Black Streaks on Your Roof: What They Are and How to Remove Them

By the Roofers Greenville Editorial Team · Last reviewed: October 2026

Black streaks on a roof are usually algae, most often Gloeocapsa magma, which thrives in warm, humid climates like the Upstate’s, according to the Asphalt Roofing Manufacturers Association (ARMA). It’s mainly a discoloration problem. ARMA recommends a gentle, diluted bleach solution, never a pressure washer, and algae-resistant shingles when you re-roof. Related: roof repair in Greenville.

In this guide:

• What are the black streaks on my roof?
• Why is it so common in Greenville?
• Does algae damage the roof?
• How do you clean algae off a roof safely?
• How do you stop algae coming back?

What are the black streaks on my roof?

In most cases, algae. Airborne algae land on the roof and grow, and their dark pigment stains shingles in streaks or patches, usually showing most on light-colored roofs. ARMA notes that the stains are often mistaken for soot, dirt, moss or tree droppings; moss and droppings usually leave localized spots rather than streaks.

Why is it so common in Greenville?

ARMA says algae growth occurs across much of the US, especially in warm, humid areas, and is worst along the Gulf States and the Eastern Seaboard. Greenville’s hot, humid summers and heavily treed lots give it the moisture and shade it likes.

Does algae damage the roof?

ARMA describes it as a discoloration problem, not a structural one. Moss is different: it grows thicker and holds moisture against the roof, so it’s worth dealing with sooner. Either way, have a roofer look if you also see curling, missing shingles or leaks; see roof repair.

Want the roof checked while you are at it? Get a free estimate from a local roofer, with no obligation.

How do you clean algae off a roof safely?

ARMA’s recommended method:

1. Apply a diluted household bleach (sodium hypochlorite) solution gently, at the ratio your shingle manufacturer recommends.
2. Leave it on for 15–20 minutes.
3. Rinse gently with plain water.

Don’t scrub, and don’t use a power washer, brush or broom: scrubbing loosens the protective granules, and high pressure can damage asphalt shingles. Use other cleaning chemicals only if the shingle manufacturer approves them. The treatment makes the roof slippery, so ARMA recommends hiring a professional. Protect plants below from runoff, and expect the streaks to return over time.

Check your shingle manufacturer’s warranty and care guidance before any cleaning.

How do you stop algae coming back?

• Algae-resistant shingles: ARMA recommends them when you re-roof. Many architectural lines include algae resistance; Owens Corning’s TruDefinition Duration data sheet, for example, lists a 10-year algae-resistance period. See architectural vs 3-tab shingles.
• More sun and air: trimming branches lets the roof dry out faster.
• Clear debris: leaves and pine needles in valleys hold moisture.
• Zinc or copper strips: some homeowners ask about metal strips near the ridge. ARMA’s guidance doesn’t cover them, so ask your shingle manufacturer and roofer whether they recommend them for your roof.

Thinking about a new roof anyway? See roof replacement in Greenville, and how humidity affects roof life in how long does a roof last in South Carolina.

Sources

• ARMA: Algae discoloration of roofs (June 2024)
• Owens Corning: TruDefinition Duration shingles data sheet

Roofers Greenville is a free referral service; roofing work is performed by independent contractors in our network. How we research prices: editorial policy.