Why is my chimney leaking black liquid?
Short answer. Black or brown liquid running from a chimney is usually water mixed with creosote and soot: rain getting in through a missing cap, cracked crown or failed flashing, or condensation in the flue, picks up deposits on the way down. It signals two problems at once, a water entry point and creosote buildup, and the fireplace should not be used until a chimney professional has inspected it.
Where the water comes from
- Rain down the flue: no cap, or a damaged one.
- Cracked crown or open liner joint: water enters the top of the masonry and runs down inside.
- Failed flashing: water enters where the chimney meets the roof and runs down through deposits in the masonry.
- Condensation: moist exhaust or humid air condenses on a cool flue, common with gas appliances vented into older chimneys and with slow, smoldering wood fires.
Why it is black
Creosote is a tar-like byproduct of burning wood that coats the inside of the flue. Water running through it carries the residue along, leaving black or dark brown streaks on the fireplace face, the ceiling or the outside of the brick. The liquid is acidic and often smells smoky, especially in humid weather.
Why it matters
- Creosote is flammable, and heavy buildup is the main fuel for chimney fires.
- The acidic water breaks down mortar, flue liners and metal dampers.
- The stains are stubborn and tend to bleed back through paint unless sealed with a stain-blocking primer once dry.
What to do
- Stop using the fireplace and any appliance vented into that chimney.
- Have a chimney professional clean and inspect the flue and liner.
- Fix the water entry: a cap or crown repair at the top, or new flashing where the chimney meets the roof.
- Clean and seal stained surfaces after everything has dried.
For the roof side of the problem, see chimney leaks and crickets.