Green Algae vs Black Algae: What Is Growing on Your House?
- Nick Corbelli
- Mar 3
- 4 min read
Green algae is the bright green film that grows on siding, fences, and shaded walls. Black algae is actually a cyanobacterium called Gloeocapsa magma that creates dark streaks on your roof shingles. Both thrive in humid areas like Wilmington and Leland, but they grow on different surfaces, cause different types of damage, and require different cleaning methods.
What Is Green Algae and Where Does It Grow?
Green algae is the bright green or greenish-brown film you see on vinyl siding, fences, concrete walkways, and north-facing walls. It grows wherever moisture and shade come together. Homeowners in Carolina Beach and Kure Beach often notice it first on the shaded side of their home where direct sunlight rarely reaches.
Green algae feeds on moisture, not on your siding material. It sits on the surface and can be removed with the right cleaning approach. But ignoring green algae lets it spread, hold moisture against the surface, and create conditions where mold and mildew can take root.
What Are the Black Streaks on Your Roof?
Those dark streaks running down your roof are not dirt or tar. They are caused by Gloeocapsa magma, a cyanobacterium that feeds on the limestone filler in asphalt shingles. As the colony grows, it develops a dark UV-protective outer coating that creates the signature black streaks homeowners notice from the ground.
Gloeocapsa magma is airborne. It spreads from roof to roof through wind and rain. Once it lands on a shingle, it starts feeding on calcium carbonate and grows outward. Homes in Landfall and across the Wilmington area are especially vulnerable because our warm, humid summers create ideal growing conditions.
The Asphalt Roofing Manufacturers Association confirms that this growth can hold moisture against shingles and contribute to premature granule loss, shortening the lifespan of your roof.
Green Algae vs Black Algae: Key Differences
Why Both Types Grow So Fast in Wilmington
Wilmington sits in one of the highest humidity zones on the East Coast. Between salt air, afternoon thunderstorms, and warm temperatures from April through October, algae growth here outpaces almost anywhere else in North Carolina.
Homeowners in Southport and along the coast often deal with both types at the same time. Green algae coats the siding while black streaks spread across the roof. Pine trees and live oaks add shade and trap moisture, creating the exact conditions both organisms need to thrive.
If you moved here from a drier climate, the speed of growth can be surprising. A house that looked clean in January can have visible green film on the north wall by April and dark streaks on the roof by midsummer.
How Each Type Is Treated
Green algae on siding is treated with a professional soft wash. A crew applies a diluted sodium hypochlorite solution, lets it dwell on the surface, and rinses at low pressure. The process kills algae at the root and leaves the siding clean without damage. You can learn more about how soft washing protects your landscaping during this process.
Black algae on the roof requires a separate soft wash treatment formulated specifically for shingles. No one should pressure wash a roof. High pressure strips the protective granules off asphalt shingles and voids most manufacturer warranties. Soft washing treats the Gloeocapsa magma chemically and lets rain wash the residue away over the following weeks. For more on this method, read our guide on what soft washing is and why Wilmington homes need it.
At Window Cleaning Wizards, we treat both types of algae regularly for homeowners across Wilmington, Leland, and Brunswick County. Our 150+ five-star Google reviews come from homeowners who wanted their house and roof looking like new again.
Frequently Asked Questions
What is the green stuff growing on my siding in Wilmington NC?
The green film on your siding is algae. It grows in shaded, moist areas and is especially common on north-facing walls in Wilmington's humid climate. Professional soft washing removes it safely without damaging vinyl or painted surfaces.
What causes black streaks on roofs in coastal North Carolina?
Black roof streaks are caused by Gloeocapsa magma, a cyanobacterium that feeds on limestone in asphalt shingles. It spreads through airborne spores and thrives in warm, humid climates like southeastern North Carolina. Soft washing is the safe, manufacturer-approved method to remove it.
Is green algae or black algae more damaging to my home?
Black algae causes more long-term damage. It feeds on shingle material and holds moisture against the roof, which can shorten the lifespan of your shingles by years. Green algae is primarily cosmetic but can trap moisture and encourage mold growth on siding if left untreated.
Can I pressure wash algae off my roof?
No. Pressure washing a roof strips the protective granules from asphalt shingles, voids manufacturer warranties, and causes more damage than the algae itself. Soft washing uses a low-pressure chemical treatment that kills the growth safely without harming your shingles.
How often should I have my house and roof cleaned in Wilmington NC?
Most homeowners in the Wilmington area benefit from having their siding soft washed once a year and their roof treated every two to three years. Homes in heavily shaded areas or near the water may need more frequent cleaning.
Stop Guessing About What Is Growing on Your House
You do not have to figure it out alone. Call Nick and Chris at 910-727-4336 for a free estimate on house washing, roof washing, or any exterior cleaning service in Wilmington, Leland, or Brunswick County. We will walk your property with you and explain exactly what needs attention.
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