What Can Cause Damp on a Chimney Breast?

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what causes damp on a chimney breast

Whether it is still functioning or just for decoration, a chimney breast can add a lot to a room. Unfortunately, they can also become magnets for unsightly patches of damp and mould.

There are a few different causes for a damp chimney breast. However, once you figure out which is behind your problem, it can be simple to fix. Adding some damp proofing measures will prevent the damp from becoming a big problem. First, you’ve got to figure out what can cause damp on a chimney breast.

Damp on a chimney breast is caused by moisture building up and soaking into the wall. This can be from rainwater getting in or condensation. It could even be from salts drawing moisture into the walls themselves. These are the most common causes of damp on a chimney breast. Once you determine which of these causes is to blame, you can take some steps to stop the damp.

Common Causes of a Damp Chimney Breast Explained

A damp chimney breast can be from a few different things. These are the common causes that might be to blame:

  • Condensation – Condensation can build up in and around your chimney. Hot moist air settles on the cold surfaces of your chimney breast and turns into moisture. This can soak into your wall and cause damp and mould.
  • External Water Like Rain – Outside water like rain can get onto the walls of your chimney through the top of your chimney stack. If your walls are unprotected, it can soak in and become damp.
  • Salts Drawing Moisture In – Hygroscopic salt is a by-product of burning fuels like coal and gas. Unfortunately, when you burn coal in your chimney, these salts can end up on your chimney breast. These salts have the nasty habit of drawing moisture in. This can lead to damp forming.
  • Roof Leaks – In some situations, the problem might be caused by defects on your roof. This could be problems around the chimney, or the stack itself. Holes, cracks, and damage to things such as pointing, and flashing will let rainwater in.

How to Find the Cause of Damp on a Chimney Breast

Since there are a few common causes of a damp chimney breast, it is important to figure out which is to blame for your problems. Below are a few ways to figure out what is to blame:

  • Check Your Roof – It is a good idea to have a quick look at your roof. If possible, stand back from your property, and try to see if there are any visible problems with your mortar, tiles, or flashing around the chimney. This could be the cause of your damp.
  • Top of Chimney – Check the top of your chimney and see if they have caps fitted. If they don’t, or if the existing caps are damaged, this could be letting water in. In this case you will need to add a cap, or replace the existing cap. Also, if your chimney is completely sealed, there is a chance this will be trapping condensation inside.
  • Check Loft Space – If you have access to your loft space, head in there to check your chimney stack. If it is damp that high up, then the culprit is probably rainwater getting in. Once you know this, you should have a good idea of how to treat it. In most cases this will involve getting help from a professional roofer.
  • Salts – If you’re burning fossil fuels like coal in your chimney, then hygroscopic salts might well be the cause. In more severe cases these salts will be physically visible to see on the chimney breast.
  • Condensation – If the cause of your damp is just condensation, it could be present in other areas of the room. This is usually a sign of poor circulation in the room itself. If your chimney breast is cold, it will create a dew point. This means it is like a magnet to warm moist air.

Going through those tests should help you identify which type of damp you have in your chimney breast, which will make it easier to fix.

Ways to Treat a Damp Chimney Breast

How you go about treating a damp chimney breast is going to depend on what the cause for your damp is. You need to deal with the root cause of the damp first and then proceed with adding the necessary protection. Below are some solutions to each type:

Rain Entering The Chimney

The process of fixing damp from rain and outside weather will require you or a professional to access the roof. The first thing to do is check your chimney pots. If these are broken, they’ll need repairing or a cap added to protect them. Open chimney pots may even need a cap fitting if they are not damaged. It is important to not seal the chimney off entirely though. This will make your airflow substantially worse and lead to damp from condensation. Fixing the chimney pots should allow you to stop the cause of the damp.

Problems with Roofing or Chimney Defects

If the problem is being caused by defects on your roof, it will be necessary to repair any cracks or holes that have formed up there. If you’re not comfortable doing this yourself, getting a roofer in to fix things can be a low-cost way to fix the problem.

Condensation Damp

Condensation damp is caused by hot air that has built up hitting a cold surface. There are many things you can do to cut down on this. However, ventilation and good air circulation inside the room are the main things to address for condensation.

Also, make sure that your chimney isn’t blocked, poor ventilation inside the chimney can cause hot air to build up. This can cause condensation internally. Fitting a vent, or a cap to your chimney pot will allow air to escape, which makes it one of the best ways to treat this problem. This might be a bit more costly, but it is a permanent solution to the problem.

Hygroscopic Salts

If the cause of your damp chimney breast is from hygroscopic salts when you’re burning fuel, then there is a simple solution and a harder solution. The first is to stop burning coal. Although, this is probably not going to be the solution that you want!

You can instead fit materials to your chimney stack to protect it against these salts. This way you can continue to use your chimney.

You will need to treat the walls of your chimney stack and the chimney breast with a salt-inhibitor. This is generally a first layer to adding some damp proofing too. When combined, they will protect against hygroscopic salts.

Stopping Damp Penetrating Your Chimney

As well as addressing any issues with the ventilation or roofing, you can damp proof the walls of your chimney breast. This will make sure that moisture doesn’t soak in.

Common ways of waterproofing a chimney breast will include painting with a damp proof paint, or installing a damp proof membrane.

Adding a membrane or damp proof paint to the surface, may require some additional cosmetic repairs, such as replastering and repainting. However, it should completely waterproof your wall against any future damp.


Damp in a chimney breast can be an unsightly problem with quite a few common causes.

Finding out which one is at the root of your damp will mean doing a little investigative work. However, once you find the cause, you can go about fixing the source of the damp and protecting your chimney breast against future damp.