The cause
What causes the black streaks?
According to the Asphalt Roofing Manufacturers Association (ARMA), the streaks are caused by Gloeocapsa magma, a blue-green cyanobacteria that ARMA calls "probably the most prevalent" cause of roof discoloration. It produces a dark, pigmented sheath to protect itself from the sun's UV rays — and that dark sheath is exactly what you see as black or brown streaking.
It spreads easily. ARMA notes that the spores are "carried by the wind or by animals and can quickly spread from one rooftop to another," which is why you'll often see it appear on several houses in the same neighborhood. It takes hold wherever moisture lingers on the roof — shaded, slower-drying surfaces are the usual starting point, and ARMA notes it occurs to varying degrees in every region of the U.S.
You may read that the algae "eats the limestone filler" in shingles. That mechanism is widely repeated in the roofing trade and is consistent with why copper and zinc deter it, but it isn't stated in ARMA's own technical bulletins — so we'll call it a likely explanation rather than a hard fact.
The honest answer
Does roof algae actually damage your roof?
Here's where a lot of cleaning companies overstate things. ARMA is direct about it: "There is no scientific evidence that algae is damaging to asphalt shingles." The effect is primarily cosmetic. So no, the streaks aren't silently eating through your roof, and you don't need to panic.
That said, there are three real, documented reasons homeowners clean it:
- Curb appeal and resale. A streaked, blotchy roof reads as "neglected" to buyers and neighbors. If you're thinking about selling, the roof is one of the first things people see.
- Cooling costs. Research from the Florida Solar Energy Center found algae and dirt cut a roof's solar reflectance by about 17%. On lighter, reflective roofs, ARMA agrees the buildup "will undoubtedly have a negative impact" on energy performance.
- It only spreads. Left alone, the staining keeps expanding and seeding nearby roofs. It's far easier to clean a light case than a roof that's gone fully dark.
What you should be skeptical of is the claim that algae "shortens your roof's life." That isn't supported by ARMA — the genuine lifespan threat comes from cleaning it the wrong way.
Right way vs. wrong way
How to remove it safely
⚠ Never pressure wash an asphalt roof
ARMA is explicit: "Never use a pressure washer to clean an asphalt shingle roof, as this will cause granule loss and very likely premature failure of the roof system." Those granules are what protect the shingle from UV. Blast them off and you really do shorten the roof's life — and high-pressure washing can void shingle warranties from GAF, Owens Corning, and CertainTeed.
The method ARMA recommends instead is soft washing: applying a cleaning solution (ARMA references a 50:50 mix of laundry-strength liquid chlorine bleach and water), letting it dwell for 15–20 minutes, then rinsing with low-pressure water. ARMA also warns against scrubbing, since the friction loosens granules too.
This is exactly how Cable cleans roofs — low-pressure soft washing with the right solution and a gentle rinse, never a pressure washer on shingles. It's the method that actually removes the algae while keeping your roof and your warranty intact. You can estimate roof soft-wash cost here.
Keeping it gone
How to prevent it coming back
Cleaning removes the stain, but spores will return. Two proven ways to slow regrowth, both recommended by manufacturers:
- Zinc or copper strips near the ridge. ARMA lists this as a prevention step — every time it rains, trace metal ions wash down the roof and inhibit algae growth. (You can often see the clean stripe under an old galvanized vent or flashing for this exact reason.)
- Algae-resistant shingles. If you're already replacing the roof, shingles with copper-coated granules resist staining for years. Owens Corning's StreakGuard and GAF's StainGuard carry algae warranties up to 25 years.
What it costs
What does roof soft washing cost?
As a ballpark, professional roof soft washing runs roughly $0.15–$0.80 per square foot, with a typical U.S. roof cleaning averaging around $460 (commonly $295–$625), per aggregate cost data from HomeGuide and Angi. Your price depends on roof size, pitch, access, and how heavy the staining is — so treat these as estimates, not quotes.
Common questions