How to Clean Stains Off Chimney Brick, Gently

Name the stain before you scrub

Each mark on chimney brick has a cause, and the cause points to the right cleaning approach. The table covers the stains found most often on fireplace faces and on chimneys that run down an outside wall.

Inside: the fireplace face and firebox

Lay down drop cloths and vacuum loose ash and soot from the brick and hearth first. Then wipe the black film with a dry soot sponge, a dense rubber sponge that lifts soot rather than smearing it. Work in strokes in one direction and turn the sponge as it loads up. Much of the stain comes off at this stage without any water.

What is left usually responds to warm water with a little mild detergent or a cleaner made for fireplace brick, worked in with a stiff nylon brush and wiped with a damp cloth. Use as little water as you can; brick soaks it up and dries slowly. Firebrick inside the firebox will not return to new color and does not need to; the aim there is removing loose deposits, not whitening it.

Smoke stains above the opening point to a draft problem

A fan of soot across the face above the fireplace is a symptom, not just dirt. Smoke that escapes into the room on its way up has a reason: a damper not fully open, a cold flue at startup, a firebox opening too large for the flue, a house that is pulling air down the chimney, or a partial blockage above. Cleaning the brick without addressing the cause means the stain comes back with the next fire.

Heavy, glossy black buildup inside the firebox or visible in the throat is a different material altogether. That is creosote, it is flammable, and it calls for a sweep rather than a scrub brush. A sweep can also tell you whether the deposits have reached the glazed stage, which takes more than brushing to remove.

Brick you can reach from the ground

Chimneys that run down an outside wall often have several feet of brick within easy reach. Choose a mild, overcast day above freezing, so the brick does not dry with cleaner on it and water does not freeze in the joints overnight. Wet the area first with a garden hose so the brick does not drink in the cleaner.

Apply the cleaner made for the stain, leave it for the time on the label, scrub with a nylon brush and rinse well. Work small sections from the top of your reach downward so runoff does not streak cleaned brick below. Protect nearby plantings, windows and metal trim. Anything that needs a ladder onto the roof is out of scope for a homeowner.

Efflorescence: dry first, water last

White salt on brick is often scrubbed with more water, which can make it worse. The salts dissolve, soak back into the brick and reappear as it dries. A stiff dry brush on a dry day removes most of it. Whatever remains can be lifted with a cleaner made for efflorescence, used sparingly.

The deposit itself is harmless, but its message is not. Efflorescence on an outside face means water is moving through the masonry from somewhere, such as a cracked crown, open joints or failed flashing. White deposits on the chimney breast indoors, or on basement brick, mean water is traveling through the whole wall and deserve a closer look.

What damages old brick

Many chimneys on prewar houses in the region were laid in soft, low-fired brick with lime mortar. That brick has a fired outer skin over a softer core, and once the skin is gone the brick absorbs more water and weathers faster. Several common cleaning shortcuts strip it.

• Acid cleaners, which etch brick, burn lime mortar and leave their own stains
• High-pressure washing, which cuts into the brick face and blows out joints
• Wire brushes and steel wool, which scour and leave rust spots behind
• Strong bleach mixtures, which can leave salt deposits and harm plants
• Sandblasting or grinding, which removes the fired surface for good

Stains above the roofline

Most stains worth worrying about sit on the stack above the roof: rust streaks under a cap, soot around a flue outlet, green growth on a shaded face. Cleaning there means water and chemicals on a steep, slick surface, which is no place for a homeowner. More usefully, a technician up there can find what caused the stain, whether a rusting cap, a corroding chase cover, failed flashing or a crown letting water in, and fix the source so the brick stays clean.

From the yard you can still take notes for that visit. Look at where streaks start and which way they run, whether they appeared after a storm or slowly over seasons, and whether the same face of the chimney stains inside the house. Those details shorten the search for the source and help decide whether the job is cleaning, metalwork, masonry or all three.

After cleaning: should the brick be sealed?

On sound brick with firm joints, a breathable water repellent made for masonry can reduce how much rain the face absorbs, which slows staining, efflorescence and freeze-thaw wear. It lets water vapor out while shedding liquid water. Paint-like or glossy sealers that form a film are a different product: they trap moisture inside the brick and can cause the face to pop off in winter.

Repellent is applied only after any cracks, open joints or crown problems are repaired, since it cannot bridge a gap. If you are unsure whether your chimney is a candidate, a free written estimate covers the cleaning, any repairs and whether a repellent makes sense.