Black Stains on a Brick Chimney: What They Mean

Read the pattern before anything else

Where a stain starts, which way it runs and what its surface looks like tell you more than its color does. Stand across the street or at the far end of the yard on a dry day and look at the chimney with the light behind you. Note whether the dark area begins right under the crown or the cap, whether it sits in patches partway down, and whether it favors one side of the stack.

Then go inside and look at the chimney breast, the ceiling beside it and any closet or attic space the chimney passes through. A stain that shows up outside and inside at roughly the same height is a different problem from one that only exists on the outer face.

Soot streaks from the top of the flue

The most common black stain starts right below the crown or the cap and fades as it runs down the face. That is soot from the exhaust. Wind drops it on the top courses and rain carries it downward, so the streaks are usually heaviest on the side the prevailing wind pushes the smoke toward. Chimneys that serve a wood fireplace and those that serve an oil boiler show it most.

A light, even film that builds slowly over the seasons is normal for a working chimney. A sudden change is the thing to watch. A boiler flue that turns black in one winter often means the burner is not burning cleanly, and an oil burner out of adjustment can put out far more soot than a tuned one. That calls for the heating technician as well as the chimney sweep.

Mildew, algae and lichen on damp brick

Patchy dark growth, often with a green or gray tint, tends to appear low on the chimney, on the side that gets little sun, or near a gutter or downspout that keeps the brick wet. It is living growth on the surface rather than exhaust, and it often comes with moss on the crown or the roof around the stack.

The growth itself does little harm. The moisture behind it does. Brick that stays wet for weeks takes the full force of freeze-thaw cycles each winter, and that is how faces start to flake off and mortar joints start to wash out. Look for the source of the water: an overflowing gutter, a cracked crown, missing flashing or a cap that was never installed.

Tar-like patches: when the flue is leaking into the brick

The stain that matters most looks different from the other two. It is shiny, sticky or tar-like, sits in patches in the middle of the chimney rather than running from the top, and often follows mortar joints. It may come with white crust nearby, or with brown runs that look like coffee stains.

That pattern means exhaust is getting out of the flue and into the masonry. On a wood fireplace, creosote and moisture soak through cracked flue tiles or open liner joints and work their way to the surface. On an unlined or poorly lined boiler flue, wet exhaust condenses inside the cold masonry and carries soot and acids out through the brick. Either way, hot gases are reaching places they should not, so the flue should be inspected before the fireplace or appliance is used again.

Stains on the chimney breast indoors

Brown or black stains on the plaster of a chimney breast, sometimes with a smell of old smoke on damp days, are the indoor half of the same problem. They show up often in brownstones and rowhouses where a chimney runs through several floors, and they frequently get blamed on a roof leak. Water from above can stain plaster too, but water stains are usually tan with a hard edge, while flue stains tend to be darker, greasier and spread along the line of the flue.

Painting over a flue stain hides it for a season at most. The stain bleeds back through because the source is still feeding it.

Black on the cap and crown

A cap with blackened mesh and dark streaks below it is showing you what leaves the flue. Light soot is expected. Mesh that is caked and nearly closed is a problem, because a clogged screen holds exhaust in the flue. That can push smoke back into the room from a fireplace, or cause poor venting and spillage at a boiler. Heavy buildup on the cap is a reason to have the flue swept and the appliance checked.

Cleaning old brick without damaging it

Surface soot and growth can be cleaned, but the method matters on older brick. Much of the brick in pre-war houses in this region is softer than modern brick, and high-pressure water or strong acid cleaners can strip the fired face and erode the mortar joints, leaving the brick more absorbent than before. A gentle cleaner suited to masonry, low water pressure and a soft brush do the job with less risk.

Chimney cleaning also means working at the roof edge, which is a job for a crew with proper roof gear rather than a homeowner on a ladder.

Fixing the source

Soot stains respond to a swept flue and a properly tuned burner. Growth responds to drying the brick out: fixing the gutter, the crown, the flashing or the cap. Stains bleeding through from the flue are fixed at the flue, usually by repairing the liner or relining it, and only after that is it worth cleaning the outside.

Sealers are where people go wrong. A film-forming coating on brick that is getting wet from the inside traps the moisture, and the next winter it freezes behind the coating and pops the faces off. If a water repellent is used at all, it should be a breathable one, applied after the leak from the flue is fixed.