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Damp On A Chimney Breast
Staining, a brown tide or blown plaster on a chimney breast, often on an upstairs wall or in the loft.
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What's happening?
A chimney breast is the most reliably damp wall in a British house, because it has vulnerabilities no other wall has. It runs from the ground to above the roof, so it collects rain over the whole height of the building and brings it down inside. Its masonry has spent decades absorbing combustion products, leaving the brickwork loaded with hygroscopic salts that pull moisture out of the air on their own. And when a fireplace is closed up, the flue that used to be ventilated by a fire becomes a sealed, unventilated shaft, so any moisture that gets in has no way out. That is why damp on a chimney breast is usually a combination of causes rather than one, and why sealing it up more thoroughly makes it worse.
What usually causes it?
- Missing, cracked or defective flaunching and chimney pot, letting rain straight down the flue
- Failed lead flashing or a defective apron and back gutter where the stack passes through the roof
- Porous or badly pointed stack brickwork, exposed on all four sides above the roofline
- Hygroscopic salts from combustion products in the flue brickwork, holding moisture from the air
- Fireplace closed up with no ventilation, so the flue cannot dry and moisture condenses in it
- Redundant flue not capped at the top, or capped without a vented cowl
- Condensation within the flue where a modern appliance vents into an unlined masonry chimney
Is it dangerous?
The damp is not, but two things around it are worth taking seriously. A stack whose flaunching and pointing have gone is deteriorating structurally in an exposed position, above a roof. And where the flue serves any working appliance, a blocked or compromised flue is a combustion safety matter rather than a damp one, and belongs with a suitably registered engineer rather than a damp proofer.
Can I fix it myself?
Two useful things. First, look at the stack from the ground with binoculars — cracked flaunching, a missing pot, open joints and slipped flashing are all visible, and knowing which you have shapes the job. Second, if a fireplace has been sealed with no vent, that is a real contributor and a vent in the closure is a recognised measure. What not to do: do not seal the breast with a waterproof coating or line it with foil-backed paper, which traps moisture in salt-laden masonry, and do not remove a chimney breast or cap a flue without understanding what else uses it.
When should you call a damp proofer?
Call a damp proofer where the pattern is salts and long-term staining on the breast itself, and the stack looks sound. Call a roofer where the water is arriving at the stack — flaunching, pot, pointing or flashing — because that is roof work and no internal treatment will fix it. Where the flue serves a working appliance of any kind, that appliance and flue want checking by the appropriately registered engineer first.
What will the damp proofer actually do?
- Establish which mechanism is operating: water coming down the flue, water crossing at roof level, or hygroscopic salts in the masonry
- Check whether the flue is ventilated at both ends and whether a redundant flue was capped without a vent
- Test the masonry for salt contamination rather than treating a meter reading as dampness
- Ventilate a sealed-up fireplace and cap a disused flue with a vented cowl so the shaft can dry
- Remove salt-contaminated plaster and replaster to a specification that resists salt migration
- Refer stack, flaunching, pointing and flashing defects to a roofer before internal work is done
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