Those dark streaks are usually a type of algae that thrives in warm, humid climates like Florida's. The streaks themselves are mostly cosmetic, but the safe way to remove them is a low-pressure soft wash. Pressure washing or scrubbing can damage the shingles.
What's causing the streaks
The most common culprit is a blue-green algae, often identified as Gloeocapsa magma, whose spores travel on the wind and settle on roofs. Warm temperatures, high humidity, frequent rain, and shade give it everything it needs. It tends to show up first on shaded slopes and spreads downward as rain carries it, which is why it looks like streaks.
Is it actually hurting my roof?
Algae itself is mostly a cosmetic problem, but it's not something to ignore forever. Heavy growth and moss can hold moisture against the roof surface, and a stained roof drags down your home's curb appeal. If you're in an HOA, check your rules, since some require roofs to be kept clean.
What not to do
- Don't pressure wash it. High pressure can strip the protective granules off shingles and push water underneath them. The Asphalt Roofing Manufacturers Association (ARMA) says not to use a power washer on a roof to remove algae.
- Don't scrub it. ARMA also advises against brushes and brooms, since scrubbing can pull granules off.
- Don't climb up there unless you have to. Wet or algae-covered roofs are slippery, and falls from roofs are serious.
- Be careful with DIY chemicals. Cleaning solutions can harm landscaping and corrode some metals, so protecting plants and rinsing correctly matters.
How a professional cleans it
The standard approach is a soft wash: a cleaning solution is applied at low pressure, given time to break down the algae, and then gently rinsed. Plants and landscaping around the house are protected. The streaks often keep fading over the following days and weeks as the treatment finishes working and rain rinses the roof.
How to keep the streaks from coming back
- Trim back overhanging branches so the roof gets more sun and airflow.
- Keep gutters clear. Debris and standing water hold moisture near the roof edge. See how often to clean gutters in Florida.
- Consider zinc or copper strips along the ridge. Rain carries trace metal ions down the roof, which can help slow algae growth below them. Results vary, and a roofer can install them.
- Choose algae-resistant shingles the next time you replace the roof. ARMA recommends this for roofs that look vulnerable.
Even with all of that, growth in Florida's climate tends to return eventually. How soon depends on shade, roof type, and how humid the year has been.
Not sure what your roof needs?
Our roof & gutter cleaning and home soft wash pages explain what we do. If you're not sure what your roof needs, call or text (425) 977-5156 and we'll tell you what we can do for it.
Frequently asked questions
Is roof algae dangerous?
Roof algae is generally considered a cosmetic problem rather than a health hazard. The main concerns are appearance and, with heavy growth or moss, moisture held against the roof surface.
Will rain wash the streaks away?
No. Rain tends to spread algae downhill, which is how the streaks form in the first place.
Could cleaning my roof void the shingle warranty?
It can if the wrong method is used. Check your shingle manufacturer's cleaning guidelines, and avoid pressure washing, which many manufacturers advise against.
Sources & further reading
About Shine Otter
Shine Otter is a family owned and operated exterior cleaning company serving Southwest Florida, from Englewood and Venice to Cape Coral and Fort Myers. We handle window cleaning, roof and gutter cleaning, soft washing, driveway and patio cleaning, window screen cleaning, storm debris cleanup, and solar panel cleaning. Call or text (425) 977-5156 for a free quote. Meet the owners.