A reader wants to get this moss cleared off an asphalt roof without damaging the shingles. (Reader photo)

Q: We have a “green roof” that is not meant to be green. Moss (I guess it is moss) is collecting on one side of the asphalt roof. The roof is otherwise in good shape. What I have learned so far is that the roof should not be power-washed. How should one remove the growth, and who does this type of work? I am not interested in getting on the roof myself.

Falls Church

A: The Asphalt Roofing Manufacturers Association distinguishes between moss, a plant with leaves that can grow into thick clumps, and mold and mildew, which are common names for types of algae that simply discolor the surface. Algae creates ugly black or brown streaks but doesn’t damage a roof. Moss — which is what shows in the pictures you sent — is more worrisome because it can damage asphalt shingles. A thick growth of moss works like a sponge, keeping the roofing damp for long periods, and it can lift edges of shingles, making them vulnerable to blowing off in a windstorm.

To clean either algae or moss, the association recommends putting on protective gear and protecting plants, spraying with a 50-50 mix of household bleach and water, allowing that to sit for at least 15 to 20 minutes and then rinsing with low-pressure water. The bleach will kill the algae or moss, but rinsing won’t necessarily leave the roof sparkling clean. “Algae will disappear and wash away with subsequent rains,” the association advises. “Moss will loosen over time and may be removed with a leaf blower.”

Ted Saunders, owner of American-ProTech, a roof-cleaning company with offices in Falls Church and Montgomery County (571-250-9650 for Falls Church; theroofcleaningcompany.com), echoes the warning not to expect the moss to disappear instantly. “It will look much worse before it looks better,” he says, adding that the moss will turn white or yellowish at first. Depending on the weather and sun exposure, he says, it may take eight weeks to six months for the dead moss to decompose and be rinsed or blown away by weather.

A reader hopes to remove the tea stains from this teacup. (Reader photo)

Don’t be tempted to rush getting the moss off faster by having someone power-wash your roof, or scrub in the bleach solution, or brush or scrape off the moss — even though you may find advice on the Web recommending these as long as you are gentle. All of these methods can take off the roofing granules, exposing the shingles to the full brunt of the sun’s UV rays. Then the roofing is likely to fail prematurely.

Saunders agrees with the basic advice from the roofing manufacturers’ association, especially warnings not to power-wash, scrape or scrub, but he instructs his crews to deviate in one detail: His workers do not rinse off the bleach solution because that creates a lot of caustic runoff that damages plants. He says his company has never found any downside to just leaving the bleach solution on the roof. The chlorine in the bleach evaporates into the atmosphere, leaving salty residue behind, but that’s no more salt than if the roof had been rinsed.

American-ProTech usually brings an articulated lift, so workers rarely need to be on the roof. The company’s minimum charge is $350. On a typical home, the company can usually clean the north side (often the only side that needs it) for a price in the mid-$400s, Saunders said.

Jon Quinn, co-owner of Smart Wash in Alexandria (703-595-4000; washsmarter.com), said his company uses a similar approach, except that his crews work from ladders. A typical charge for a single-family home ranges from $450 to $750, he said. The company’s website touts use of “a custom cleaning solution, specially blended to meet demands of each project.” But in most cases, the solution used on roof moss is bleach and water, he said. If a customer does not want that, the crew offers another option.

I have a couple of Corelle coffee mugs with deeply ingrained tea stains. I’ve tried soaking them in vinegar and in vinegar mixed with baking powder. Nothing worked. I even tried cleaning with Scotch-Brite without success. Any suggestions?

Chantilly

Most pieces of Corelle dishware are made of Vitrelle, which consists of two types of glass laminated as three layers. Originally introduced by Corning Glass Works in 1970, Corelle is now manufactured and sold by World Kitchen (worldkitchen.com).

However, the cups and mugs that come with Corelle sets aren’t glass because the lamination process does not work well for these shapes. The cups and mugs are stoneware or porcelain, depending on the style. For both materials, the company recommends removing tea stains with a non-abrasive cleaner such as Bon Ami or by filling each cup one-fourth full with vinegar and then rubbing with a paper towel. The company warns not to use chlorine bleach, which it says will lighten the stains but not remove them.

A call to the World Kitchen help line yielded a few other suggestions. Katie Stewart, a consumer care representative, suggested using lemon juice and baking soda, or baking soda alone, or Bar Keepers Friend, a powdered cleaner that contains oxalic acid. You’ve already tried vinegar, which is chemically similar to lemon juice, but lemon juice is often more acidic than vinegar, so there’s a chance it would work. Baking soda is alkaline, so using it alone may make it more effective than mixing it with something acidic.

If the stains persist, consider them permanent. Stewart said this could happen for several reasons. Many people don’t realize that the company warns against leaving stoneware or porcelain soaking in water for a long period. The moisture gets past the glaze and into the clay, and if heat is applied after the cups soak, the glaze can crack. That would allow the tea to stain the clay — a permanent change. Or the clay could be stained because you accidentally scratched the glaze with an inappropriate type of Scotch-Brite pad. Scotch-Brite is a brand for numerous types of cleaning products. Some cleaning pads don’t scratch and are fine to use on porcelain or stoneware. But others, such as the green mesh Heavy Duty Scrub Sponge, are as aggressive as steel wool and do scratch ceramic glaze.

If the stains are permanent, you might want to buy replacement mugs. Stewart looked at the picture you sent and identified your pattern as Apricot Grove. It is not currently shown on the World Kitchen website, but you can call the World Kitchen Consumer Care Center (800-999-3436) to order it, as item No. 1079909, at $3.99 per mug.

Have a problem in your home? Send questions to localliving@washpost.com . Put “How To” in the subject line, tell us where you live and try to include a photo.