Chimney safety guide

What Is Creosote? Stages, Dangers and How to Prevent It

The tar-like residue behind most chimney fires: how it forms, how to recognise each stage, and how to keep your flue clear.

Book a chimney inspection Call 0330 133 2121

What is creosote?

Creosote is the dark, flammable residue left inside a chimney when wood smoke cools and condenses on the flue walls. It builds up layer by layer, and fastest when you burn damp wood, keep the stove turned right down, or the flue runs cold.

It forms in three stages. The further it goes, the harder it is to remove and the more likely it is to catch fire.

Stage 1 · lower risk

Flaky soot

Dry, powdery deposits that brush away easily. A standard sweep removes it completely.

Easy to remove
Normal with regular use

Stage 2 · higher risk

Crunchy and tarry

Black, flaky-to-tarry deposits that stick to the flue. It needs a thorough sweep, sometimes with rotary tools.

Harder to remove
Burns readily in a hot flue

Stage 3 · high risk

Glazed creosote

A hard, shiny, tar-like coating that brushes won't shift. It needs specialist rotary or chemical treatment.

Specialist removal only
The main cause of chimney fires

Think you've got glazed (stage 3) creosote? Stop using the stove and have the flue inspected before lighting it again.

Why creosote is dangerous

Chimney fires

Creosote can ignite in a hot flue and burn at temperatures that can exceed 1,000°C, cracking liners and brickwork and spreading to the roof space.

Poor draw and wasted fuel

Build-up narrows the flue, so the stove draws badly, gives out less heat and burns more fuel.

Smoke and carbon monoxide

A restricted flue can push smoke and carbon monoxide back into the room.

Repair bills and insurance

Fire-damaged liners and stacks are expensive to fix, and insurers may ask for proof of regular sweeping.

What causes creosote to build up

Wet or unseasoned wood

Wood above 20% moisture wastes heat boiling off water and makes cool, smoky fumes: perfect conditions for creosote.

Stove turned down too low

Shutting the air controls to make fuel last longer gives a slow, smouldering burn and more tar in the flue.

A cold or oversized flue

Smoke cools quickly in an unlined or oversized chimney and condenses on the walls. A correctly sized liner helps.

Not sweeping often enough

Each layer insulates the last, so deposits harden over time. Regular sweeping stops stage 1 turning into stage 3.

Signs of creosote build-up

A strong tar or bonfire smell from the fireplace, especially in warm or damp weather
Black, sticky or shiny deposits around the stove door, flue pipe or register plate
Brown or black streaks on the chimney breast, or dripping from the flue pipe joints
Smoke coming back into the room, or the stove struggling to get going
Flaky black debris falling onto the stove baffle

How to prevent and remove creosote

Burn dry wood
Use wood marked Ready to Burn, or seasoned logs with less than 20% moisture. A cheap moisture meter tells you in seconds.
Burn hot, not slow
Get the stove up to temperature quickly and don't shut the air right down. A stove thermometer on the flue pipe helps.
Sweep regularly
At least once a year, and twice if you burn wood regularly. Book a sweep from £95.
Get it inspected
If you've found stage 2 or 3 deposits, a CCTV inspection shows how far the build-up goes and whether the liner is damaged.

Can't remember your last sweep?

We'll sweep the flue, tell you which stage any creosote is at, and deal with it.

Book an inspection Call 0330 133 2121

Quick answers

Can I remove creosote myself?

You can buy creosote-reducing logs and powders, but they're not a substitute for sweeping and won't shift glazed deposits. Stage 2 and 3 creosote needs a professional sweep, and sometimes specialist treatment.

What's the best wood to burn to avoid creosote?

Well-seasoned hardwood such as ash, oak or beech, with less than 20% moisture. Look for the Ready to Burn logo when buying in small quantities.

How often should I sweep a wood burner?

At least once a year. If you burn wood most days in winter, twice a year: before and after the burning season. More in how often you should sweep your chimney and getting your log burner ready for winter.