Why is water leaking from the chimney breast?
Short answer. Damp or dripping on the chimney breast, the part of the chimney that projects into the room around the fireplace, usually comes from rain getting in at the top or the roofline: failed flashing, a cracked crown, a missing cap, or porous brick soaking up wind-driven rain. Condensation in an unused or poorly lined flue can also leave damp patches, so note whether it appears during rain or on humid, dry days.
Rain-related causes
- Flashing at the roofline: water enters where the chimney passes through the roof and runs down the masonry, showing up on the breast below the ceiling.
- Crown and cap: cracks in the crown let water into the brick core, and an uncapped flue lets rain fall straight down the inside.
- Porous brick and mortar: brick absorbs wind-driven rain, and eroded joints let in more. A breathable masonry water repellent can help once joints are sound.
- Chase covers: on framed chimneys, a rusted metal cover pools water and leaks into the chase.
Not rain at all
- Condensation: a flue sealed off without ventilation can trap moist air, and gas appliances venting into an oversized or unlined flue produce water vapor that condenses on cool masonry.
- Old flue deposits: in older chimneys, soot and salts can draw moisture from humid air, leaving damp, stained patches that come and go with humidity rather than rain.
- Nearby plumbing or HVAC lines.
How to narrow it down
- Write down when it appears: during rain, a day after rain, or on muggy dry days.
- Check the attic where the chimney passes through for wet wood or masonry.
- Look at the top of the chimney with binoculars for a cap, crown cracks and missing mortar.
- Have the roofline flashing inspected, and have a chimney sweep inspect the flue and liner.
Fix the source first and let the masonry dry, which can take weeks, before repairing plaster or paint. See chimney leaks and crickets and roof flashing explained.