Picture two neighbors in your driveway, both trying to help.

The first spent decades roofing. He eyes the black streaks on your north slope. “Never wash shingles,” he says. “Leave it.”

The second runs a soft-wash business out of his pickup. He hands you a card. “I can make that roof look new without hurting a single granule.”

This September, a new homeowner on a roofing forum asked whether to accept soft-wash quotes, some with a multi-year “no moss” guarantee. Most replies said don’t. So who’s right? Both, in part.

What roofers worry about

Roofers see the aftermath. Think of an asphalt shingle as a frosted cake: the granules are the frosting, protecting the asphalt underneath. A high-pressure wand blasts the frosting off.

The industry backs them up. The asphalt roofing industry group says flatly: “DO NOT use a power washer or any type of brush or broom.” It adds that high-pressure washing is “likely to damage asphalt roofing.” So when a roofer says “never wash,” he’s picturing a wand in the wrong hands. Fair, but “never pressure-wash” isn’t “never clean.”

What washers say

Soft-wash crews see a humid street where every roof has the same dark streaks. The industry group says Gloeocapsa magma is probably the most prevalent of several algae species, all carried through the air and settling on roofs.

The method makers describe is a gentle spray of bleach and water, then a gentle rinse. Done that way, soft washing is closer to the makers’ advice than the roofers’ “never” suggests. The catch is the word “soft.” One homeowner on the same forum said a contractor told them a lower-pressure power wash was “the same as soft wash.” It isn’t. A wand is a wand. If you are thinking of doing it yourself, see a safe DIY soft wash setup.

What the shingle makers actually say

Here’s where the argument settles down. The makers’ own guidance lands in the gentle middle.

Source Cleaning method What it warns against
ARMA (asphalt roofing industry group) Bleach (sodium hypochlorite) and water, gently dispersed; leave 15 to 20 minutes; gentle rinse Power washers, brushes, brooms, scrubbing; other chemicals without the shingle maker’s approval
Owens Corning roofing care page 1:1 bleach and water, sprayed on; leave about 10 to 20 minutes; flush the roof and plants Scrubbing or high-pressure power washing, which loosens granules and shortens the roof’s life
  • The streaks are mostly cosmetic. One maker says it hasn’t seen evidence that algae affects how its shingles perform, calling it “more of an aesthetic problem.”
  • Cleaning doesn’t last forever. Both sources say the result is temporary and the staining may come back.
  • Mixes differ by maker. The industry group says directions vary, so ask whose recipe the crew follows.
  • Protect the yard, and hire for the climb. The maker says to cover plants under the roofline, and the industry group encourages using professionals to clean shingles.

So the real disagreement isn’t roofers versus washers. It’s pressure versus patience.

When to clean, when to leave it, when zinc strips fit

Consider cleaning if:

  • The streaks bother you, or you’re selling and want curb appeal.
  • The roof is still in good shape: no curling, cracking or missing tabs.
  • The crew can name the maker’s method it follows and rinses gently instead of using a wand. Our questions to ask a house washing company help on that call.

Consider leaving it alone if:

  • The roof is near the end of its life, with brittle shingles or heavy granule loss.
  • The staining is faint, or only on a back slope no one sees.
  • Money is tight. By the makers’ own account, algae is a looks problem.

When zinc strips make sense

Zinc strips along the ridge release a little metal with each rain. University moss guidance, passed along by a University of Washington garden library, says they “appear to have effect up to 15 feet below” the strip. Most strips last up to a year; galvanized flashing can last decades. They work best before growth is well developed, so the guidance suggests removing existing moss first, then installing strips. Keep the runoff away from fish ponds and gardens, since some zinc washes off.

Moss is its own problem: one maker notes it isn’t the same organism as the black streaks, and the industry group says to ask your shingle maker about it.

Where the streaks show up most

The industry group says algae staining is most widespread in the Gulf States and along the Northwest and Eastern Seaboards, especially areas “subjected to warm, humid conditions.” Charleston, South Carolina, fits that picture. NOAA’s 1991–2020 normals put the city’s average July low at about 74°F, with about 52.5 inches of rain a year, nearly 20 of them in summer. You still don’t have to clean. But since cleaning is temporary, a home like that is a natural fit for prevention afterward.

So, is roof cleaning worth it?

Yes, when the method matches your shingle maker’s and the roof is healthy enough to deserve the care. No, when someone chases stains with a pressure wand.

Think of a wool sweater. You can wash it, gently and in cold water. Toss it in a hot machine and you pull out a toddler-sized version. Roofs work the same way.

For roof soft-wash prices, see our pressure washing cost guide. For other ways a wash goes wrong, see pressure washing mistakes homeowners make; for humid-climate upkeep, Louisiana heat and humidity and your house exterior. How we check facts: our methodology page.

Sources