Why Does My Roof Have Black Streaks?

It usually starts small. You’re outside one morning, maybe cleaning up after a storm, and you notice dark streaks running down the roof. They weren’t there a year ago. You figure it’s dirt or maybe soot from the fireplace, but even after weeks of rain, those marks don’t fade.

At Robb’s Roofing, we see this almost every day in Tacoma. Those black streaks aren’t dirt at all — they’re a form of algae that loves our damp, cloudy weather. It’s common around the Puget Sound and harmless at first, but over time, it holds on to moisture and slowly wears away your shingles.

What’s Really Causing the Streaks

The streaks come from a type of airborne algae called Gloeocapsa magma. The spores float through the air, land on roofs, and start feeding on the limestone found in asphalt shingles. Once it finds a damp, shaded area, it spreads quietly.

Here’s how it works:

• Moisture sticks around after Tacoma’s long rains, keeping the roof damp.
• Shaded areas dry slowly, giving the algae more time to grow.
• Minerals in shingles act like food, helping it spread further each season.
• Rainwater washes the algae downward, and those black lines grow longer with every storm.

Quick Guide

Quick answer: The black streaks are algae — common in Tacoma’s damp climate.

Try this: Clean using a gentle algaecide and install zinc or copper strips along the ridge.

Stop signs: Don’t use pressure washing — it strips shingle protection.

Typical cost: Roof cleaning averages $300–$700, depending on roof size and buildup.

Why Tacoma Roofs Get It So Often

The Puget Sound climate gives algae everything it needs — moisture, shade, and time. Even newer roofs can start streaking after a couple of wet seasons. Our steady drizzle means the roof rarely dries out, and once the algae gets comfortable, it stays for good.

It’s not poor maintenance or a bad roof. It’s just Tacoma weather doing what Tacoma weather does — staying damp longer than most places.

How to Clean and Prevent Roof Algae

• Use a soft wash with a gentle algaecide made for roofs.
• Add zinc or copper strips near the top; when rain hits, it releases ions that stop algae from growing.
• Trim branches so sunlight can reach the surface.
• Keep gutters clear so water drains properly after rain.
• Skip the power washer — it’ll do more harm than good.

How Robb’s Roofing Handles It

When we clean a roof, we use soft-wash methods that remove algae without damaging shingles. Then we apply a long-lasting treatment to slow regrowth. If we find weak spots or moisture damage, we’ll take care of those too.

Final Thoughts

Black streaks aren’t a sign of a bad roof — just a damp one. With the right care, you can clean them off and keep them from coming back.

Call Robb’s Roofing at (253) 820-2163 for safe, professional roof cleaning in Tacoma. We’ll bring back your roof’s color and keep it protected through every rainy season.