problem. The second is a leak, and there is usually more mold on the hidden side of the drywall than on the side you see.

Bathroom ceilings

The ceiling above a shower is the most common place for ceiling mold in any house. Steam rises, condenses on the cooler ceiling and, without enough exhaust, the surface stays damp long after the shower. Small spots clean off, but they return until the ventilation is fixed: a fan big enough for the room, ducted outdoors rather than into the attic, and run for about 20 minutes after each shower. Bathroom mold removal covers the details.

Cold corners and thin insulation

Mold in a band along the top of an exterior wall, or in the corners of upstairs rooms, usually marks a cold spot. At the eaves, attic insulation is often thin, compressed or blown back by wind coming through the soffit vents, so the ceiling there gets cold in winter. Indoor moisture condenses on it and mold follows. Sometimes faint gray stripes appear across a ceiling at regular spacing; those often line up with the framing above, where the ceiling is slightly colder and dust or mold collects. Adding insulation at the eaves, with baffles to keep the soffit vents clear, and keeping indoor humidity in check solves it.

Leaks from the attic or the floor above

A ring-shaped stain with mold around it means water came through. In summer, the usual source in Triangle houses is an attic air handler whose drain clogged and pan overflowed. After storms, it is the roof, often a cracked rubber boot around a plumbing vent or failed flashing. Under an upstairs bathroom, it is a toilet seal, a tub drain or a shower pan. The ceiling leak page covers how to read the timing, and AC leaking water covers the air conditioning side.

Mold above the drywall

When a ceiling has been wet from above, the growth you can see is a fraction of what is there. The back of the drywall, the insulation on top of it and, in an attic, the framing and roof sheathing may all be affected. Before deciding on a fix, someone needs to look from above: in the attic, or by opening a small section of ceiling between floors. If the mold extends onto the roof deck, attic mold removal covers that part.

Removing ceiling mold

A few spots of surface mold from steam can be cleaned with detergent and water, wearing gloves, goggles and an N95 respirator, then dried. Mold on drywall that got wet from above is cut out, because it has grown into the paper and onto the back of the board. Overhead removal is messy, so the room is contained with plastic, the floor and contents protected, and HEPA filtration run while the ceiling comes down. Wet or moldy insulation above it is removed, the framing is cleaned and dried, and the area is checked before new drywall goes up. Mold remediation explains the full process.

Keeping the ceiling clean

• Run bathroom fans during and after showers, and confirm they vent outdoors.
• Keep indoor humidity below about 60 percent.
• Top up attic insulation at the eaves, with baffles so the soffit vents still breathe.
• Have the AC drain flushed each spring, and test the float switch on an attic unit.
• Repaint a cleaned bathroom ceiling with a mildew-resistant bathroom paint over a stain-blocking primer.