Q: Our chimney is badly stained from weather. A chimney sweep told us chemicals and power washing would not guarantee removing these stains, and a heavy power wash could even damage the brick. How can we safely remove the stains?

Germantown, Md.

A: The stains could be mold, dirt, algae or soot, judging from the assessments offered by four chimney sweeps who respond to calls in the area.

“Trust me, it’s mold,” said Jim Shortley, owner of My Chimney Sweep Service in Potomac, Md., (301-760-6712; mychimneysweepservice.com). The remedy, he said, is to spray with a bleach-based product labeled for removing mold and mildew, wait 10 to 15 minutes, then rinse. Sometimes he also has to scrub stubborn stains using a synthetic-fiber brush. To keep from bleaching more than the chimney, he covers nearby roofing with a heavy tarp first.

Carlos Aragon, owner of Aragon Chimney Services in Derwood, Md., (301-706-8564; aragonmsi.com) also said the stains are probably mold. He would inspect to make sure the bricks and mortar were in good condition, then use a pressure washer to remove the stains. That’s the “easiest and fastest” way to clean the exterior of a chimney, he said.

Carmine Ferraro, owner of Colonial Chimney in Germantown (800-685-5225; colonialchimneyhvac.com), said he thinks the problem is algae and he’d use an algae-removing 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 the cleaning to help keep the problem from recurring.

Steve Coates, owner of Chimney Masters in Germantown (301-972-9158; chimneymasters.com), said the black stains could be dirt blown off a nearby tree, soot, or a combination of soot and dirt. He would use an acidic cleaner, which bubbles up as it works and requires keen attention to safety and washing everything from the bricks on down to the ground afterward. “Dilute the hell out of things,” he said, adding that if any of the rinse water looked like it were headed to a storm drain, he would direct a hose into the runoff to ensure good dilution. Asked whether a stain that’s just dirt might come off with a gentler cleaner, he said that might work if the stains are not deeply embedded, but it’s a waste of time otherwise. “You’d be up there all day getting nothing done.”

Both Shortley and Coates warned against using a power washer, especially on old bricks. “If you power wash old brick structures, you’re going to blow the mortar out or peel off a layer of bricks,” Coates said.

At each of these companies, the cost varies depending partly on height and access, as well as whether the two sides of the chimney visible in the picture you sent are representative of the stains on the other two sides. Aragon estimated $50 to $150. Ferraro at Colonial Chimney said $200 to $500. Coates at Chimney Masters said $250 to $400. And Shortley at My Chimney Sweep Service said $200 to $1,000.

Given the different opinions, which approach makes sense? You definitely don’t want to damage the bricks or mortar while fixing a cosmetic problem, and you’re dealing with surfaces high up, where it’s hard for you to inspect. So take a cautious approach.

Spraying on a bleach-based product, waiting a few minutes and rinsing it away with water from a hose won’t hurt the bricks or mortar. Hire a chimney sweep and specify that you want the stains treated with a bleach-based cleaner first. Or, if your chimney isn’t too high and you are comfortable being on a ladder, you can do a test patch on your own. The cleaner Shortley uses, Mold Armor Instant Mold & Mildew Stain Remover, costs $5.98 for a 32-ounce bottle at Home Depot. If you want a synthetic-fiber scrub brush, which he uses, also buy the HDX Gong Scrub Brush for $6.98. Be sure to wear goggles in case the spray drifts, as well as rubber gloves and clothing that covers your skin.

Using an acidic cleaner poses more risk both to your plants and chimney. On new masonry, these cleaners are great for removing smeared mortar, but that’s not your situation. The Brick Industry Association, a trade group, warns against using unbuffered muriatic (hydrochloric) or hydrofluoric acid, and it says to always test first. That’s especially important with light-colored bricks, which you have, because they are more susceptible than dark bricks to staining from acidic cleaners.

If the person you hire insists on using a pressure washer, make sure the pressure isn’t so high that it could damage the bricks or mortar. Under the “Read & Research” section of the Brick Industry Association’s website (gobrick.com) a primer titled “Technical Notes on Brick Construction” suggests first saturating the dirty bricks and those below with water at very low pressure (less than 100 pounds per square inch). It says to apply cleaning solution with a brush or a pump sprayer with no more than 30 to 50 psi, followed by a thorough rinse at low pressure (100 to 400 psi). Never exceed 400 psi, it says.

If the stains turn out to be soot, you’ll need a cleaner up to that task. Shortley uses Rutland Brick and Stone Cleaner ($7.16 for a 16-ounce bottle from rutland.com).

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