Question 1

Why is my chimney breast damp?

Accepted Answer

Three usual causes: water coming down the stack because the flaunching or pointing has failed, water coming in at the flashing where the stack passes through the roof, or — very commonly — a disused flue that was capped without being ventilated and is now sweating from the inside. The chimney is the most exposed masonry on the house. It stands clear on all four sides, has nothing above it, and 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 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 emerges on the chimney breast, often a floor or two below. From the ground it is invisible. The second is the pointing. Eroded joints let the brickwork take up water, and a saturated stack transmits that damp inwards. Stacks repointed at some time with a cement mortar harder than the brick are particularly prone, because the water cannot escape through the joint and instead 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 proper set of lead weatherings — an apron at the front, soakers and a stepped flashing at the sides, and a back gutter behind — all chased into the mortar courses, wedged and pointed. What you find far more often is a mortar fillet or a stripe of black bituminous paint, both of which crack. The fourth is the one that catches people out and costs them the most. If the fire has been taken out and the flue capped without being ventilated, the flue sweats: moisture from the masonry and from the house has nowhere to go and condenses inside. That produces damp staining on the breast that behaves nothing like a leak, and no amount of work to the roof will change it. The answer is a ventilated cap at the top and an air brick at the bottom — a cheap job. One more complication worth knowing. A chimney that carried a fire for a century holds hygroscopic salts from the combustion products, and those draw moisture out of the air — so a breast can look damp in humid weather long after the ingress has been fixed. Persistent staining after a genuine repair is often salt contamination in the plaster, and the remedy is replastering with a salt-retardant system rather than more roof work.

Question 2

Damp above the fireplace

Accepted Answer

Three usual causes: water coming down the stack because the flaunching or pointing has failed, water coming in at the flashing where the stack passes through the roof, or — very commonly — a disused flue that was capped without being ventilated and is now sweating from the inside. The chimney is the most exposed masonry on the house. It stands clear on all four sides, has nothing above it, and 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 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 emerges on the chimney breast, often a floor or two below. From the ground it is invisible. The second is the pointing. Eroded joints let the brickwork take up water, and a saturated stack transmits that damp inwards. Stacks repointed at some time with a cement mortar harder than the brick are particularly prone, because the water cannot escape through the joint and instead 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 proper set of lead weatherings — an apron at the front, soakers and a stepped flashing at the sides, and a back gutter behind — all chased into the mortar courses, wedged and pointed. What you find far more often is a mortar fillet or a stripe of black bituminous paint, both of which crack. The fourth is the one that catches people out and costs them the most. If the fire has been taken out and the flue capped without being ventilated, the flue sweats: moisture from the masonry and from the house has nowhere to go and condenses inside. That produces damp staining on the breast that behaves nothing like a leak, and no amount of work to the roof will change it. The answer is a ventilated cap at the top and an air brick at the bottom — a cheap job. One more complication worth knowing. A chimney that carried a fire for a century holds hygroscopic salts from the combustion products, and those draw moisture out of the air — so a breast can look damp in humid weather long after the ingress has been fixed. Persistent staining after a genuine repair is often salt contamination in the plaster, and the remedy is replastering with a salt-retardant system rather than more roof work.

Question 3

Staining on the chimney breast

Accepted Answer

Three usual causes: water coming down the stack because the flaunching or pointing has failed, water coming in at the flashing where the stack passes through the roof, or — very commonly — a disused flue that was capped without being ventilated and is now sweating from the inside. The chimney is the most exposed masonry on the house. It stands clear on all four sides, has nothing above it, and 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 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 emerges on the chimney breast, often a floor or two below. From the ground it is invisible. The second is the pointing. Eroded joints let the brickwork take up water, and a saturated stack transmits that damp inwards. Stacks repointed at some time with a cement mortar harder than the brick are particularly prone, because the water cannot escape through the joint and instead 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 proper set of lead weatherings — an apron at the front, soakers and a stepped flashing at the sides, and a back gutter behind — all chased into the mortar courses, wedged and pointed. What you find far more often is a mortar fillet or a stripe of black bituminous paint, both of which crack. The fourth is the one that catches people out and costs them the most. If the fire has been taken out and the flue capped without being ventilated, the flue sweats: moisture from the masonry and from the house has nowhere to go and condenses inside. That produces damp staining on the breast that behaves nothing like a leak, and no amount of work to the roof will change it. The answer is a ventilated cap at the top and an air brick at the bottom — a cheap job. One more complication worth knowing. A chimney that carried a fire for a century holds hygroscopic salts from the combustion products, and those draw moisture out of the air — so a breast can look damp in humid weather long after the ingress has been fixed. Persistent staining after a genuine repair is often salt contamination in the plaster, and the remedy is replastering with a salt-retardant system rather than more roof work.

Question 4

Chimney leaking inside

Accepted Answer

Three usual causes: water coming down the stack because the flaunching or pointing has failed, water coming in at the flashing where the stack passes through the roof, or — very commonly — a disused flue that was capped without being ventilated and is now sweating from the inside. The chimney is the most exposed masonry on the house. It stands clear on all four sides, has nothing above it, and 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 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 emerges on the chimney breast, often a floor or two below. From the ground it is invisible. The second is the pointing. Eroded joints let the brickwork take up water, and a saturated stack transmits that damp inwards. Stacks repointed at some time with a cement mortar harder than the brick are particularly prone, because the water cannot escape through the joint and instead 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 proper set of lead weatherings — an apron at the front, soakers and a stepped flashing at the sides, and a back gutter behind — all chased into the mortar courses, wedged and pointed. What you find far more often is a mortar fillet or a stripe of black bituminous paint, both of which crack. The fourth is the one that catches people out and costs them the most. If the fire has been taken out and the flue capped without being ventilated, the flue sweats: moisture from the masonry and from the house has nowhere to go and condenses inside. That produces damp staining on the breast that behaves nothing like a leak, and no amount of work to the roof will change it. The answer is a ventilated cap at the top and an air brick at the bottom — a cheap job. One more complication worth knowing. A chimney that carried a fire for a century holds hygroscopic salts from the combustion products, and those draw moisture out of the air — so a breast can look damp in humid weather long after the ingress has been fixed. Persistent staining after a genuine repair is often salt contamination in the plaster, and the remedy is replastering with a salt-retardant system rather than more roof work.

Why is my chimney breast damp?

Got a roofing job in Macclesfield?

Got a roofing job? Let's get it sorted.