What’s inflicting these stains on the chimney?

Question: Our chimney is badly polluted by the weather. A chimney sweep told us that chemicals and power wash would not guarantee the removal of these stains, and a strong power wash could even damage the brick. How can we safely remove the stains?

Answers: The stains could be mold, dirt, algae, or soot, according to reviews from four Maryland chimney sweeps.

“Trust me, it’s mold,” said Jim Shortley, owner of My Chimney Sweep Service in Potomac, Maryland. The remedy, he said, is to spray with a bleach-based product labeled to remove mold and mildew, wait 10 to 15 minutes, then rinse. Sometimes he also needs to remove stubborn stains with a synthetic fiber brush. So that nothing more than the chimney fades, he first covers the nearby roofs with a heavy tarpaulin.

Carlos Aragon, owner of Aragon Chimney Services in Derwood, Md., Also said the stains are likely mold. He checked that the bricks and mortar were in good condition, then used a pressure washer to remove the stains. This is the “easiest and fastest” way to clean the outside of a chimney, he said.

Carmine Ferraro, owner of Colonial Chimney in Germantown, Md., Said he believed the problem was algae and he would use an algae removal cleaner, “the same stuff we use on the roof,” and a pressure washer. Both Ferraro and Aragon said they would also coat the bricks with a water repellent after cleaning to prevent the problem from recurring.

Steve Coates, owner of Chimney Masters in Germantown, said the black spots could be dirt from a nearby tree, soot, or a combination of soot and dirt. He would use an acidic cleaner that bubbles up as he does his job and requires a lot of attention to safety and washes everything from the bricks to the floor afterwards. “Dilute these things,” he said, adding that he would run a hose down the drain to ensure good dilution if the rinse water looked like it was going down a rain drain. When asked if a stain that is just dirt could be removed with a gentler cleaner, he said that this might work if the stains are not deeply embedded, but otherwise, it’s a waste of time. “You’d be up there all day doing nothing.”

Both Shortley and Coates warned against using a pressure washer, especially with old bricks. “When you machine wash old brick structures, you blow out the mortar or peel off a layer of brick,” Coates said.

Which approach makes sense in view of the different opinions? There’s no way you want to damage the bricks or mortar while you’re fixing a cosmetic problem and you’re dealing with high-altitude surfaces that are difficult for you to inspect. So proceed carefully.

Spraying on a bleach-based product, waiting a few minutes and rinsing it away with water from a hose will not damage the bricks or mortar. Hire a chimney sweep and indicate that the stains should be treated with a bleach-based detergent first. Or, if your chimney isn’t too high and you’re comfortable on a ladder, you can make a test patch yourself. The cleaner that Shortley uses, Mold Armor Instant Mold & Mildew Stain Remover, is $ 5.98 for a 32-ounce bottle at Home Depot. If you want a synthetic fiber scrub brush that he uses, also buy the HDX Gong scrub brush for $ 6.98. Be sure to wear safety glasses in case the spray drifts, as well as rubber gloves and clothing that covers your skin.

Using an acidic cleaner puts you at greater risk to both your plants and your chimney. On new masonry, these cleaners are great for removing smeared mortar, but this is not your situation. The Brick Industry Association, a trade group, warns against using unbuffered hydrochloric or hydrofluoric acid and says always test first. This is especially important with light-colored bricks that you have, as they are more prone to staining from acidic detergents than dark bricks.

If the person you’re hiring insists on using a pressure washer, make sure the pressure isn’t so high that it could damage the bricks or mortar. Under the “Reading & Research” section on the Brick and Tile Industry Association’s website (gobrick.com), an introduction entitled “Technical Notes on Brick Building” suggests first taking the dirty bricks and the ones underneath with water at very low pressure (less than 100 Pound per.) To soak square inch). It is said to apply cleaning solution with a brush or pump sprayer at no more than 30 to 50 psi, followed by a thorough rinse at low pressure (100 to 400 psi). They say never exceed 400 psi.