# Black streaks on the roof: what they are and what to do

Updated: 2026-10-24 · https://pros411.com/en/guides/roofers/black-streaks-on-roof

## Short answer

Black streaks on a roof are usually algae that feed on the shingles and grow where the roof stays shady and damp. They are mostly a looks problem, but moss can lift shingles. Do not pressure wash the roof or climb up to scrub it. Trim the branches over the roof and keep the gutters clean. A roofer can soft wash it with a gentle cleaner and add zinc or copper strips.

> Safety first: Do not climb on the roof to clean it: algae and cleaners make shingles very slippery. Roof cleaners can harm you, your plants and your pets, and some must never be mixed. Leave them to a pro.

## What to do about black streaks on the roof

![Cartoon house with black algae streaks running down its shaded roof](https://pros411.com/img/guias/black-streaks-on-roof.webp)

1. **Dark streaks run down the roof.** They show up first on the shady, damp side and get darker every year.
2. **It is algae, not dirt.** Tiny algae feed on the shingles and turn dark to protect themselves from the sun. Green, fuzzy clumps are moss, which holds water.
3. **No pressure washer, no climbing.** A pressure washer blasts off the granules that protect the shingles, and a wet roof is slippery. Skip both.
4. **Let the sun in and keep gutters clean.** Have the branches over the roof trimmed back, working from the ground. More sun and air slow the algae down.
5. **A pro soft washes it and adds zinc.** A low-pressure wash with a gentle cleaner removes the streaks. Zinc or copper strips near the top keep them from coming back.

## Why it happens

The algae travel on the wind and spread in warm, humid weather. They feed on the limestone filler in many asphalt shingles, so the streaks follow the water down the roof. Shade from trees and a north-facing roof make it worse. Many newer shingles are made to resist algae, with copper granules mixed in.

## Streaks or moss?

- Black streaks: algae. Mostly a looks problem, but a darker roof can soak up more heat.
- Green, fuzzy clumps: moss. It holds water against the roof and lifts the shingles, so have it removed.
- Dark patches with bare spots and sand in the gutters: worn shingles, not algae. Have the roof inspected.

## What not to do

- Do not pressure wash the roof or scrub it with a stiff brush.
- Do not climb up to spray anything, not even from the edge.
- Do not mix cleaners: some combinations make a toxic gas.

## When to call a roofer

- Green moss is growing in clumps or lifting shingles.
- The streaks cover most of the roof and you want them gone.
- You see bare spots, or lots of granules in the gutters.
- Your HOA has sent a notice about how the roof looks.

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## Frequently asked questions

### What causes black streaks on my roof?

Algae, usually a kind called Gloeocapsa magma. It spreads in warm, humid air and feeds on the filler in asphalt shingles, so it grows most on shady, damp parts of the roof.

### Are black streaks on a roof harmful?

Mostly not right away: they are a looks problem. Over many years algae can shorten the life of the shingles a little, and moss can lift shingles and hold water.

### Can I pressure wash black streaks off my roof?

No. Pressure washing strips the granules that protect the shingles and can void the warranty. A low-pressure soft wash by a pro is the safe way.

### Do zinc strips stop roof algae?

They help. When it rains, a little zinc washes down the roof and slows the algae. They work best near the top of the roof, on new or freshly cleaned roofs.

### Will the streaks come back after cleaning?

Usually, within a few years, unless you add zinc or copper strips or let more sun reach the roof. Algae-resistant shingles help when the roof is replaced.