You've seen them: those dark, dirty looking streaks running down a roof, worst on the shaded side. Most Brooklyn homeowners assume it's dirt, soot, or just an old roof. It isn't. Those streaks are living algae, and while they start as a cosmetic problem, ignoring them for years actually ages your roof. Here's what the streaks really are, whether you should worry, and how to get rid of them the right way.
The black streaks on most asphalt roofs are a blue green algae called Gloeocapsa magma. It feeds on the limestone filler baked into modern shingles, spreads through airborne spores that drift from roof to roof, and thrives wherever a roof stays damp and shaded. That's why the streaks almost always run downward (following the water) and show worst on north facing slopes that get the least sun. Because it's a living organism, not loose grime, spraying it with a hose does nothing.
Early on, it's mostly cosmetic. But left for years, algae does real work against your roof:
⚠️ Never pressure wash an asphalt roof. A high pressure washer blasts off the protective granules, strips years of life from the shingles, and can void the manufacturer warranty. The dark streaks might come off, but you've traded a cosmetic problem for a structural one. This is the single most common roof cleaning mistake we see.
The correct method is a low pressure soft wash with a roof safe cleaning solution. The solution kills the algae at the root and lets it rinse away gently, without touching the granules. It's the method roofing manufacturers actually endorse. Because it involves ladders, roof safe chemicals, and steep or slick surfaces, roof cleaning is one of those jobs that's genuinely safer, and more effective, left to a pro than tackled as a weekend DIY.
| Prevention | How It Works |
|---|---|
| Zinc or copper strips at the ridge | Rain washes trace metal down the roof, inhibiting algae growth, the most durable fix |
| Trim overhanging branches | More sun and airflow means the roof dries faster and algae struggles |
| Keep the roof clear of debris | Leaves and dirt hold moisture that algae feeds on |
| Algae resistant shingles | Choose them at your next reroof. They have copper granules built in |
Sometimes what looks like algae is masking a real problem, worn granules, moss lifting shingles, or an aging roof near the end of its life. If your roof is 15+ years old, heavily streaked, and losing granules into the gutters, cleaning is a band aid; the smarter move is to have someone assess whether it's worth cleaning or nearing replacement. A quick inspection tells you which one you're dealing with.
They're a blue green algae called Gloeocapsa magma that feeds on the limestone filler in modern shingles and spreads by airborne spores. It thrives on damp, shaded areas, so streaks run downward and show worst on north facing slopes. It looks like dirt but it's alive, which is why hosing it off does nothing.
Over time, yes. Algae holds moisture against the shingles, feeds on the protective granules, and darkens the roof so it runs hotter. The bigger risk is the moss and lichen that follow it, which lift shingles and trap water. Early algae is cosmetic, but years of it contribute to premature aging.
With a low pressure soft wash using a roof safe solution, never a pressure washer, which strips the granules and can void your warranty. Soft washing kills the algae and rinses it away without harming the shingle. Because of ladders, chemicals, and slick surfaces, it's usually safest done by a pro.
Installing zinc or copper strips near the ridge is the most durable fix, rain washes trace metal down the roof to inhibit algae. Trimming branches and keeping the roof clear so it dries faster helps, and algae resistant shingles at your next reroof prevent it long term.
We'll take a look, tell you honestly whether it's a simple soft wash or a roof nearing replacement, and give you a written quote either way. Serving Brooklyn, Queens, Manhattan, Staten Island, the Bronx, and Long Island.
📞 (516) 713-9199 Get a Free Estimate