Damp patches and brown staining on a chimney breast are very common in older homes, and they often get worse in autumn and winter. There are a few usual causes, and getting the diagnosis right is the key to a lasting fix.
1. Damp from the top of the chimney
An uncapped chimney pot leaves the flue open to the weather. Cracked flaunching (the mortar around the pots), eroded pointing on the stack, and failed lead flashings where the stack meets the roof all cause damp too. Damp from these sources often shows highest near the ceiling and gets worse after heavy rain.
2. Condensation in a blocked-up flue
When a fireplace is blocked up without ventilation, moist air trapped in the flue can condense on the cold masonry inside. The flue needs a vent at the bottom and a ventilated cap at the top so air can move through it.
3. Hygroscopic salts
Years of burning coal and wood leave salts in the brickwork of the flue. These salts absorb moisture from the air, so a chimney breast can look damp on humid days even when there is no defect outside. They often show as brown or yellow stains that come through paint and wallpaper.
4. Rising damp
Damp at the base of a chimney breast can also be rising damp, particularly if the damp proof course was never extended across the hearth area.
How each is fixed
- Rain: fit ventilated caps to unused flues, and repair flaunching, pointing and flashings
- Condensation: add ventilation at the bottom of a blocked fireplace and at the top of the flue
- Salts: remove contaminated plaster and replaster with a salt-resistant system, once the moisture source is dealt with
- Rising damp: install a damp proof course and replaster
Why painting over it doesn’t work
Stain-block paint may hide the marks for a while, but the moisture and salts remain and the staining usually returns.
Our surveyors work out which cause, or combination of causes, is to blame. See our damp proofing and property maintenance services.
