What Is Creosote and Why Does It Matter?
Creosote builds up inside chimneys from burning wood. Here is what it is, why it matters, and how to manage it.
Reviewed by John Freeman, chimney and fireplace professional with 20+ years of field experience.
Last updated
Short Answer
Creosote is the black or brown residue that wood smoke leaves on the inside of a chimney as it cools and condenses. It is highly combustible, which is why chimneys are swept. The Chimney Safety Institute of America (CSIA) recommends sweeping a masonry fireplace chimney at 1/8 inch of buildup, and sooner if any glaze is present. NFPA 211 calls for an inspection at least once a year.
How Creosote Forms
CSIA explains that burning wood produces smoke, water vapor, gases, unburned wood particles, hydrocarbons, tar fog, and minerals. As these rise into the relatively cooler chimney, they condense and stick to the inner walls. That residue is creosote. CSIA says restricted air supply, unseasoned wood, and cooler-than-normal chimney temperatures all speed buildup, and that the longer smoke stays in the flue, the more likely creosote is to form. Closed glass doors, a damper that is not opened wide, and stove air inlets closed too soon all restrict the air. Overloading a wood stove for a longer burn contributes too.
Forms of Creosote
CSIA says creosote can be crusty and flaky, tar-like and sticky, or shiny and hardened, and that all forms often occur in one chimney. People often call these stages, but different sources number them in different orders, so the description matters more than the number.
| Form | What it means for cleaning |
|---|---|
| Crusty and flaky | Brushing removes it. CSIA says that if it fractures when hit, rotary cleaning can help. |
| Tar-like, drippy, and sticky | CSIA says gummy creosote can generally be dealt with only by a chemical treatment or an acid application. |
| Shiny and hardened (glazed) | CSIA says a hard or tacky layer cannot be removed by normal brushing, and that professionals use chemical products to change it into a form that can be brushed off. Any glaze means sweeping sooner than 1/8 inch. |
Why It Matters
CSIA says creosote in sufficient quantity, with a high enough internal flue temperature, can cause a chimney fire, and that masonry chimney fires burn at around 2000 degrees F. They can melt mortar, crack tiles, and damage the surrounding masonry. The US Fire Administration (USFA) counted an average of 34,200 heating fires a year in residential buildings from 2017 to 2019. About 49 percent of them were chimney or flue fires confined to the chimney, which USFA classes as small fires that rarely cause serious injury or large losses. The concern is the damage a fire can leave behind unseen: CSIA says the majority of chimney fires go undetected.
Signs of a Past Chimney Fire
If a chimney fire is happening, with loud cracking or popping, dense smoke, or an intense hot smell, call 911. After the fact, CSIA lists signs a sweep looks for:
- Puffy or honeycombed creosote
- Warped metal on the damper or connector pipe
- Cracked or collapsed flue tiles
- A discolored or distorted rain cap
- Creosote flakes or pieces on the roof or ground
- Roofing material damaged by hot creosote
- Cracks in exterior masonry, or smoke escaping through mortar joints
Burn Practices That Slow Buildup
EPA's Burn Wise guidance says to season wood for at least six months, keep it at a moisture content under 20 percent, and test it with a moisture meter before burning. It also says to build hot fires, because for most appliances a smoldering fire is not safe or efficient, and to store wood outdoors off the ground with only the top covered. Local firewood such as madrone, oak, Douglas-fir, and pine is judged by the same measure, moisture content, whatever the species. The Rogue Valley has wet winters, so wood left uncovered or stored on the ground will stay wet. EPA also lists materials that should never be burned, including household trash, plastics, painted or pressure-treated wood, and plywood.
Removing It
CSIA says the best method is mechanical brushing combined with a complete evaluation of the system. Chemical creosote products are not bad in themselves, but CSIA says they are not an adequate substitute for mechanical cleaning and inspection, because loosened debris still has to be removed from the smoke shelf, the baffle, and any offsets. If you are in Southern Oregon, the $349 Complete Chimney Sweep & Inspection covers the sweep and an inspection together. Severe glazed creosote can change the price, and any added charge is explained before work proceeds.
Sources
- NFPA 211: Standard for Chimneys, Fireplaces, Vents, and Solid Fuel-Burning Appliances
- Chimney Safety Institute of America: Homeowner FAQs
- Chimney Safety Institute of America: Preventing Chimney Fires
- US Fire Administration: Heating Fires in Residential Buildings (2017-2019)
- US EPA Burn Wise: Best Wood-Burning Practices
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