Why Is Water Running Down My Chimney Breast?

What the chimney breast is

The chimney breast is the part of the chimney that projects into the room, usually around the fireplace and running up through the floors above. Behind the plaster is brick, and behind the brick are one or more flues. Water that gets into any of those layers can work its way out through the plaster and paint, where it shows as a damp patch, bubbling paint, a tide line or a brown stain.

In brownstones, rowhouses and older brick houses in the city and its older suburbs, the breast often runs through three or four floors and may carry flues for fireplaces on every level plus the boiler. A leak at the top can show up on the top floor only, or travel down inside the masonry and appear lower.

Reading the timing

When the stain appears tells you more than where it appears. Keep a simple note for a few weeks: the date, the weather, whether the heat was running, and how the patch looked. That record often points straight to the source.

• Damp that grows after rain and shrinks in dry spells: the crown, cap, joints or flashing, fixed at the entry point
• Damp in dry weather all year: a sealed flue with no airflow, solved by venting the flue and capping the top
• Yellow or brown stains through new paint: salts from old soot, sealed with a stain-blocking primer once dry
• Damp worst in the coldest weeks while the heat runs: boiler or water heater condensation, solved with a sized liner
• Damp after snow melts: the crown, flashing or ice at the roof line

Rain coming through the masonry

If the damp patch grows after storms and shrinks in dry weather, rain is the likely source. It gets in through a cracked crown, an uncapped flue, worn mortar joints, porous brick or loose flashing, then moves down through the brick. Wind-driven rain from one direction often explains why the stain appears on one side of the breast and not the other.

The patch is usually highest on the top floor, often in the upper corners or right below the ceiling. Paint bubbles, plaster softens and you may see a tide line where the water stopped. On the floor below, the same leak may show as a stain at the ceiling line beside the chimney.

Sealed fireplaces and stained plaster

Many old houses have fireplaces that were boarded up long ago, with the flue left open at the top and sealed at the bottom, or sealed at both ends. A flue with no airflow traps moisture. Damp air condenses on the cold brick inside, and the breast stays damp no matter the weather.

Old soot from generations of coal and wood fires also holds salts that pull moisture from the air. Those salts can bleed through new paint as yellow or brown stains, even when no water is coming in. Repainting covers them for a while, then they come back. A small vent in the board over the fireplace opening, with the flue capped at the top, lets air move through and often ends this kind of damp.

Boiler flues and condensation

If a gas or oil boiler vents into a flue in the same chimney, its exhaust carries water vapor. In a cold, unlined or oversized flue, that vapor condenses on the inside of the brick. The water is acidic, often mixed with soot, and can soak through to the breast on any floor the flue passes. This kind of damp gets worse in the coldest weeks, when the boiler runs most and the chimney is coldest, and fades in summer.

It is also common after a boiler replacement, when a newer, more efficient boiler sends cooler exhaust into an old flue, or when a water heater is left alone in a flue sized for two appliances. A boiler flue that is damaged enough to leak water can also leak exhaust. Keep CO alarms working near sleeping areas; an alarm, or anyone feeling sick, is a reason to leave the house and phone 911 or the gas company.

What a technician checks

Tracing a damp breast starts at the top: the crown, every flue cap, the condition of the brick and joints above the roof, and the flashing. Then the flues are identified, floor by floor, and each one is checked for what it serves and whether it is lined, open, sealed or wet inside. The base of any boiler flue is opened to look for water, scale and fallen tile.

A moisture meter on the plaster, read on different floors, shows how far the water travels. The aim is to name the source, or sources, before any repair is proposed, since an older chimney often has more than one.

Expect the findings in writing, flue by flue, with each proposed repair tied to the source it stops. That makes it easier to decide what to do first if the work has to be phased.

How a damp chimney breast is fixed

First the water is stopped at its source: a cap on every flue, crown repair, repointing, new flashing, or a liner sized for the boiler. A closed-off fireplace gets a vent so air can move through the flue, and an unused flue is capped at the top so rain stays out while air still moves. Breathable water repellent on sound brick can help where the brick itself is porous.

Then the plaster is allowed to dry before it is repaired. That can take weeks on an old, thick wall. Painting over a wet or salt-stained breast just hides the problem until the next season.

What you can check yourself

From the street, with binoculars, check whether every flue has a cap and whether the crown looks cracked. Inside, feel the damp patch on a dry day and again after rain. Note whether the boiler was running hard. If there is a sealed fireplace on that breast, check whether the board over the opening has any vent. Bring those observations to the inspection.

• Caps on every flue, seen from the ground
• Damp after rain only, or in dry weather too
• Worse when the heat runs hard
• Sealed fireplace with no vent on that breast
• Stains returning through fresh paint