Damp chimney breasts & staining
In short
Damp on a chimney breast has three usual causes: water coming down the stack because the flaunching or pointing has failed, water coming in at the flashing where the stack meets the roof, or a disused flue sweating from the inside because it was capped without being ventilated.
Also called: Chimney breast damp · Staining above fireplace · Damp round chimney · Chimney leaking
Signs you need this
- A damp or stained patch on the chimney breast, often upstairs
- Staining that shows in the room below an unused fireplace
- Wallpaper lifting or plaster blown around a breast
- White crystalline salts on the plaster surface
- Visible cracks in the mortar at the top of the stack
- Eroded or missing pointing on the stack brickwork
The chimney is the most exposed masonry on the house. It stands clear of the roof on all four sides, it has nothing above it, and it takes wind-driven rain from every direction — so it is both the commonest source of a roof leak and the commonest source of internal damp that gets blamed on the roof covering.
The first suspect is the top. The flaunching is the mortar fillet on the head of the stack that holds the pots, and it is the single most weather-beaten piece of mortar on the building. Once it cracks, water runs straight down the inside of the stack and comes out on the chimney breast, often a floor or two below. From the ground a cracked flaunching is invisible; from a scaffold or a drone it is obvious.
The second is the pointing. Eroded mortar joints let the brickwork itself take up water, and a saturated stack transmits that damp inwards. Old stacks that have been repointed at some point with a cement mortar that is harder than the brick are particularly prone — the water cannot escape through the joint, so it goes through the face of the brick and spalls it.
The third is the flashing where the stack passes through the roof. It should be a stepped lead flashing chased into the mortar courses, wedged and pointed, with a separate soaker or apron detail — not a mortar fillet, and not black bituminous paint, both of which are extremely common and both of which crack. This is the classic 'the roof leaks near the chimney' fault.
The fourth is different and catches people out. If the fire has been taken out and the flue capped, but the stack was not ventilated top and bottom, the flue sweats: moisture from the masonry and from the house has nowhere to go and condenses inside. It produces damp staining on the breast that behaves nothing like a leak, and no amount of work to the roof or the flashings will change it. The answer is a vented cap and an air brick at the bottom.
Salts complicate diagnosis, usefully. A chimney that has had a fire in it for a century holds hygroscopic salts from the combustion products, and those salts draw moisture out of the air — so a breast can look damp in humid weather long after the water ingress has been fixed. Persistent staining after a genuine repair is often salt contamination in the plaster, and the fix is replastering with a salt-retardant system rather than more roof work.
What the job covers
- Inspection of the stack head, flaunching and pots
- Pointing condition assessed on all four faces
- Flashing at the roof junction checked properly, not from the ground
- Ventilation of a capped flue checked top and bottom
- Salt contamination distinguished from live ingress
- A clear answer on which of the causes is actually yours
What it costs
We don't publish a price for this yet, because a number pulled out of the air is worse than no number at all — your roofer has to honour it. On roofing it would also be close to meaningless: access is often the biggest single factor, and it varies enormously house to house. You'll get a firm price for your actual job when they ring you back, before any work starts. The quote itself is free and there's no obligation either way.
Common questions
The fire was removed years ago. Why is it damp now?
Very likely because the flue was capped without being ventilated. A sealed flue has no airflow, so moisture from the masonry and from the house condenses inside it and comes out on the breast. It needs a vented cap at the top and an air brick at the bottom — a cheap job that fixes a problem people spend a lot on the roof trying to solve.
Is it the flashing or the stack?
Position is the clue. Damp appearing well down the breast, on a lower floor, usually means water coming down the inside of the stack from a failed flaunching or pointing. Damp concentrated at ceiling level in the top room, near where the stack passes through, points at the flashing. Both get checked, because a stack that has failed at the top has often failed at the flashing too.
Why does the damp come back after repointing?
Frequently because it is not damp any more — it is salt. A flue that carried smoke for a century holds hygroscopic salts, and those pull moisture from the air and stain the plaster in humid weather even with the roof perfectly sound. The remedy is replastering with a salt-retardant system, not further roof work.
My chimney is shared with next door. Does that matter?
Yes. A stack sitting on the party wall is usually a party structure, so work to it falls under the Party Wall etc. Act 1996 and the neighbour has to be given written notice — two months for work to a party structure. It is the homeowner's duty rather than the roofer's, and it is much easier to do at the start than to argue about afterwards.
Need this doing in Macclesfield?
Tell us the job and it goes straight to our local roofer — one call back, usually the same day. Free quote, no obligation.