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

The dark stripes are a colony of algae living on the shingle surface. The Asphalt Roofing Manufacturers Association (ARMA) says Gloeocapsa magma is probably the most prevalent of several algae species that cause roof discoloration, and that natural pigments these algae produce can turn white or light-colored roofs dark brown or black over time.¹

The algae is carried through the air and collects and grows on roofing surfaces.² Malarkey, a shingle manufacturer, points to dew, humidity and sunlight as the conditions that let it take hold.³ It spreads where water lingers and drains, which is why it shows up as streaks that run down the slope instead of as random spots. Lighter shingle colors show it first because the contrast is greater.

ARMA also notes that the discoloration may be mistaken for soot, dirt, moss or tree droppings.² Moss and lichen are different organisms with a different look, raised and textured rather than a flat stain. They are worth having looked at separately.

By themselves, no. ARMA's guidance calls the discoloration "an aesthetic irritant to many people."² Malarkey describes the streaks as altering a roof's color and marring its appearance.³ A University of Georgia Extension housing specialist put it plainly in 1996: these stains "don't mar the roof's strength or service life."⁴

That is why a streaked roof is not a reason to file a claim or to replace a roof that is otherwise sound. The waterproofing is the asphalt and the mat under the granules. A thin film sitting on the granule surface does not open a path for water.

The caution is that some dark areas are not algae. Black spots where the granules are gone and the bare asphalt shows through are a real concern; IKO's technical bulletin says granule loss becomes a performance concern when bare spots of coating asphalt are exposed.⁵ Fresh hail hits can also look dark at first. If you are seeing granules piling up in your gutters or dark spots after a storm, read understanding hail damage before assuming it is algae. And if water is coming through a ceiling, the streaks are not the cause; that is a roof repair question.

ARMA's method uses a solution of liquid household bleach (sodium hypochlorite) and water.¹ The steps it describes:

1. Mix to the manufacturer's directions. ARMA notes that mixing directions vary among shingle manufacturers, so use the one for your shingle.
2. Apply gently. Disperse the solution gently over the roof surface. Apply it carefully to avoid damaging other parts of the building and the landscape.
3. Time it. Leave the solution on for at least 15 minutes and no more than 20.
4. Rinse gently. Spray the surface with plain water to rinse it off.¹

To protect plants, cover shrubs below the eaves with plastic and dilute any solution that reaches the ground by spraying it with water.⁴

Two warnings from ARMA matter. The process makes the roof slippery, and the effect is only temporary: the discoloration may come back.¹ A wet, bleach-slick roof is a fall hazard even for people who work on roofs every day. If you would need a tall ladder to reach it, read ladder safety first, or hire it out.

If you hire it out, ask four questions before anyone sets foot on the roof. What solution will you use, and does it follow the shingle manufacturer's directions? How long will it sit before rinsing? How will you protect the plants and the siding below? What fall protection will your crew use on a wet roof? Any answer that includes a pressure washer or a stiff brush is a reason to keep looking, however good the before-and-after photos look.

ARMA's guidance is direct: "DO NOT use a power washer or any type of brush or broom to clean algae from the roof surface." It adds that high-pressure washing systems are likely to damage asphalt roofing.¹ Even scrubbing by hand is a problem, because the friction may loosen and remove granules, shortening the life of the shingle.² Malarkey makes the same point: pressure washing can knock loose the granules that protect the shingle from damage and UV aging.³

Granules are not decoration. They shade the asphalt from the sun. Research from the Insurance Institute for Business & Home Safety (IBHS) explains that once granules are displaced, the exposed asphalt is hit by ultraviolet light and becomes more brittle, speeding up natural aging.⁶ A pressure washer turns a cosmetic stain into real granule loss. You trade a stripe you can see for wear you cannot undo.

The durable answer is built into the shingle. ARMA notes that several types of algae-resistant shingles are commercially available, designed to inhibit algae growth for extended periods.¹ The common approach is copper: ARMA's guidance says copper is being incorporated into shingle granules and that appropriate levels of copper effectively block algae growth on roofs.² Malarkey, for example, uses copper-coated granules from 3M, at least a 10% blend in its shingles with Scotchgard Protector, which release copper ions that help inhibit blue-green algae.³

Read the fine print on what "algae-resistant" covers. Malarkey's algae protection applies only to blue-green algae (Gloeocapsa magma) and excludes other algae, mold, mildew, lichens and moss.³ Ask for the product name and its algae terms in writing when you compare quotes for a roof replacement.

Metal at the ridge is an older trick. JLC, a building trade publication, notes that roof areas below lead, zinc or copper flashing stay unstained while the rest of the roof darkens, and suggests slipping a narrow zinc strip under the ridge row so about 3/4 inch is exposed.⁷ If you try it, keep fasteners covered by the ridge shingles. A new exposed nail hole is a worse problem than a stain.

ARMA lists the Gulf States among the areas where algae discoloration is most widespread, along with regions subject to warm, humid conditions.¹ Texas is a Gulf State, and Central Texas summers are long and humid. Shaded slopes under live oaks and pecans stay damp longer after dew and rain, which gives algae more of what it needs.

Hail season adds a wrinkle. Fresh hail hits can leave dark spots, and it is easy to write them off as algae and let the window to document storm damage pass. If dark marks appear right after a storm in San Antonio or the Hill Country, have them checked against the hail damage signs before deciding they are only a stain.

Algae alone never justifies a roof replacement. If the shingles are otherwise sound, clean them gently or live with the stain. If the roof needs replacement for other reasons, choose an algae-resistant shingle and get its algae terms in writing. For other problems found during an inspection, Atrium's rule of thumb is that repair wins when it costs under 20% of a replacement. The full framework is in repair vs replacement.