What Is Creosote — and Why It Is So Dangerous

Creosote is the dark, flammable residue that wood smoke leaves on the inside of your chimney. It is the single biggest reason chimneys catch fire — and the main reason chimneys need regular sweeping.

Where creosote comes from

When you burn wood, the smoke carries unburned particles, tar, and moisture up the flue. As that smoke cools on the way up, it condenses on the chimney walls as creosote. Cooler, smokier, smoldering fires — and wet or unseasoned wood — produce the most.

The three stages of creosote

Creosote builds in stages: first a light, flaky soot that brushes off easily; then a harder, crunchy layer; and finally a shiny, tar-like glaze that is very difficult to remove and highly flammable. The longer it builds, the more dangerous and expensive it becomes.

Why it is so dangerous

• Creosote is highly flammable — a layer as thin as an eighth to a quarter inch can ignite into a chimney fire
• A chimney fire can reach temperatures that crack liners and spread into the home
• Buildup narrows the flue, hurting draft and pushing smoke and carbon monoxide back inside

How to keep it under control

Burn only seasoned hardwood, keep fires hot and well-supplied with air, open the damper fully, and have the chimney swept on schedule — usually once a year. A professional sweep removes all three stages of creosote and confirms the flue is drafting safely.

Frequently asked questions

Written by Adam Evans, owner of Chimney Sweep Brothers and a chimney technician with 12 years of experience serving La Grange, IL and the western suburbs.