What is creosote, and why does it cause chimney fires?
September 26, 2026 · 4 min read · Chimney Sweep Master, Cary, IL
The black build-up inside every wood-burning chimney, how it forms, and how to keep it from catching fire.
Every time you burn wood, smoke carries tiny bits of unburned fuel, tar and moisture up the chimney. As that smoke cools against the flue walls, it sticks. That coating is creosote, and every wood-burning chimney has some.
The three stages
- Stage one: a dry, sooty powder that brushes off easily.
- Stage two: crunchy, flaky black deposits that are harder to remove.
- Stage three: a shiny, tar-like glaze. It is the hardest to remove and the most dangerous.
Why it catches fire
Creosote burns. When enough of it builds up and a hot fire reaches it, it can ignite inside the flue. Some chimney fires roar loudly; many burn quietly and nobody notices. Either way, the heat can crack flue tiles and damage the chimney, which opens a path for fire or carbon monoxide to reach the house.
What makes it build up faster
- Burning wet or unseasoned wood
- Slow, smoldering fires with the air turned down
- A cold chimney, such as one on an outside wall
- A damper that is only partly open
How to keep it under control
Burn dry, seasoned hardwood, give your fires enough air to burn hot and clean, and have the chimney inspected every year. Sweeps use one-eighth of an inch of build-up as the point where it should come out. The EPA's Burn Wise program has more good advice on burning wood cleanly.
Want a licensed sweep to take a look?
Chimney Sweep Master, 426 Lloyd Street # B1, Cary, IL 60013 — serving Cary, Crystal Lake, Barrington and the surrounding towns since 1985. You can upload photos of your fireplace or chimney with your request.