The short answer
Creosote is a tar-like deposit that forms when smoke from burning wood cools and condenses on the inside of a chimney or flue. It is flammable and is the main cause of chimney fires, and a thick layer also narrows the flue so smoke can come back into the room. Burning dry wood, running hot fires and having the chimney swept at least once a year keeps it under control.
What creosote is
When wood burns, the smoke carries water vapour, gases and tiny unburnt particles up the chimney. As the smoke rises it cools. When it touches the cooler inner walls of the chimney or flue, some of it condenses and sticks. That deposit is creosote.
Every wood fire produces some. The question is how much, and how quickly it builds up.
How it builds up in stages
Chimney sweeps usually describe creosote in three broad stages:
- Flaky soot. A dry, black, powdery or flaky layer. It brushes off easily during a normal sweep.
- Sticky, tar-like deposit. Harder and more crunchy, sometimes shiny in places. It needs more work to remove.
- Glazed creosote. A hard, shiny, tar-like coating that sets on the flue walls. It is the most flammable stage and the hardest to remove, and it calls for a professional.
A chimney that is swept every year rarely gets past the first stage. Problems start when sweeps are skipped and the fire is regularly burned slowly with wet wood.
Why creosote is dangerous
Chimney fires
Creosote is flammable. If enough builds up, a hot fire or a spark can set the deposit itself alight inside the chimney. A chimney fire can burn very fiercely, crack the flue and spread heat or fire into the building around it. Some chimney fires are loud and obvious. Others smoulder unnoticed and only show up as damage at the next inspection.
If you ever hear a roaring sound from the chimney, see sparks or flames coming out of the top, or notice thick smoke and a hot, strong smell, get everyone out and call 112. Do not use the fire again until a sweep has inspected the chimney. Our guide to the signs of a chimney fire explains what to do.
Smoke and carbon monoxide
A thick layer of creosote narrows the flue. The fire draws less well, smoke can come back into the room, and the risk of carbon monoxide building up indoors goes up. A working carbon monoxide alarm in the room with the fire is essential.
Why Maltese homes are prone to it
Malta’s mild climate does not make creosote less of a problem. In some ways it makes it more likely:
- Fires are lit occasionally. Many homes only light the fire on colder evenings or at weekends. A chimney that is cold most of the time encourages smoke to condense on the walls.
- Holiday homes and farmhouses in Gozo and rural Malta are often used only now and again, so the flue is cold and often damp when the fire is lit.
- Damp winters. Firewood stored outside or in a damp garage picks up moisture, and wet wood is the biggest single cause of heavy creosote.
- Low, slow fires. Turning the air right down to make a fire last longer creates cooler, smokier fires that leave more deposit.
- Older chimneys. Many older townhouses and farmhouses have chimneys that were built long before modern stoves, with rough inner surfaces that deposits cling to.
Signs creosote is building up
- Black, flaky or tarry deposits around the fireplace opening or the stove door.
- A strong smoky or tar-like smell, especially when the fire is out or on damp days.
- Smoke coming into the room, or a fire that is harder to light and keep going.
- Stove glass that blackens quickly.
- Pieces of soot falling into the grate.
If you notice any of these, book a sweep before using the fire much more.
How to keep creosote down
- Burn dry wood. Seasoned firewood with a moisture content below 20% burns hotter and cleaner. A simple moisture meter tells you if your wood is dry enough.
- Never burn painted, varnished or treated wood, chipboard or household rubbish, and only burn pallet wood if it is plain and stamped HT (see pallet wood).
- Warm the flue first. Start with a brisk, hot fire with plenty of air before turning it down.
- Avoid long, smouldering burns with the air closed right down.
- Store wood off the ground, covered on top and open at the sides, out of the rain.
- Have the chimney swept at least once a year, before the first fire of the season, and again mid-season if you burn most evenings.
Our guide to chimney and stove safety before winter has a full pre-season checklist.
What a sweep does about it
A chimney sweep brushes the flue from bottom to top, removes the soot and creosote, and clears the firebox, smoke chamber and any ledges where debris collects. They will also look for nests, blockages and visible damage, and tell you if the flue needs further attention from a specialist.
Light and sticky creosote comes off with brushing. Hard glazed creosote may need more than one visit or specialist treatment, which is another reason to keep up with yearly sweeps.
When to book
Book your sweep early. Demand in Malta and Gozo peaks from October to January, and it is far better to have the chimney swept before the first cold evening than to wait for a slot in the middle of winter.
clear.mt arranges chimney sweeping and stove cleaning across Malta and Gozo. Tell us what you have, an open fire, wood burner or pellet stove, and your preferred month, and we will confirm a date.