Damp on a chimney breast is one of the most misread faults in an older house. The stain appears on the wall of a living room or bedroom, so the instinct is to treat the plaster, but the water has usually arrived from somewhere else. A chimney breast is the interior end of a long, narrow shaft that rises through the house and out above the roof. Rain can enter at the top, moisture can condense inside the flue, and salts left by years of burning fuel can draw damp out of the air. Each source leaves a slightly different mark and needs a different cure, so repainting or replastering without finding the cause only hides the problem for a season. This guide sets out the possible sources in the order we would check them: first water from outside, then condensation, then salts, then the less likely causes. It is a way of narrowing the search, and a survey on site is what settles it.
Start with what the damp looks like
The pattern on the wall is the first clue, so spend a few minutes noting it before anyone touches the chimney.
- Is the damp near the ceiling, in the middle of the breast or low down by the skirting? Water getting in at roof level tends to show higher up, while rising moisture shows at the base.
- Does it come and go with the weather? A patch that appears after a heavy downpour and dries in a spell of fine days is likely to be rain getting in. A patch that stays through dry weather and worsens in cold spells points to condensation or salts.
- Is it brown or yellow, with a tide mark at its edge? That suggests water has travelled through the masonry and carried staining with it.
- Does the surface feel powdery, or do white crystals form on it? That points to salts.
- Is there a sooty or tarry brown stain that bleeds through fresh paint? That is a separate problem with its own page, covered in our guide to tar staining on chimney breasts.
Take photographs on dry days and wet days from the same place, and write the dates on them. A record of when the damp appears is worth more than any single reading, and it tells us whether to start at the top of the stack or lower down. Rooms on the upper floors of a house can show damp from the stack above the roof, while a ground-floor breast can show damp that travelled down inside the wall from an upper room.
It also helps to know whether the flue is used. A working flue is warm and dries itself out. An unused one is cold and still, and that changes which causes are likely.
Rain entering at the top of the stack
Water from outside is the cause we rule in or out first, because it is the most common and the quickest to fix. The path is simple: rain lands on the top of a stack, finds a way in, and runs down the inside of the masonry to the room below. Our checks run from the crown of the stack downward.
Pots, cowls and the crown
An open flue lets rain fall straight down the shaft in a storm. Cowls and caps reduce this, and a pot that is cracked or missing leaves the top of the flue open. The flaunching, the sloped mortar that seats the pots, is the next suspect. Cracked or crumbling flaunching is a quick route for water, and the way it fails is set out in our flaunching guide. A damaged pot can be swapped for a new one, which is the subject of the chimney pot replacement guide.
The shaft
Weathered joints and cracked or spalled units let water into the body of the stack. The stack is exposed to rain from every side, so it takes more punishment than the walls beneath. If the joints are open and the stone or brick is soft, the whole shaft can be wet through after a storm.
Flashings and the roof junction
Where the stack meets the roof, lead flashings and soakers keep water out of the gap. A flashing that has lifted, split or been pulled from its joint lets rain in at roof level. Lead tray details, where they exist, are a further line of defence. If the lead is the fault, the damp often appears on the breast just below ceiling height, or in the loft against the stack.
The roof around the stack
Slipped tiles, cracked slates or a failed valley near the stack can send water towards the masonry. This is why a proper inspection takes in the whole area around the stack, not only the stack itself.
Condensation in the flue
If the damp does not follow the rain, condensation is next. A flue is a cold, tall tube. When warm, damp air from the house or from a gas appliance meets the cold inner surface, the water vapour turns to liquid on it. The moisture can then soak into the brickwork and show on the breast.
This is most likely in three situations.
- A disused flue that has been sealed. Closing the top of an unused flue with no way for air to move through it traps damp air inside. The moisture has nowhere to go but into the masonry. A flue that is no longer in use is capped to keep rain out and ventilated to let air pass, so that the chimney can dry.
- A flue that serves a modern appliance. A gas boiler or a stove produces moisture in its combustion products, and a flue that is too wide or lacks a liner stays cool enough for that moisture to condense on the walls. The appliance and flue should be assessed together by a suitably qualified engineer.
- A cold stack in an unheated room. A chimney breast in a bedroom that is rarely warmed can cool enough on its inner face to gather condensation from the room air, particularly when the room is damp from drying washing or poor ventilation.
Condensation damp is usually worse in winter, and it tends to be patchy and low-level rather than a single stain below the ceiling. Black mould on the surface of the paint is a signal that moisture is sitting at the wall. The wider chimney repairs page links the other jobs a stack can need, and the loft side can be checked at the same visit.
Do not seal an unused flue airtight to stop damp. It usually makes condensation worse and leaves the masonry wet.
Salts left behind by old fires
Salts are the least familiar cause and the one that most often leads to repeated redecoration. Burning coal, wood and gas leaves chemical residues in the flue, and over many years they soak into the brick and plaster around it. Some of these salts attract moisture from the air. They take up water on damp days and dry out on dry days, so the wall looks wet even when no rain has entered the stack.
The signs are quite distinct. The damp patch appears and disappears without any link to rainfall, it is often worse in humid weather, and a fine white powder or crystals may collect on the surface. New paint or wallpaper blisters and fails because the salts push through it. Staining can reappear within months of a decoration job.
Salts are not cured by treating the surface. Painting over them or applying a damp-proof paint hides the symptom briefly and then fails. The usual remedy is to deal with the source of any moisture first, then remove the contaminated plaster back to sound brick, let the wall dry and replaster with a mix that suits the wall. In an older house a breathable plaster is chosen, so the wall can release moisture and not trap it.
Because salts are carried by water, they often accompany one of the other two causes. A stack that has been wet from the top for years will have moved salts down the wall. That is why we look for the water source before recommending any plaster work, and why a replastered breast that goes damp again is frequently the sign that the stack above is still leaking.
Where the house is listed, finishes inside the building can also need consent, and a lime-based plaster is often the better material. Our guide to listed chimneys covers the consent side of work to the stack itself.
Other causes worth ruling out
If the stack, the flue and the salts have all been checked and the damp persists, there are a handful of other possibilities to consider before returning to the roof.
Plumbing and heating pipes
A leaking pipe running through or beside the chimney breast can wet the wall from behind. A pipe that drips slowly will produce a persistent patch that does not track the weather at all. Pipes in the wall are checked before any further work is done to the stack.
Gutters and downpipes
A gutter that overflows or a downpipe that leaks beside a chimney breast on an external wall can soak the masonry from outside. Because the breast is part of the external wall in many terraced houses, the damp can pass through the full thickness.
Ground moisture
Low-level damp at the base of the breast may be rising or penetrating damp from the ground or from raised outside levels. It looks like a tide mark that creeps up from the skirting. That is a building-damp question and sits outside roofing, though we mention it so you can rule it in or out.
A previous repair
Hard cement render or repointing on the outside of the stack can force water to leave through the interior face instead of the exterior. If the damp began soon after a repair, or the stack has a cement skin, that is worth raising when you talk to the surveyor.
It is rare for only one factor to be at work. A stack with weathered joints, a cracked cap and a tired flashing may have all three problems feeding the one patch on the breast. For that reason we repair the whole path of the water and not just the nearest defect.
What a survey will do and what to expect
A survey follows the same order as this guide, from the top down. We inspect the pots, cowl and flaunching, then the shaft and its joints, then the flashings and the roof around the stack. Inside the loft, the back of the stack is checked for staining, tide marks and wet timber. We send you pictures of the stack taken during the visit, so the findings are plain to see.
Often the finding is plain. A cracked flaunching and an open joint under it explain a patch on the breast beneath, and the repair is small. At other times the roof is sound and the cause is condensation or salts. In that case we tell you so, and tell you what to do about the flue, even if the next step is not roofing work.
Where the stack needs work, a fixed price is written up after the visit, listing each repair along with the mortar, lead and scaffold. For a listed stack, or one in a conservation area, we outline what consent may be needed and suggest asking B&NES planning first. Where an existing flue serves an appliance, we recommend a check by a qualified engineer once any stack repairs are complete.
After repairs, give the wall time to dry before redecorating. A breast that stayed saturated for a long spell can take weeks, or longer, to dry out, and the moisture content falls more slowly in a cold, closed room. Opening a window, keeping the room gently heated and leaving the surface unpainted until it has dried will give a better finish. The stack and roof repairs we carry out sit under our 10-year written workmanship guarantee.