What the streaks are
The black streaks on many Florida roofs are algae. ARMA says the most common type is Gloeocapsa magma, a blue-green algae that protects itself from UV light by making a dark pigmented coating. That coating is the black color you see.
ARMA says coastal and humid climates create the best conditions for algae growth, which is why Florida sees so much of it. Growth usually starts as small spots and then spreads into streaks down the roof. Atlas Roofing notes it often starts on the north side and in areas that stay damp longest.
Does algae damage a roof?
Mostly it is a looks problem. Atlas Roofing says the algae feeds on the limestone filler in shingles, but the roofing industry association (ARMA) does not make that claim, and we found no study showing algae shortens roof life. Moss and heavy growth that hold moisture are a bigger concern.
How algae-resistant shingles work
- Copper granules. 3M makes copper-containing granules that release copper ions slowly. Algae struggles to grow when enough copper is present in the water on the roof.
- Zinc or copper strips. ARMA lists metal strips near the ridge as another option. Rain washes metal ions down the roof.
- Warranties vary. Algae warranties run from about 10 years to lifetime depending on the brand. See our brand comparison.
How to clean it safely
- Use a 50:50 mix of laundry-strength liquid chlorine bleach and water, as ARMA recommends. Mixing directions vary by manufacturer, so check yours first.
- Wet plants and grass below the roof and rinse them afterward to protect them from the bleach.
- Never pressure wash a shingle roof. ARMA warns it strips granules and will very likely cause early failure. Its technical bulletin also says not to use brushes or brooms.
- Walking on a wet, sloped roof is dangerous. Consider hiring a roof cleaner who uses a low-pressure soft wash.
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Frequently asked questions
What causes black streaks on a roof?
Mostly a blue-green algae called Gloeocapsa magma, which makes a dark coating to protect itself from sunlight. It thrives in warm, humid places like Florida.
Can I pressure wash my roof?
No. The roofing industry association warns that pressure washing strips the protective granules from shingles and can cause early failure. Use a bleach and water solution or a low-pressure soft wash.
Do algae-resistant shingles work?
They use copper granules that slowly release copper, which inhibits algae growth. Warranty length varies by brand.
More on how roofs work
- How asphalt shingles are made, and why Florida sun wears them out
- Shingle wind ratings explained: what Class F and Class H really mean
- Impact-resistant shingles: what Class 4 really means
- Underlayment and secondary water barriers: the layer that saves your house
- How hurricanes damage roofs, and what keeps yours on
- Metal roofing near the coast: Galvalume, aluminum and salt air
- Concrete tile roofs explained: how they are made and what fails first
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