Flue safety

Creosote in a wood burner flue: what it is and why it matters

Updated 12 October 2026

Creosote is the main reason chimneys need sweeping and the main fuel for chimney fires. It builds up quietly, and how fast it builds depends mostly on what you burn and how you burn it. This page explains what it is, what makes it worse, the signs to look for, and when to get a sweep in.

What creosote is

When wood burns without enough heat or air, it does not burn completely. Some of the gases and tiny droplets of tar and moisture go up the flue unburnt. As they hit the cooler flue lining, they condense and stick. That deposit is creosote. It is a mix of soot, tar and other residues from incomplete combustion.

Sweeps often describe it in three rough stages:

• Stage one — dry, flaky, powdery soot. It brushes off easily during a normal sweep.
• Stage two — crunchy, crusty, black flakes or granules. Harder to shift, and a sign the stove is often burning cool or the fuel is damp.
• Stage three — shiny, hard, tar-like glaze, sometimes runny when warm. This is very hard to remove with brushes alone and is the most dangerous form, because it is concentrated fuel coating the inside of the flue.

Why wet or unseasoned wood makes it worse

Freshly cut wood can be 50% water or more by weight. When you burn it, a large share of the fire’s energy goes into boiling off that water instead of heating the stove and flue. The fire runs cooler, the smoke is wetter and thicker, and the flue stays cold. Cold flue plus wet, tarry smoke is exactly the recipe for creosote.

Other things that make it worse:

• Burning slumbered or overnight with the air turned right down. A smouldering fire is a smoky fire.
• Oversized stoves that are always run low because the room gets too hot.
• Uninsulated or external flues that stay cold, especially in an old, unlined masonry chimney.
• Burning household rubbish, treated or painted wood, or pallets, which also adds chemicals you should not be putting through a domestic flue.

The single best fix is dry fuel. In England, wood sold in bags and nets under 2 m³ must be certified Ready to Burn, which means 20% moisture or less. If you buy loose or in bulk, or season your own, a cheap moisture meter on the fresh face of a split log tells you where you stand. More on choosing fuel on our logs page.

Buy Ready to Burn kiln-dried logs from LogsOnline (affiliate link — we may earn a commission if you buy).

Signs you have a creosote build-up

• Sticky black or dark brown deposits around the stove door, the top of the stove, the flue pipe joints or inside the firebox.
• Brown or black staining or tarry streaks on the chimney breast, or tar weeping from register plates or pipe joints.
• A strong, acrid, smoky or tarry smell from the stove or fireplace, often worse in damp weather or when the stove is cold.
• A fire that is hard to start or keep going, or that smokes back into the room. A coated flue is narrower and draws less well.
• Glass that blackens quickly even with the airwash open. Often a sign of wet fuel and a cool burn.
• Flakes or crunchy black lumps falling into the stove or onto the baffle.

Any of these is a reason to stop and book a sweep rather than keep burning and hope.

Why it matters: chimney fire risk

Creosote burns. If a layer builds up and the flue gets hot enough, for example a big, hot fire after weeks of slow burning, the deposit itself can catch light. A chimney fire can be loud (a roaring or rumbling sound), throw sparks or flames out of the top of the chimney, and get hot enough to crack a flue liner, damage masonry and spread into a roof space or neighbouring property. Some chimney fires are slow and quiet and only show up when a sweep finds the damage later.

If you think you have a chimney fire: get everyone out, call 999, and do not go back in. Afterwards, do not use the stove again until the chimney has been inspected and swept. Your insurer is likely to ask when it was last swept and by whom, so keep your certificates (see insurance certificate).

A heavy build-up can also narrow or block the flue, which makes it harder for combustion gases to escape. That increases the risk of carbon monoxide spilling back into the room. Keep a working CO alarm in any room with a stove.

How often to sweep

For wood, most guidance is at least once a year, and more often with heavy use or if the fuel is not reliably dry. If your sweep finds stage two or three deposits, they may suggest sweeping more often, changing how you run the stove, or a specialist treatment for glazed creosote. See how often to sweep for a rule of thumb by appliance and fuel, and what a sweep costs.

Lighting up for the first time this autumn? Run through the first fire checklist first. Diaries fill fast from October, so it is worth booking now — see silly season.

Keeping creosote down between sweeps

• Burn only dry, seasoned or kiln-dried wood at 20% moisture or less.
• Get the stove up to temperature quickly with kindling and the air open before adding bigger logs.
• Avoid long, low, smouldering burns. Smaller, hotter fires are cleaner than one big log turned right down.
• A stove pipe thermometer helps you stay in the efficient burning zone.
• Do not burn rubbish, treated timber, pallets or cardboard in quantity.
• Keep an eye on the glass and the smell. If either changes, ask your sweep.

Need a sweep?

Find a registered sweep listed from public registers. Ask for a certificate with the date and the sweep’s registration body on it.

Disclosure: some product links are affiliate / referral links. We may earn a commission at no extra cost to you. Not affiliated with HETAS, NACS, the Guild, APICS, Sweep Safe or Ready to Burn / Woodsure. This is general guidance; follow your stove maker’s manual and your sweep’s advice. In an emergency, call 999.