Those black streaks running down your roof are not dirt. They are alive. The streaks are a type of algae called Gloeocapsa magma, and it is eating your shingles. Here is what the streaks are, why they keep coming back, and how we remove them without damaging your roof.
What the black streaks actually are
Gloeocapsa magma is an airborne algae. It lands on your roof, takes hold, and feeds on the crushed limestone that manufacturers mix into asphalt shingles. The black color is the algae’s protective coating. It shields the organism from the sun, which is why the streaks look darker and thicker every year.
One streak means the roof is already colonized. The spores spread from roof to roof on the wind, which is why you see the same streaks up and down the same street.
Why the north side always looks worse
Algae needs moisture. The north side of your roof gets the least sun, stays damp the longest after rain and dew, and gives the algae the best place to grow. Shaded sections under trees do the same thing. If your streaks are heaviest on the north face, that is textbook algae growth. We covered this on WANE 15 in the segment below.
What happens if you leave it alone
| Time untreated | What happens on the roof |
|---|---|
| Year 1 | A few faint streaks, usually on the north side. Easy to ignore. |
| Years 2 to 3 | Streaks darken and spread across the whole face. Curb appeal drops. The algae holds moisture against the shingles. |
| Years 4 to 5 | Granule loss speeds up where the algae feeds. Moss and lichen often move in behind it, and lichen roots into the shingle itself. |
| Beyond | Shingles age faster than they should. A roof that could have lasted its full rated life gets replaced years early. |
A roof replacement in northern Indiana runs well into five figures. A professional roof cleaning costs a small fraction of that. Our roof cleaning typically runs about 45 cents per square foot, based on the square footage of your home. For a 2,000 square foot house, that is around $900 to add years back to a roof that typically costs $10,000 to $20,000 to replace. That math is the whole reason roof cleaning exists as a service.
Why you should never pressure wash a shingle roof
High pressure blasts the granules right off the shingle. Those granules are the shingle’s sun protection. Losing them ages the roof faster than the algae does. Shingle manufacturers say the same thing. Their guidance calls for a low pressure chemical cleaning, never a pressure washer.
| Pressure washing a shingle roof | Soft washing a shingle roof | |
|---|---|---|
| Pressure | High. Similar to what cleans concrete. | Low. About the strength of a garden hose. |
| What does the work | Brute force | A cleaning solution that kills the algae |
| Granule loss | Yes. Permanent damage. | No. |
| Kills the algae | No. It grows back fast because the organism survives. | Yes. Killed at the root. |
| How long it stays clean | Months | Years |
| Warranty risk | Can void a shingle warranty | Follows manufacturer guidance |
This is exactly why we soft wash roofs. The solution kills the algae, the streaks disappear, and the shingles never take a hit. Metal roofs get their own process, which you can read about on our metal roof cleaning page.
Watch Brandon explain it on WANE 15
Owner Brandon Metzger walked through spring roof cleaning on Living Local 15. This is one of more than a dozen segments WANE 15 has done with us.
Can you DIY it?
You can buy roof cleaning products at the hardware store. Here is what usually goes wrong. The mix is too weak to kill the algae, so the streaks come back in months. Or the runoff hits the landscaping and kills the bushes. And every year people are seriously hurt falling off wet roofs and ladders. A wet, algae covered roof is one of the slickest surfaces you will ever stand on.
We rinse and protect the landscaping on every job, we carry the right insurance, and nobody in your family has to climb anything.
When to clean
Any time the roof is streaked. Spring and fall book fastest. If you are getting the house ready to sell, do the roof first. It is one of the biggest curb appeal wins per dollar, and we usually pair it with a house soft wash in the same visit.
Get the streaks off before they spread
We have cleaned more than 3,000 properties across northern Indiana, from Warsaw and the lakes to Fort Wayne. Over 300 five star reviews. BBB A+ accredited. Request a fast quote or call or text (260) 802-0297. Send us a photo of the roof and we will quote it fast.
Look at your north-facing roof today. How long have those streaks been there?
