# Black Streaks on Your Roof: What They Are and What to Do | Pressure Wash 901

> What causes black streaks on shingle roofs in the Mid-South, why the north side is always worst, and why pressure washing a roof is the wrong answer.

## Black Streaks on Your Roof: What They Are and What to Do

Those dark streaks running down your shingles are not dirt, and they are not a sign the roof is failing. They are alive.
Published 2026-08-16 by Pressure Wash 901 · Memphis & the Mid-South

## What the streaks are

The usual culprit is Gloeocapsa magma, a hardy algae that feeds on the limestone filler used in asphalt shingles. It spreads by airborne spores, takes hold in damp shaded areas, and streaks downward as rain carries it along the roof.
It is not soot, not dirt, and not shingle granules washing away, though it is regularly mistaken for all three.

## Why the north side is always worse

North-facing roof planes get the least direct sun, so they stay damp longest after rain and dew. That is the whole explanation for why one side of a roof can be heavily streaked while the other looks nearly new. Overhanging trees produce the same effect on any orientation.

## Does it damage the roof?

Over a long period it can. The algae holds moisture against the shingle surface and gradually breaks down the granules that protect the asphalt underneath. Dark colonies also absorb more heat, which raises attic temperatures.
It is not an emergency. It is a slow degradation worth addressing on a comfortable timeline rather than urgently.

## Why you should never pressure wash a roof

High pressure strips the protective granules straight off asphalt shingles. That is the layer standing between UV light and the asphalt, so removing it materially shortens roof life. It can also force water under shingles and into the decking.
Most shingle manufacturers void warranties on pressure-washed roofs. Anyone offering to pressure wash your shingles is offering to damage them.

## The correct method

Roofs are soft washed: low-pressure application of a cleaning solution that kills the algae, with the roof rinsed gently or in some cases left for rain to clear. No mechanical force, no granule loss.
If your roof has streaks, ask specifically whether the method is soft washing before booking anyone. If the answer involves pressure, keep looking.

## Prevention

Trimming back overhanging branches to let sun and airflow reach the roof slows regrowth more than anything else. Zinc or copper strips installed near the ridge release trace metal ions in rainwater that inhibit algae, and are worth considering if you have persistent problems.

### Free quote in 24 hours

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### Related services

- House washing

## Questions

**Q: Can you pressure wash a roof?**
You should not. High pressure strips the protective granules from asphalt shingles and usually voids the manufacturer warranty. Roofs need low-pressure soft washing.

**Q: Are black streaks on a roof a sign it needs replacing?**
Not by itself. The streaks are algae growing on the shingle surface, which is a cleaning issue rather than a structural one.

**Q: Will the streaks come back?**
In time, particularly on shaded north-facing planes. Trimming branches to increase sun and airflow slows it considerably.

## Send a photo. Get a price.

Flatwork at 12 cents per square foot, house washing around 18 cents, $149 minimum per visit. Written quote within 24 hours.

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