Black Streaks on Roof Shingles: The Cause and the Fix
Quick answer
Black streaks on a roof are mostly an algae called Gloeocapsa magma, which shingle makers name as the main cause of roof discoloration. It spreads through the air and grows on damp, shady slopes. The fix is a low-pressure roof soft wash. Pressure washing strips the granules off shingles, so roofing makers warn against it.
Around Central and Zachary, plenty of houses sit under old oaks and pines. Those roofs stay shaded and damp, and the dark streaks show up on the slope that sees the least sun. People often think the roof is old or failing. Most of the time it's growth on the surface, and it comes off.
What Causes Black Streaks on a Roof
Those dark streaks aren't dirt, soot or a failing roof. They're growth, and one kind does most of the damage to the look of a roof.
The Asphalt Roofing Manufacturers Association says Gloeocapsa magma is probably the most common of the algae species behind roof discoloration. The group notes that pigments from the algae can turn a light roof dark brown or black over time.
It travels on the air. Owens Corning describes it as airborne and most common in warm, humid areas, which is why one streaked roof on a street is often followed by more. That brings up the next question: why your roof and not your neighbor's.
Why Some Roofs Streak and Others Don't
The algae needs moisture and time. Anything that keeps a slope wet longer gives it both. Baton Rouge averages 61.94 inches of rain a year, so most roofs here have the water. Shade decides which ones streak first.
| What your roof has | What it does |
|---|---|
| A north-facing slope | Less sun, so it dries slowest. Owens Corning notes streaks favor northern exposure |
| Overhanging branches | Shade and dripping water, plus leaves that hold moisture |
| Light-colored shingles | Streaks show sooner and look darker against the color |
| Valleys and low spots | Water runs and lingers there after every storm |
| A streaked roof nearby | The algae spreads through the air from roof to roof |
A roof with three or four of these will usually streak before one that sits in full sun.
None of that means the roof was installed wrong. It means the conditions are right for growth. The cleaning method is where people get into trouble.
Why You Should Never Pressure Wash Shingles
A pressure washer will take streaks off a roof. It will also take off the granules, and that's the problem.
The granules on asphalt shingles shield them from the sun. Owens Corning warns that high-pressure washing can loosen granules and shorten a roof's life. ARMA's algae guidance says not to use a power washer on an asphalt roof at all.
Scrubbing with a stiff brush causes the same kind of damage. What roofing makers point to instead is low pressure and a cleaning mix that does the work.
How a Roof Soft Wash Works
A roof soft wash puts the cleaning mix on at low pressure, gives it time to work on the growth, then rinses it off gently. Here's how a job runs:
- Look at the roof, in person whenever we can, or measured online if we can't make it out first
- Cover the plants under the roofline and seal the outdoor outlets
- Move anything sitting on the walks and driveway below
- Apply the mix at low pressure and give it time to work
- Rinse gently, then take the covers off and water the plants
Shingle roofs and metal roofs both get low pressure. You can read more on our roof cleaning in Baton Rouge page. The next thing people ask is how long it lasts.
How Long a Clean Roof Stays Clean
Honestly, it depends. ARMA notes that cleaning results are temporary, and the discoloration is likely to come back, because the shade and moisture that caused it are still there.
You can slow it down. Trim branches that hang over the roof so it dries faster after rain, and keep gutters and valleys clear of leaves. When the streaks start to show again, call before they cover the whole slope. Siding goes green for the same reasons; see why siding turns green in Baton Rouge.
What Roof Cleaning Costs in Baton Rouge
There's no honest single price for roof cleaning, because no two roofs are the same job. These are the things that move the price:
- The size of the roof and how steep it is
- How many stories the house is
- How heavy the streaking and growth are
- How easy the roof is to reach around trees and beds
- Adding the house, driveway or windows to the same trip
We look at the roof and give you a free quote. Once you approve it, that's the final price. See how our quotes work for the details, or meet Jaylan before you call.
Roof Streak Questions We Get Around Baton Rouge
These come up on almost every roof call. If yours isn't here, call (225) 503-5675.
Are black streaks on my roof mold?
Mostly no. Roofing makers say the dark streaks are mainly an algae called Gloeocapsa magma. It looks like dirt or soot, but it's growth on the surface of the shingles, and it spreads through the air from roof to roof.
Do black streaks mean I need a new roof?
Not on their own. Streaks are growth on the surface, so the roof can look much older than it is. A roof soft wash takes the streaks off. If you have missing or curling shingles, that's a question for a roofer.
Can I pressure wash my roof to get the streaks off?
Roofing makers say no. High pressure loosens the granules that shield asphalt shingles from the sun. ARMA's guidance on algae says not to use a power washer on an asphalt roof. A low-pressure soft wash is the method they point to.
Will the streaks come back after cleaning?
They can. ARMA notes that cleaning results are temporary, because the shade and moisture are still there. Trimming branches over the roof helps it dry faster. When streaks start to show again, call before they spread across the slope.
Do you clean metal roofs?
Yes. Metal roofs collect dirt and mildew too, most of all on the shady side. They get the same low-pressure soft wash as shingle roofs. Send a photo or call if you'd like to know what to expect.
How much does roof cleaning cost?
It depends on the roof's size and pitch, the number of stories and how heavy the streaking is. We look at the roof and give you a free quote. Once you approve it, that's the final price, with no surprises.
About the author
Jaylan Gerald
Owner, Gerald Washing Solutions · Firefighter · Central, LA
Jaylan Gerald is a firefighter and the owner and operator of Gerald Washing Solutions. He soft washes houses and roofs, pressure washes driveways and cleans windows across Central, Denham Springs, Zachary and the Baton Rouge area.
This article is written from how Jaylan cleans houses, roofs and concrete on the job. Outside facts link to where they came from:
- Asphalt Roofing Manufacturers Association: Algae Discoloration of Roofs
- Owens Corning: Why are there dark streaks on my roof?
- NOAA U.S. Climate Normals, 1991 to 2020 (Baton Rouge)
More about JaylanCall (225) 503-5675
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