The short answer
Brown staining on a chimney breast is usually either tar from the flue or water coming in from above. Tar stains are oily, dark and often smell of smoke in damp weather, and they bleed through fresh paint. Water stains show tide marks and get worse after rain. Both can be present together, so the flue and the stack both need checking before the wall is decorated.
How tar reaches the wall
When wood, coal or other solid fuel burns slowly or with damp fuel, the smoke is cool and wet. It condenses on the inside of the flue and leaves a sticky deposit of tar and creosote. Over time the deposit soaks into the flue wall, and in an unlined flue it can pass through the brick or the mortar joints and reach the plaster of the chimney breast on the other side.
The result is a brown or black mark that tracks the path of the flue. It may be a patch at one height or a run that tracks up the breast. The surface can feel slightly greasy or shiny, and in humid weather there may be a smell of old smoke or tar. Paint does not cover it. The tar migrates through emulsion within weeks and shows as a brown halo.
Flues that are no longer used can still do this, because old deposits remain in the structure and soften in damp conditions. The stain does not need a fire to reappear, and a damp winter is enough to bring it back.
How rain reaches the wall
The other cause is water. Rain finds its way into the stack through eroded pointing, a cracked flaunching, a flue left open without a cap, or a gap in the lead around the base. It runs down the masonry, collects dissolved salts and iron compounds on the way, and shows on the breast as a brown tide mark when it dries.
Water stains look different. They have a soft edge that grows after wet weather and fades a little in dry spells, and they are often accompanied by a damp, cold feel in the plaster, flaking paint or white salt deposits. They are most obvious on the upper floor and near the ceiling, where the stack meets the roof, though water can run a long way down inside a flue and appear lower.
Our page on the life of chimney flashing is a good place to start if the stain is near the roof line. It does not have to be a dramatic fault. A small gap in the lead can wet a breast for years.
Telling tar from water at home
You cannot diagnose a stain with certainty from a photograph, but a few observations help.
- Smell. A smoky or tarry smell, stronger when the air is damp, points to the flue.
- Texture. An oily, glossy or tacky patch suggests tar. A dull, chalky or powdery patch with salts suggests water.
- Weather. A stain that grows soon after heavy rain is probably water. One that varies with humidity and fires is more likely tar.
- Position. Tar tends to follow the flue line. Water often starts near the ceiling or the roof.
- Fuel. A history of wet wood or slow-burning coal makes tar more likely.
If the loft hatch is easy to reach, put your head through and look there as well. A wet patch on the timber around the stack, or damp brickwork, shows that water is entering above. If the staining returns in the same place after being sealed and painted, that is a sign the source has not been dealt with. Which trade to call first is the subject of roofer or sweep for chimney problems.
Dealing with the cause before the stain
Cosmetic work comes last. For tar, a sweep should clean the flue, and a lined flue or a liner suited to the fuel will stop further bleeding. Burning dry, well-seasoned fuel at a proper burn rate cuts the amount of tar produced. Once the source is controlled, the stained plaster can be cut back and replastered, or treated with a stain-blocking sealer before decoration. Standard paint on its own will fail.
For water, the roofer fixes the entry point: repointing the stack, renewing the flaunching, replacing the lead, fitting a cap or a cowl. Then the breast needs time to dry out, which can take a long while in a thick masonry wall. Decorating a damp wall too soon traps the moisture and brings the stain back.
Where render on the stack is cracked, water can run behind it and show as staining below. Our guide to rendering a chimney explains how render fails and when it helps.
When the answer is to take the stack down
Sometimes a chimney is surplus to the house. If the flues are unused, the stack is damaged beyond sensible repair, and removing it would also stop the staining, taking down the stack may be the cleaner solution. Removal above the roof line is different from removing the breast inside, and each has its own planning and structural points. Taking the stack down is covered in chimney stack removal, and in a listed building or conservation area the council should be asked first.
If you are unsure which cause you have, the stack is the place to start, because it is the part we deal with. The free survey puts someone on the roof to photograph the masonry, pots and lead, and you are shown what the pictures reveal. That evidence tells you whether the stack is admitting water.
If it is, the repair is priced and fixed in writing, and the stained plaster inside should be left to dry out fully before it is sealed and redecorated. If the stack proves sound, you will know the next step is the flue, and a sweep is the right call. Our work begins at the chimney repairs hub.
