Creosote in Your Chimney and Why You Don’t Want It

Creosote is another name for tarring inside your chimney, it is a build-up of a sticky substance that is not only smelly but is also an extremely combustible material. Creosote begins to build-up when the gasses that are given off during the wood burning process combine and as such begin to condense.

Creosote is formed in four stages; Condensation, Liquification, Solidification and Friable, let’s explore this in a little more detail.

  • Condensation – Often people try to reduce the amount of air entering the combustion chamber as they burn wood. This is to try to increase efficiency and burn less fuel. It can be counterproductive and can have serious repercussions. As the burning process is slowed down the appliance can begin to smoulder and the smoke cools as it enters the chimney. As the smoke no longer maintains such a high temperature it is not driven up the flue at sufficient speed to escape through the chimney stack – the sap then turns the condensation inside the chimney to creosote.


  • Liquification – As you might expect, the condensation turns creosote into liquid, this sticky substance can be very damaging at it seeps into mortar joints and masonry. Creosote by its very nature is corrosive and can cause structural damage.


  • Solidification – Just as the creosote can be liquid, it can also solidify. The varying temperatures inside your chimney causes the substance to continually heat up and cool down again. In liquid form the creosote spreads and covers a larger area but as it cools it then sets and turns solid. This build up is often described as tar, it significantly reduces the surface area inside the chimney, it can even totally block a flue. It is important to note that creosote in its solid form is highly flammable and serious chimney fires are a very real concern.


  • Creosote becomes friable – When solid creosote is burned it leaves behind a residue that is crisp and dry. The dryness occurs as the oils disperse which makes it less gunky. The cleaning of this dry build up is a very important part of the job your chimney sweep does.


Is creosote inside the chimney serious and difficult to remove?

There are varying degrees to which the creosote coating can be serious and sometimes downright dangerous. If you call a chimney sweep they will evaluate to which degree the coating is causing concern.

  • The first degree is usually caused by ash; it is easy to sweep away and is usually the result of burning medium or hard woods that are well seasoned. It is important to note that the level of build-up is often dependent on the quality of the wood that has been burned.



  • The second degree is often referred to as “clinker” this build up is a little more developed than the above example. It presents itself as a flaky deposit and is fairly easy to sweep away with a little elbow grease.


  • The third degree of creosote is often described as tar, it is sticky, drippy and causes lots of damage. This is not an easy deposit to remove and it will take time and skill.



How to reduce creosote build-up and help prevent a chimney fire?


“Chimney fire” is a term that strikes fear into even the most experienced of chimney owners and as such people look for ways to prevent it.

So, what can you do to prevent the build-up of highly combustible creosote within your chimney?


  • Use good quality wood, do not burn soft wood, painted or treated wood. Stick to well-seasoned, dry hard wood logs. Do not attempt to light wet firewood as this by its very nature takes up more energy and will result in a lower burning temperature, this in turn keeps the flue cool which results in steam which contributes to condensation.


  • When the fire is lit, exercise control so that the fire does not get too big and reach the creosote area within the chimney.


  • Use smokeless coal alongside your wood, this can be a little more expensive but very much worth it.


  • Do not throw paper or cardboard into the fire as this can result in a large flame reaching the inside of the chimney.


  • If you use your wood burning stove overnight, ensure you do a hard burn for a minimum of half an hour each morning, this will heat up the flue. Always do your hard burn with the stove door open.


  • Do not restrict the air flow of your fireplace. If you reduce the air supply by using glass doors or by not opening your damper up wide enough the hot smoke will not travel up through the chimney and this can cause a rapid build-up of creosote.

It is important to note that once it is formed creosote is difficult to remove and this is a specialist job. If you have signs of creosote or have not had your chimney inspected do not risk it, the possibility of a chimney fire taking hold is a very real one.

Always have your chimney swept and inspected at least once a year to help keep you and your home safe.

We hope that this information has been helpful. If you need a chimney sweep or have any questions, please get in touch.