How to Diagnose and Fix a Damp Chimney Breast (2024)

A damp chimney breast problem can present itself in a number of different ways and for a number of different reasons.

Some signs include brown plaster staining on your chimney breast or you might have damp patches on your chimney breast that come and go.

Other visual signs can be a damp chimney breast and ceiling, damp walls either side of your chimney breast, damp in your loft or attic space around the chimney area or even damp halfway up a chimney breast.

Whatever the visible signs of damp in your chimney, in this project we’ll help you diagnose what’s causing it and guide you through the steps to resolve the problem for a successful damp cure or chimney repair.

Are Chimney Damp Issues the Same as Other Damp Issues?

Damp chimney breast issues are normally caused, diagnosed and fixed in a similar manner to ordinary damp issues, but with a few subtle differences and potential causes.

Due to the job that a chimney does, namely removing smoke, fumes and soot from heating sources (fuel burning etc.) the particles and by-products of this can cause their own damp problems to appear in or on the chimney.

It’s a good idea to firstly establish that you have a damp problem at all, and this can be done in two ways:

• Relative Humidity Hygrometer – This is a piece of kit that can be used to test the moisture content of the air inside your room, which can be useful in helping you to understand which moisture source is causing your damp
• Back of Hand – Another method is to use the back of your hand to firstly check the affected chimney area and then reference it against an unaffected dry area. Use your judgement to assess how damp the area is

For a reasonably accurate measurement of the moisture content in your chimney breast you can use what’s called a "Damp Meter".

The meter itself usually features two little prongs on its base and you stick the prongs into the damp area and on the face of the meter it will show a percentage of damp content in that area.

Before you can go about putting right any damage caused by the damp in your chimney you will first have to ensure that the source leak/moisture point is located and repaired.

If the origin of the issue is not completely fixed then any repairs will be short lived, wasting the time that it has taken to patch up the damage and also any funds spent, as a few months later you will just have to do it all over again.

Your checks will have to be thorough as what’s causing your damp chimney can in some situations be a little hard to spot, and of course it might even be more than one issue. Below you will find a list of items and areas to look at.

You may be thinking that some of the chimney’s features are going to be hard to see without physically climbing onto your roof and you would be right!

If you do indeed have the correct ladders, the correct knowledge of where to step as to not cause any further damage, know how to use a ladder safely (see our ladder safety project for details) and have the confidence to climb on to your roof then it is best do this, but if you do not have ALL of the aforementioned skills and items then do not attempt this as you will likely cause damage to your roof and also most likely to yourself!

If you’re not able to get up onto your roof then borrow or hire a decent set of binoculars as these will allow you to view your chimney almost as well.

Damp Chimney Breast Causes – What to Check

As we have mentioned, there are numerous causes for a damp chimney breast but before any fixes can be put in place you first have to identify the actual cause. Here follows a selection of the most common causes.

Open or Capped Chimney Stack

The first thing to check here is to see if your chimney stack or chimney pots are open-topped or capped.

If they are open-topped and your chimney is not in use there is the chance that moisture is entering through here and travelling to the base of your stack, causing damp in your chimney.

The tell-tale signs of this are damp around the base of the chimney breast, as the moisture rises up from the inside.

This can make it seem like a DPC (damp proof course) problem but in fact the issue is coming from the top as opposed to the bottom.

The best way to check that this is the problem is to remove a brick from the base of the chimney breast and access the inside to see if there is moisture present.

If there is moisture present it may be worth considering adding a vented cap or cowl to your chimney pots and seeing if this resolves the issue.

For a cheaper option, you can instead remove your pots and add a paving slab on top of the stack instead, raising it up on all sides by about 2-3 inches to provide ventilation.

If you’re planning on using the chimney in the future, cowls are a better option. If there is debris in your chimney breast clear this at the same time as this may be causing DPC bridging.

Similarly, if your chimney pots or stack are capped and sealed at the top, are they ventilated? (i.e. is there a sealed cap or mortar covering the top of the pot?).

If not then this could be causing condensation to build up in the chimney stack due to cold air being trapped in the void, meaning that when your house warms up, condensation then collects to form damp on the chimney breast surface.

Again, the way to remedy this is to replace your caps with either a chimney cowl on each of your chimney pots or a raised paving slab directly on top of your chimney stack.

If your fireplace is bricked up and you haven’t got any ventilation on the wall area directly in front of the fireplace, you’ll need to introduce this.

You can do this by adding an airbrick roughly 300mm above the level of your floor. For more information on how to replace a brick see our Replacing a Brick Project page.

Chimney Stack Mortar

Is the mortar around your chimney stack or at the base of each pot in good condition where it enters the chimney? Can you see any splits or cracks in the mortar?

If you can see cracks then there is the chance that water or moisture is entering through here and either running down the inside of the stack causing damp or then running out of mortar cracks inside your property and causing damp issues on your chimney breast this way.

Even if this is not the root cause of your issue, it’s certainly not helping so will need repointing to prevent future problems.

Shared Chimney

Do you share a chimney with your next door neighbours? If so then there is the chance that any work that they have carried out is the cause of your damp chimney.

They might have fitted a word burner without inserting the correct flue or they may have blocked up internal ventilation.

Have a polite chat and ask them to confirm any work they have carried out relating to the chimney area, even going back a few years as occasionally chimney damp issues take quite some time to become apparent.

In the event that they have carried out some work on the chimney and it looks like this may be the cause of your damp issues you will have to come to an agreement on how it is fixed.

Be as amicable as possible as there is huge potential for disagreements, especially where money is concerned.

Check Your Roof Area Around the Chimney Stack

Lead Flashing

The lead flashing used on your roof is effectively there to seal any joints around the roof where it meets a wall or a chimney stack.

Check to see if the lead used to seal your roof to your chimney stack is cracked, coming away from the surface or looks in poor condition.

Typically lead is lapped and layered, soldered or sealed using silicone to create a continuous run, and over time these joints can become weak and split, or in fact the lead itself can become weak and crack.

You should also check whether inside your roof space you can see any day light coming in around the stack – this is where the lead has failed.

You should also check inside your roof for any signs of water leakage or a build-up of a white salt-like substance (hygroscopic salts) as this could indicate flashing failure.

Typically lead is used in flashing because it is very malleable (easy to shape), long lasting, low maintenance, good with extremes in temperature and is relatively wind resistant.

All of these features help to prevent the penetration of water via wind pressure, gravity and also tension on a surface.

Although lead has quite a few advantages in terms of sealing it can be very expensive and as you might imagine there are now quite a few substitutes available on the market that can be used instead, but in actuality when it comes to flashing there is no better substance than lead itself.

If you find that your lead is damaged then it will need either replacing or repairing.

As mentioned replacement could be very expensive but to ensure a proper fix this is your best option (more information in our Chimney Flashing project).

If due to the cost this is not an option, you can buy "Flashing Tape" from most builders’ merchants or your local DIY store. In the past we have used flashing tape from Sylglas which we have found to be rather good.

You can then stick the tape over the split or crack to seal it – more information on how to use flashing tape to carry out this repair is available in our Repairing Lead Flashing Project page.

Sealers (silicone etc.) can also be used to seal any other small gaps but as said most of these substitutes are no match for using lead and doing the job properly.

Guttering

Broken or blocked gutters can cause rainwater to filter and collect in unwanted areas causing water ingress and damp in your chimney.

If you have a broken gutter, in heavy downpours water can run down the face of a wall and seep into any splits or cracks in mortar, presenting as damp in your chimney breast.

If you live in an area where rain is common then the sheer act of lots of water running down your walls can over time can cause also cause your chimney to become damp as water can be physically sucked into it (by capillary action).

You should also take note of whether your guttering downpipes drain away from the property or they simply create a huge puddle right next wall, as this may also be causing your damp issues.

Inspect your guttering for any splits or cracks, poor joints on any down pipes or broken brackets (sections of guttering can sag and cause joints to fail when you have broken or faulty brackets).

If there are issues get these fixed as soon as possible to minimise the chance that they’re causing your damp. You can find out how to replace plastic guttering in our Replacing Plastic Guttering project page.

Damaged Tiles

As you may have guessed from the title this can cause many problems including damp in your chimney.

Tiles are used to create a watertight covering for your roof, and if one or more become damaged or dislodged then this can give moisture and damp a direct route into your property.

In some cases the damp issues on your chimney breast are quite a distance away from the actual source of the leak so make sure you check the quality of your tiles across your entire roof.

If you find you have any split, broken or missing tiles then the best course of action is to replace the tile with a new one. Replacing a tile can be a bit of a tricky job so it may be best to bite the bullet and call in a roofer.

Inspect the Chimney Breast Itself to Identify the Cause of the Damp

Ventilation (capped or sealed chimneys and bricked up fireplaces)

The ventilation for a blocked up fireplace can usually be found on the wall in front of the fireplace itself at ground level somewhere around 300mm up from the floor (occasionally in properties with an under-floor void, the vent will be under the floor as this helps to reduce draughts).

To find out more about closing off or blocking up a fireplace, have a look at this project.

When a chimney is capped and blocked up or an internal fireplace is bricked up this then removes some of the original ventilation, and if this is not compensated for then this can create condensation and then damp issues within the chimney stack or on the face of the chimney stack e.g. in your loft space.

A poorly ventilated sealed chimney can cause new chimney damp issues in the form of condensation build up or in some cases make existing damp issues worse.

The chimney should be vented at both the top and the bottom. This then prevents cold air from being trapped in the chimney void as air is still allowed to flow through the chimney.

Any capping, sealing or bricking up that a chimney has had should itself be ventilated.

You can ventilate a capped chimney by adding vented caps or cowls to your chimney pots or by adding a paving slab directly on top of your chimney stack. This will need to be raised by about 2 or 3 inches to allow air to circulate.

Ventilating your chimney top can be a little tricky as it will involve you climbing onto your roof and fixing the vented cap, cowl or slab to the chimney stack. You may need the services of a roofer to do this.

You can ventilate a blocked up fireplace by adding an airbrick or vent to the wall in front of the fireplace.

This can be added to the wall about 300mm up from floor level or you can add it below your floor if there is an underfloor space to minimise draughts in your living area.

The addition of air bricks and vents is a little easier than adding ventilation to your chimney top.

To do this you will need to remove a brick or bricks from the chimney breast or bricked up fireplace and insert an air brick in its place to increase ventilation within the chimney void (a link to our Removing a brick project can be found above).

This should help minimise any damp in your chimney breast that has been caused by poor ventilation.

Wood Burning Stove/Gas Stove not Functioning Properly

If you have a wood burning or gas stove in use you have to ensure that the chimney is correctly lined.

The act of burning fuels (wood, gas etc.) can create massive quantities of water vapour that progresses up the chimney stack, getting cooler as it moves upwards.

If your chimney is not correctly lined then this will be allowed to rest on the inside of the stack, condense on the colder areas towards its top and form condensation, potentially causing damp in your chimney breast.

In this case any issues should be more evident towards the top of the chimney stack as opposed to near the base, and will in most situations appear as water droplets, damp patches, white salt crystal build-up or brown plaster staining on your chimney breast.

NOTE: Burning fuels produces fumes that can be potentially fatal if not removed from the property correctly or if there insufficient air within a room space. This can cause a potentially fatal amount of Carbon Monoxide to build up, so please take precautions to minimise the level of fumes present in your living space.

If you are suffering from damp issues in your chimney area and you have either a wood or gas burning stove installed then get your flue inspected right away!!

There may be no issue with the flue and the cause may be poor ventilation up the chimney stack itself, but do not chance it – get it inspected now.

You can find a qualified engineer to check that your chimney is working correctly over on the National Association of Chimney Engineers website.

Damp Proof Course

Does your chimney breast have a DPC (Damp Proof Course – layer of slate or plastic between brick courses about 6 inches above ground level)? Some older properties in particular were built with no DPC around the chimney breast.

If you don’t have a DPC around your chimney breast and water and moisture gets inside the stack and is allowed to sit at the base, or the ground beneath your chimney breast is damp, then this damp can rise up into the walls of your chimney breast (rising damp).

If you think that this may be the cause of your damp chimney breast, you will have to insert a chemical DPC into the base of your chimney.

This is a relatively easy job and can be done by a DIYer. For more information one this have a look at our project on injecting a damp proof course for larger jobs and also our project on smaller chemical DPC jobs.

You will need to drill some holes into the brick of the chimney breast at 6 inches up from ground level and inject a damp proofing cream using either a skeleton gun or a manual pump.

This cream soaks into the brick and the chemicals in it combine with one another to form a waterproof layer in the brickwork – your new DPC.

Another cause of your chimney damp could potentially be a bridged DPC in your chimney breast.

This is where rubbish and debris within the chimney or surrounding cavities rises above the level of the DPC in your chimney breast.

This can give moisture a direct route around the DPC and up into the chimney breast, causing damp in your chimney. Make sure that any debris has been removed to make sure this isn’t causing the damp problem.

Visual Signs of Chimney Breast Damp and How to Repair Them

In the section below you will find a description of a variety of visual ways that damp around a chimney breast can manifest itself and how each of these can be diagnosed and fixed:

Damp Patches, Brown or Yellow Stains or Marks on Walls and Ceilings Around Chimney Breast

The Issue With Damp Patches

If you have yellow or brown stains or marks that are appearing on ceilings that join to your chimney breast and spreading outwards then you may have a water leak.

Check the flashing on your roof around the chimney stack for splits and cracks and repair or replace your flashing if required as mentioned above.

You should also access your loft space and check the chimney breast and see if it’s damp.

If it is, then this could be damaged pointing that needs repairing or possibly even open chimney pots that could do with a chimney cowl being added to prevent the rain from entering whilst allowing ventilation.

Note that the issue of rain entering chimney pots is only an issue if your chimney is not in use, as an active chimney will evaporate this moisture.

Also be aware that just because there is no evidence of a leak around the immediate chimney area, water or moisture may be getting in and running along to the chimney area to form damp so ensure that your checks are thorough!

In the event that you can’t find any evidence of water or moisture penetration then another damp problem inherent to chimneys and the areas around chimneys is the presence of hygroscopic salts (sometimes known as Chimney Salts). These are usually deposited as a result of burning fuels.

As we all know fuels generally come from the ground (e.g. coal, wood etc.). During the time that these materials spend in the ground they can absorb salt minerals, sulphates, nitrates etc.

When the materials are burnt these salts and minerals are released and settle on the inside of the chimney.

Over a period of years these salts can then penetrate through the brick or block-work of your chimney breast and onto your inner walls.

The salts themselves are hygroscopic (moisture attracting) and when present on your chimney area they attract the moisture within a room space and can create damp patches on your chimney breast.

Any staining caused on these patches, in most cases, is the result of soot and other particles that have also managed to creep through the wall over a period of years and this is also occasionally due to the type of room e.g. kitchens with generally high humidity from cooking where water vapour can contain food particles.

Another typical indicator of hygroscopic salt contamination is the fact that the damp patches will in most cases be more apparent during wet weather or more humid conditions.

The Fix for Damp Patches

There are several ways to remedy this problem with varying degrees of difficulty.

Most of the internal walls of a house are plastered and once damp and moisture have been allowed to sit on or penetrate into a plastered surface for a period of time (including plasterboard), that surface will never be the same again. It will all need to be removed and you will have to re-plaster from scratch.

Before you can re-plaster you will have to deal with the salt issue itself.

Remove all the damp plaster from the affected chimney area and any surrounding areas and take it right back to the bare wall behind.

Allow the wall to dry out thoroughly, giving it as much time as is required, in some case this can be several months. Decent ventilation, fans and a dehumidifier will help.

Once your wall is as dry as it can be, you will then need to apply a salt neutralizer (allowing to dry and penetrate in accordance to manufacturers guidelines). There are quite a few available on the market with varying degrees of quality.

You can also get salt neutralisers that are already in the plaster you use to replaster your wall or they can be added to the plaster. Once all is dry and treated you can now go ahead and replaster your chimney area and redecorate as required.

In very extreme situations where the salt contamination is extensive it may also be necessary to apply an Air Gap Membrane.

This is a lining membrane (supplied on rolls similar to wallpaper) that is fixed over the wall’s surface after you have applied salt neutraliser.

The membrane then acts as a barrier preventing any further salts from penetrating through. More can be found about this in our lining damp walls project here.

In the event that your original chimney breast plaster survives and dries out okay through the use of ventilation, fans or a dehumidifier you may be okay to leave it without replacing it.

Apply the aforementioned salt neutralizer to the plaster surface (after stripping any wallpaper or paint) and allow it to dry (following manufacturer’s guidelines) correctly. You may need to give it up to a few weeks!

Once dry you can then redecorate again but beware this may be a short lived fix. Ultimately the best course of action is to knock all the plaster off and do the job properly!

Stains and Marks around the Base of Chimney Breast (Rising Damp)

The Issue With the Chimney Base

If you have damp patches that appear just above your skirting boards that appear no higher up the wall than about 1.5m then the chances are that you have rising damp.

There are several different potential causes of this the first being that you may not have a DPC (damp proof course) installed into your chimney breast.

As mentioned above some older chimney breasts do not have a Damp Proof Course installed.

To check this access the area around the chimney breast from inside and outside if possible, the DPC should be visible.

If you are unable to see it then the chances are that you do not have one, although it may be worth a second opinion from a reliable tradesman.

Another issue could be that the DPC is being bridged. During the build process or over time, rubbish and debris can collect in the base of the chimney stack or in adjacent cavities and any moisture or water that descends down can seep up through the rubbish, allowing it to travel around the DPC and back up into the wall creating the damp in your chimney breast.

To check whether this is occurring you will need to remove one or two bricks and access the chimney breast to have a look inside it for any debris – do not remove more than two bricks without consulting a reliable tradesman or property surveyor.

The Fix for the Chimney Base

If you have found that you do not in fact have a Damp Proof Course in your chimney breast then it is a good idea to add one.

You can do this by drilling holes into your chimney breast about six inches up from ground level and injecting a chemical cream DPC into the holes.

This is absorbed into the brick to form a waterproof chemical layer in the brickwork.

There are quite a few DIY chemical DPC kits available today that can be quite cheap and easily to install by anyone with a bit of DIY knowledge There are quite a few products of this type on the market, one we tend to use is mentioned above.

Once the Damp Proof Course has been added to your chimney breast you will have to allow the area to dry out thoroughly for as long as needed.

Depending on how bad the damage to plaster, wallpaper etc. is you may then need to remove it all and then replaster and redecorate. If there are any white salt deposits present, be sure to add a salt neutraliser as mentioned above.

In the case that you have debris and rubbish in the chimney breast void the only choice is to clean it out.

After removing one or two bricks (do not remove any more than two bricks without the express permission of a reliable surveyor or builder) use a torch, mobile phone, video camera etc. and get some images of inside the breast.

If debris is present, clean it out thoroughly wearing tough gloves to protect your hands from any sharp objects and allow it to dry properly. You can then replace the bricks and redecorate.

Also confirm that water is not being allowed to access through the stack or pots. If water is entering your chimney pots this way, simply add some chimney cowls to your chimney pots – these stop the rainwater ingress whilst allowing ventilation.

Plaster Blowing off Walls or White Salts Present on Surfaces

The Issue With Blowing Plaster

If you are suffering from very bad damp issues in your chimney, then in most cases this will cause some damage to the plaster as plaster once applied to walls and dried does not mix well with water.

This means that any of the damp causes above could be causing your plaster damage.

If you are experiencing a severe level of plaster damage then this is most likely down to a water leak so you will have to use the checks listed above to trace and find the leak and fix it.

If you have a white, salt-like staining present on plastered surfaces or even on bare brick or block surfaces then the damp you are seeing could be down to an extreme case of Hygroscopic Salts as mentioned above, so also be aware of this if you are unable to find any leaks.

Hygroscopic salts are deposits created as a result of burning fuel in your fireplace – these can travel to the surface of your chimney breast.

Hygroscopic Salts attract moisture in the air within a room and create visible damp patches or make the surface damp to touch. These patches usually come and go.

If the salty deposits you’re seeing are crunching and stand out from the wall, these as deposits known as ‘salts’. These are created when water in your damp wall evaporates on the surface of the wall, leaving behind salts. These can erupt through your plaster, blowing it off the wall.

The Fix for Blowing Plaster

Once you have resolved the cause of the damp (and either fixed the leak source or established that it’s down to a salt issue) remove all the damaged plaster from the chimney area and any affected surrounding areas (if any) and allow to dry thoroughly for as long as needed.

If the cause is leakage, once dried out correctly you can replaster and then redecorate, but if the damp was caused by hygroscopic salts or you had ‘salt’ deposits present then you will need to first add salt neutralizer to the wall’s surface to prevent any further salts penetrating through.

Strip back your plaster to the wall and you can then either add your salt neutraliser directly to the wall or add it to your replastering plaster. You can also get plaster mixes with a salt neutraliser already added.

We use the salt neutraliser manufactured by PRS as we have found it to produce consistently good results.

In extreme cases where you have massive salt build up you will also need to add a membrane to the wall’s surface before replastering to further prevent any future salt ingress (as mentioned above). You can then replaster over the membrane.

Once you have added the salt neutralizer (and membrane if required) and allowed it to dry (following manufacturers guidelines) you can then go ahead and plaster and redecorate your chimney area. More information on salt neutralizer and membrane can be found above.

If after examining the damp in your chimney area following the guidelines above you are still unsure about the cause of the damp and how to fix it, it’s then best to call in the professionals to get any issues resolved for you.

All project content written and produced by Mike Edwards, founder of DIY Doctor and industry expert in building technology.