The short answer
Damp on a chimney breast usually comes from one of three causes: rain getting into the stack through failed flashing, flaunching, pots or mortar; condensation inside an unused, unventilated flue; or salts and tar left by past fires that draw moisture and stain the plaster. Which one it is shows in the pattern, so look at when the stain appears, where it sits and what colour it is before paying for the wrong repair.
Rain getting in at the stack
A chimney stands in the weather on every side, and rain finds the weak points. On the roof, the lead flashing around the base of the stack can lift or split. The back gutter behind the stack, which carries water around it, can clog or crack. The flaunching and the pots at the top can open up. The brickwork can have open joints or spalled faces that soak up rain like a sponge.
Stains caused by rain tend to follow the weather. They darken or spread after heavy or prolonged rain and fade a little in a dry spell. They often sit high on the breast, near the ceiling, or in the loft where the stack passes through, and the wall can feel cold and wet to the touch.
To tell whether the water comes from the roof, the next step is to trace it outside. Our guide to why Stroud chimneys leak follows the water along the roof and shows where it most often enters. The stain inside tells you a leak exists; the roof shows where.
Condensation in an unused flue
A fireplace that is blocked off and a chimney that is capped at both ends can look tidy and still cause trouble. The flue is a tall, cold space. Warm, damp air from the house meets cold masonry, water condenses inside, and with no airflow to carry it away, it soaks into the breast.
This type of damp looks different. It is steadier, less tied to the weather and often worse in cold months. It can appear at a low or middle height on the breast, and it may come with a musty smell. It tends to affect a chimney that has been sealed for years, and the stain does not clear in a dry week.
The remedy is ventilation: an air brick or grille at the base of the breast and a vented cap at the top, so air moves through the flue. Our page on removing a chimney notes that taking a stack out is another way to deal with an unused flue, although it needs consent first. For most houses, a vented cap and a vent are far simpler.
Salts and tar from past fires
Fires leave more than soot. Over the years, burning coal, wood and other fuels deposits sulphates, tar and other salts in the flue walls. If the flue gets damp, those salts dissolve and travel through the brickwork to the surface of the plaster, where they dry as stains. Typical signs are brown or yellow-brown marks, sometimes with a sticky or oily feel, and a white, powdery bloom of salts. Fresh paint can bleed brown through within weeks.
Salt stains can persist even after the water source has been stopped, because the salts stay in the wall and draw moisture from the air. They tend to mark the line of the flue and come back through new decoration. The cure is to fix the water first, let the wall dry, then treat or replaster with a suitable lining before redecorating.
If you can smell old smoke or tar when it rains, or see marks the colour of tea, salts are likely part of the picture. A moisture reading or a closer look at the plaster will show how far they go.
Other causes worth ruling out
Not every stain on a chimney breast starts in the chimney. A leaking gutter or downpipe on the gable wall can soak the masonry from the outside. A damaged slate or tile close to the stack can let water in without touching the flashing. A plumbing pipe running through a breast can leak into it. And a loft with poor ventilation can add condensation to the upper part of the stack.
Rising damp is a different thing. It starts at ground level and climbs, and it is rarely the cause of a stain high on a chimney breast. A patch near the ceiling points to the roof, and one near the floor to something else.
These are the reasons a good diagnosis looks at more than the stain. At a survey, we check the roof, the stack, the flashing, the gutters and the pot together, working from the stain outward until the path of the water is clear.
Working out which cause you have, and what to do
A few simple observations narrow it down. Note when the stain appears: during or after rain suggests a leak, constant damp points to condensation, and stains that bleed through paint suggest salts. Note the height: high stains lean toward the roof, mid-height toward the flue. Note the colour: brown or yellow marks are likely salts, grey or dark patches with a musty smell are likely moisture and mould.
Then call a roofer to look at the stack from outside. Replacing a cracked or missing pot may be part of the answer; our note on chimney pot replacement explains that job. Photographs of what we find go with the written quote, which deals with the roof repair and any ventilation work together. Repairs carry a 10-year workmanship guarantee in writing, and for the bigger picture of stack work, see chimney repairs in Stroud.