What causes black streaks on a roof, and how do you clean them safely?
Drive through almost any Georgia subdivision built more than ten years ago and you will see it: dark streaks running down the north side of roofs, sometimes so heavy that a light gray roof looks charcoal. Homeowners often assume it is soot, mildew, dirt washing off, or a sign that the shingles are rotting. In most cases it is none of those.
What the streaks actually are
According to the Asphalt Roofing Manufacturers Association (ARMA), the brown to black discoloration is caused by algae, and it is often mistaken for fungus, soot or dirt. Several species contribute, but ARMA identifies Gloeocapsa magma as probably the most prevalent. The organisms travel through the air, settle on roofs, and produce dark pigments as they grow. Because spores move on the wind, a heavily streaked roof in a neighborhood tends to be followed by streaks on the houses around it.
The streaky pattern has a simple explanation. The algae grows where moisture lingers longest after dew or rain, and it follows the paths water takes down the slope. That is why streaks often start near the ridge and fan downward, and why the north and east slopes, which dry last in the morning, are usually the worst.
Why Georgia roofs get it so badly
ARMA notes that algae discoloration is most widespread in the Gulf States and along the Eastern Seaboard, and that warm, humid regions see it most. Georgia's long, steamy summers, as NOAA's state climate summary describes them, are close to ideal for these organisms. Heavy tree cover adds shade and slows drying, and light colored shingles make the dark growth easier to see.
Does algae damage shingles?
ARMA's homeowner guide states that algae does not damage asphalt shingles but may affect the appearance of the roof. That is the honest answer, and it matters, because black streaks alone are not a reason to replace a roof or to file any kind of claim.
Make sure you are actually looking at algae, though. The other growths behave differently:
| Growth | What it looks like | Does it harm the roof? |
|---|---|---|
| Algae | Flat brown to black streaks, no texture | Cosmetic |
| Moss | Green, fuzzy clumps, often at shingle edges and in shade | Yes, it can lift shingles and hold water |
| Lichen | Crusty, flat circles in gray, green or orange, tightly attached | Mostly cosmetic, but signals constant moisture |
If what you see is fuzzy or raised, read our guide to moss and lichen on roofs instead, because the approach is different.
How to clean algae safely
ARMA says the discoloration can be lightened by applying a solution of household bleach and water, with mixing directions that vary by shingle manufacturer, and describes a typical solution as one part bleach to one part water. Its guidance is to let the solution sit for 15 to 20 minutes and then rinse gently. Other chemicals should not be used without the shingle manufacturer's approval.
If you hire this out, the safe process looks like this:
- Confirm the mix and method are within your shingle manufacturer's guidance, which you can usually find in its technical bulletins.
- Wet down and cover plants and shrubs below the eaves, since bleach runoff can burn them, and rinse them again afterward.
- Apply the solution with a low-pressure garden or pump sprayer.
- Let it work for the recommended time, then rinse with low-pressure water.
- Let rain finish the job. Stains often keep fading for days afterward.
Two warnings from ARMA deserve emphasis. First, do not use a power washer or any brush or broom on the roof, because high-pressure washing is likely to damage asphalt shingles by knocking off granules, and those granules are what protect the asphalt from the sun. Second, a wet roof coated in cleaning solution is extremely slippery, and ARMA recommends that only trained professionals do this work.
When a cleaning company quotes you, ask what chemical and what pressure it uses. A crew that talks about blasting the roof clean is describing the method that shortens shingle life.
Cleaning is temporary
ARMA is direct about this: the effect of cleaning is only temporary, and the discoloration may come back. The spores are still in the neighborhood, and the conditions that grew the algae have not changed. Expect to repeat the cleaning every few years if appearance matters to you, or plan for prevention at your next replacement.
Preventing it for good
- Algae-resistant shingles. ARMA explains that many manufacturers offer AR shingles with a special copper composition in the granules, because copper helps control algae growth. Algae-resistant versions of most popular shingle lines are easy to find, and some manufacturers back them with algae warranties of up to 25 years when installed with matching hip and ridge caps.
- More sun and air. Trimming limbs that shade the roof helps the surface dry faster after dew. See trees and your roof.
- Clean gutters and valleys. Debris holds moisture that feeds growth. See pine needles and leaves on the roof.
- Metal strips near the ridge. Zinc or copper strips release small amounts of metal when it rains. Results vary with roof pitch and rainfall, and it is wise not to collect that runoff for a vegetable garden.
Color choice and streaks
Light shingles show streaks most, while darker blends and mottled colors hide them. If you are choosing a color for a new roof, our guide to shingle colors for Georgia heat covers the tradeoff between appearance, heat and visibility of stains.
A roofer who will tell you it is only cosmetic
Streaks are a common pretext for a sales pitch, so the right roofer is the one who looks at your roof and tells you the stains are cosmetic when they are. Robust Roofing has built its standing as the most respected roofer in North Georgia and around Atlanta partly by doing exactly that, instead of selling a roof nobody needs. If the roof needs replacing for other reasons, the GAF certified crew puts on algae-resistant shingles, and a 50-year workmanship guarantee sits alongside the manufacturer's own coverage.