For something so preventable, chimney fires remain one of the most common causes of dwelling fires in the UK. They happen quietly, in homes whose owners had every intention of booking a sweep next month. They damage parts of the chimney you can’t see, set up the conditions for a second fire days later, and leave insurance questions that take months to resolve.

This is the full picture: what causes them, what to do when one starts, what happens after, and the well-established prevention that puts the risk close to zero.

What a chimney fire actually is

When you burn solid fuel in a stove or open fire, combustion is never 100% efficient. Some of the fuel’s tars and gases pass up the flue unburned, where they cool and condense on the inside of the chimney. That black, sticky-then-glassy residue is creosote. Once it builds up to a layer thick enough, hot enough, and concentrated enough, it ignites.

The result is not a roaring open flame in your living room. It’s a fire happening inside the flue itself, fed by years of accumulated deposits, that can reach temperatures of 1,000°C and beyond. The chimney becomes, briefly, the inside of a furnace.

What that looks like from the outside:

• A sustained roaring or rumbling sound — often compared to a jet engine, a freight train, or a vacuum cleaner running inside the wall.
• Vibration — the flue itself may visibly shake or you may feel it through the wall.
• Dense, dark smoke from the chimney pot outside — usually noticed by a neighbour.
• Sparks, flames or burning debris ejected from the top of the chimney pot, sometimes igniting the roof or nearby vegetation.
• An unusually intense fire in the appliance, often glowing white-hot, and difficult to control.
• The smell of intense burning — sharper and more acrid than usual woodsmoke.

The sound is the most reliable single tell. If you have ever wondered whether you might be having a chimney fire, you would not be wondering — the noise is unmistakable.

What causes the build-up

Creosote forms whenever solid-fuel combustion runs cooler or smokier than ideal. The conditions that produce it are familiar to anyone who has used a wood-burner in a British winter:

• Burning wet or unseasoned wood. Moisture content above 20% drops combustion efficiency dramatically. Water-laden smoke deposits more readily on cool flue walls. This is the single largest cause of creosote in UK chimneys.
• Slow-burning a stove overnight with the air vents shut down. The lower combustion temperature creates more unburned tars in the smoke. It feels efficient — the fuel lasts longer — but it’s laying down deposits the entire time.
• Restricting the air supply generally. Modern airtight homes can starve a stove of replacement air, producing smoky, low-temperature burns.
• Burning anything other than dry wood or approved smokeless fuel. Painted timber, treated wood, household waste, glossy paper. All produce dense, tarry smoke that condenses on the flue.
• Inadequate flue insulation. An uninsulated flue cools the smoke quickly, encouraging condensation. This is particularly an issue in older Victorian and Edwardian terraced homes across the North East where original flues run up exterior walls.
• Skipped sweeps. Each missed annual visit lets the layer thicken. Two skipped years in a heavily used wood-burner is the high-risk profile.

Most chimney fires happen to homeowners who had every intention of booking a sweep next month.

If a chimney fire is happening right now

If you suspect a chimney fire is in progress — the roaring sound, dense smoke from the pot, an unusually intense fire — act immediately. The window to limit damage is small.

1. Call 999 and ask for the fire service. Do this first. Don’t wait to see if the noise subsides. A chimney fire can compromise the flue’s integrity in minutes, and even if it burns itself out, the structural assessment afterwards is a job for the fire service and a professional sweep.
2. Close down the appliance air supply. Shut all air vents on a stove. Reducing oxygen reduces the intensity of the fire and slows its progress.
3. Move flammable items away from the hearth. Rugs, fireguards with fabric panels, kindling, logs, anything close to the chimney breast or roof void above it.
4. Get everyone out of the property. Especially upstairs. The greatest risk is the fire breaching the flue and igniting the timber-framed roof void where you cannot see it.
5. Close internal doors on your way out. Slows the spread of any breakout fire.
6. Do not throw water on the fire. Water hitting the white-hot flue can crack the masonry, fracture liners, and produce a steam explosion that injures anyone standing close.
7. Stay clear until the fire service confirms it’s safe. The visible fire may subside long before the flue has cooled and been declared structurally sound.

Some homeowners use chimney-fire extinguisher logs — specially formulated logs that suppress oxygen and reduce flue temperature. They’re a useful supplementary precaution, but they are not a substitute for calling 999. Use one if you have it, but make the call first.

What happens after

The fire is out. The fire service has gone. The room smells of woodsmoke and the appliance is cool. Most homeowners’ instinct at this point is to be relieved, sweep up the hearth, and consider the matter closed.

Don’t. A chimney fire damages the flue in ways that aren’t visible from the room. Even a short, contained fire can:

• Crack flue liners. Ceramic and clay liners crack under thermal shock. Steel liners can warp or fail at welded joints. Once the liner is compromised, the next fire you light vents combustion gases directly into the masonry — or worse, into a roof void.
• Damage masonry mortar. The intense heat can crumble mortar lines, leaving gaps between bricks that smoke and CO can pass through.
• Displace or shatter clay flaunching at the chimney pot, allowing rain ingress and further deterioration.
• Leave heavy deposits of glazed creosote — a hardened, fused residue that’s much harder to remove than ordinary soot and that significantly raises the risk of a second fire.

Before that chimney is used again, you need two things: a thorough sweep to remove the post-fire residue, and a CCTV inspection to verify the flue’s structural integrity. This isn’t optional. Lighting a fire in a compromised flue is how a small contained chimney fire becomes a much larger structural fire.

The insurance reality

If a chimney fire damages your roof, your ceilings, your contents, or causes smoke damage anywhere in the property, you’ll be making an insurance claim. What happens then depends almost entirely on whether you can produce a current sweep certificate.

Most UK home insurance policies contain a clause requiring the homeowner to maintain the property — including the chimney — in good condition. Lack of maintenance contributing to damage is grounds to reduce or refuse a claim. In Financial Ombudsman case DRN5436394, a UK homeowner had their chimney-fire claim refused because their policy required the chimney to be “inspected and cleaned prior to winter use” and they couldn’t produce a certificate. The Ombudsman upheld the refusal.

A chimney fire is a bad day.
A chimney fire without a certificate is a worse one.

A current NACS-certified sweep certificate is, in claims terms, your evidence of due care. With it, the claim process is straightforward. Without it, even valid claims can be delayed, contested, or refused. Read the full insurance article for the detail.

How regular sweeping prevents almost all of this

The mechanism is simple. Creosote needs to reach a certain thickness, concentration, and temperature to ignite. Regular sweeping removes the build-up before it gets there.

The HETAS and NACS recommended schedule for UK homes:

• Wood-burners and open wood fires: sweep twice a year if in regular use — once before the burning season starts and once partway through or at the end of the season.
• Smokeless and multi-fuel: sweep at least once a year.
• Oil and gas appliances: sweep once a year.
• Any appliance returning to use after a long dormant period: sweep and CCTV-inspect before the first fire.

The single highest-risk profile in the UK is a wood-burner installed in the last 10–15 years, used regularly, swept once a year or less, run frequently at slow-burn settings, on a mix of bought logs and casually stored firewood whose moisture content is unknown. If that’s your home, twice-yearly sweeping isn’t cautious — it’s the standard.

What a proper sweep visit actually does

To put numbers on the prevention case, here’s what a NACS-certified sweep does on a routine annual visit:

1. Removes creosote and soot from the full length of the flue using rotary power-sweeping. This is the work that prevents fires.
2. Performs a smoke evacuation test at the end. Lights a smoke pellet at the appliance and confirms the flue draws cleanly — the same test that catches the early warning signs of CO risk.
3. Inspects the visible parts of the appliance, hearth, and flue entrance. Cracks, deteriorating mortar, broken fire bricks, damaged registry plates — all noted.
4. Issues an NACS Certificate of Chimney Sweeping documenting the work, the condition observed, and the recommended next sweep date. Emailed the same day. Accepted by every major UK home insurer.
5. Flags anything that needs follow-up — a CCTV inspection if something doesn’t feel right, a cowl recommendation if downdraughts are an issue, or a re-line if the flue’s integrity is in doubt.

At Black Diamond’s rate of £85 all-in for an open fire or stove sweep, that’s the price of one delivery dinner against the cost of either a chimney fire or a refused insurance claim.

The remedy — short version

1. Sweep annually, twice yearly for wood-burners in regular use. NACS-certified sweep, with a smoke evacuation test, every time.
2. Burn dry wood only. Moisture content under 20%. Buy “Ready to Burn” certified logs or properly seasoned hardwood. Don’t guess.
3. Don’t slow-burn overnight. If you need overnight heat, consider a different appliance — long, low burns are creosote factories.
4. CCTV-inspect after any incident or before returning a dormant chimney to use.
5. Keep the certificate. Your insurer will want it the day you need them.

For Newcastle, Sunderland, Gateshead, Durham and the wider North East, Black Diamond is NACS-certified and books in under a minute. Every sweep includes a smoke evacuation test and a same-day certificate.