Why is my chimney breast damp?
The six routes water and salts take into a chimney breast, how to tell an active leak from decades-old combustion salts, and why sealing a flue at both ends makes it worse.
- Difficulty
- intermediate
- Time
- 1 hr
- Read
- 5 min
- Safety
- restricted
Short answer
Six routes: cracked flaunching at the top, an uncapped pot, the exposed stack brickwork, failed flashings, hygroscopic salts left by decades of smoke, and condensation in a flue that has been sealed at both ends. The first four follow the rain; the last two do not, and that difference tells you which you have.
A chimney is a column of masonry standing above the roof, wetted on four faces, warmed on none, with a hole running down the middle of it into your house. Even in good condition it is the wettest part of the building. Add a working life of combustion products soaked into the lining, and a chimney breast can stain and bloom long after the roof above it has been made watertight.
Safety
What you'll need
- Binoculars
- Torch
- Access to the loft
Step by step
- Establish whether it tracks the weather.Mark the patch and date it. Growing after heavy rain and shrinking a week later means water is getting in from above. Unchanged by rain, but darker in muggy weather, means salts.
- Look at the stack with binoculars.Cracked or missing flaunching around the pots, open pots on a disused flue, spalled or missing bricks, failed pointing, and any lean. Photograph it at maximum zoom from two directions.
- Look at the flashings.The apron at the front, the stepped flashings up the sides, the back gutter behind. A cement fillet where lead should be is a confirmed future leak. Any of these lets water into the roof void and down the breast.
- Go into the loft and look at the stack there.Staining on the brickwork inside the roof space, wet timbers around the stack, or daylight around it all localise the entry point. This is the safest and most informative view you will get.
- Check the flue's ventilation.A disused flue that has been sealed at the top and blocked at the fireplace has no airflow. Moisture in the masonry cannot leave, so it comes into the room. This is a very common cause of damp on a chimney breast in a house where the fires were closed up.
- Look for the salt signature.A brown or yellow tarry stain, or a white bloom, that comes back through paint within weeks, is worse in humid weather, and is unaffected by rain. That is hygroscopic contamination from combustion products, and no roof repair will change it.
- Have the stack repaired properly if it is leaking.Roofer's work: reflaunch, repoint or rebuild the stack as needed, replace cement fillets with lead, and cap or cowl a disused flue. Do not attempt any of it yourself.
- Ventilate a disused flue at both ends.A vented cowl or a capping that allows airflow at the top, and an airbrick or a proprietary vent at the bottom in the blocked-off fireplace. The flue must be able to breathe or the damp returns.
- Replace salt-contaminated plaster once the water is stopped.Hack off to at least 300mm beyond the visible contamination and replaster to a specification appropriate to the wall. Painting over it does not work, and a stain blocker only buys time on a plaster that is still contaminated.
Tips
- If a chimney has been removed above roof level but the breast is still in the house, check what was done at the roof. A stack taken down and simply tiled over frequently leaves an unventilated column of masonry that goes damp.
- Removing a chimney breast in one room while leaving it in the room above requires proper support and building control approval. Unsupported breasts left in lofts are a recognised structural hazard.
- Have any flue that is still in use swept and checked before winter, and fit a carbon monoxide alarm in any room with a fuel-burning appliance.
Common mistakes
- Sealing a disused flue airtight at top and bottom — The masonry stops drying and the moisture inside it, plus any that gets past the cap, comes through into the rooms as damp patches. A flue needs a small continuous airflow whether it is in use or not.
- Repainting a tarry chimney stain with ordinary emulsion — Combustion residues bleed through emulsion indefinitely. Until the plaster is replaced, every repaint fails within weeks.
If it doesn't work
Damp on the breast in the loft and the top bedroom, after rain
Cause: Flaunching, stack brickwork or flashings — Fix: Roofer, from proper access. Inspect from the ground and the loft first so you can describe what you have seen.
Damp on a breast in a house where the fires were blocked up
Cause: Sealed unventilated flue — Fix: Fit a vented cowl or ventilated cap at the top and a vent at the bottom, and let the flue dry.
Brown or yellow stain returning through fresh paint
Cause: Hygroscopic salts and tar from combustion — Fix: Replace the plaster; sealing and painting will not hold. Address any active water first.
White bloom on a chimney breast that is worse in summer
Cause: Hygroscopic salt contamination pulling moisture from room air — Fix: Same as above — the plaster is the problem, and the room's humidity makes it visible.
Questions people ask
How can it be salts if there is no fireplace any more?
The salts are in the masonry and the old flue lining, not in the fireplace. Decades of smoke drove sulphates and chlorides into the brickwork, and they stay there long after the fire, the grate and the surround have gone.
Should I have the flue lined?
If it is to be used, yes, and that is a job for a competent installer to a current standard. If it is disused, lining is not the answer to damp — ventilation and a weathertight top are.
The damp is only on one side of the breast. Does that help?
Yes. It usually points at a specific flashing or a specific face of the stack rather than the flue. Compare which side of the roof the damp side corresponds to, and inspect that flashing first.