Those black streaks running down your roof shingles are gloeocapsa magma — an airborne algae that eats the limestone filler in modern shingles and spreads streak by streak across the shaded, damp side of your roof. They shorten shingle life and gut your home's curb appeal, and the one thing you absolutely cannot do is pressure wash them off: that strips the protective granules and can void your shingle warranty on the spot.

The black streaks on your roof are an algae — gloeocapsa magma — that feeds on the limestone filler in asphalt shingles. It spreads down-slope wherever moisture lingers, which is why it shows up first on the north-facing plane and under tree cover. It will not wash off with rain, it keeps growing season over season, and it's eating the surface of the roof you paid five figures for.

The good news: it's completely removable. The catch is the method.

Why Pressure Washing a Roof Is the Expensive Answer

Every shingle manufacturer's warranty says the same thing: do not high-pressure wash asphalt shingles. The granules glued to the shingle surface ARE the product — they're what protects the asphalt from UV. A pressure wand at close range strips them off in visible lines, and once granules are in your gutter, they don't come back. That's not cleaning; that's giving your roof an early funeral, and it hands the warranty fight to the homeowner who pulled the trigger.

The safe method is a soft wash: a properly diluted algaecide treatment applied at garden-hose pressures, given dwell time to kill the algae at the root, then rinsed clean. The streaks don't get blasted off — the growth dies and washes away, and the shingles keep their granules. This is exactly how our roof washing in Fort Wayne is built, and it's the only method we'd let near an architectural shingle roof.

What the Black Streaks Are Actually Doing to Your Roof

• Granule consumption — the algae feeds on the shingle filler, loosening the protective granule layer as it colonizes
• Higher cooling costs — dark streaking absorbs heat that a clean roof reflects; a heavily streaked roof runs measurably hotter in summer
• Neighborhood spread — airborne spores mean one streaked roof seeds the block; it's why whole streets in Fort Wayne go black section by section
• Resale drag — a streaked roof reads as "roof problem" to buyers and appraisers even when the shingles underneath have years of life left

The 5-Minute Self-Check

1. The gutter granule test. Next rain, check your gutter downspout exit for a pile of sand-like granules. Heavy granule loss combined with streaking means stop waiting — the algae is actively consuming your roof.
2. The shade-side look. Compare your north/shaded plane to the sunny one. If the sunny side is clean and the shaded side is streaked, it's algae (environmental), not shingle failure (structural) — very good news, fully treatable.
3. The age question. Shingles under ~20 years with streaking are almost always worth treating. Black streaks on a 25-year roof still deserve a professional look before anyone assumes replacement.

If the sunny AND shaded sides are uniformly losing granules and curling, that's wear, not algae — different problem. But streaking concentrated in shade is the treatment case, and it's the one where soft washing saves roofs.

After Treatment: Keeping Streaks Off

Post-treatment, regrowth prevention is about removing what algae loves: trim back branches that shade the roof plane, clear debris from valleys, and ask about a maintenance treatment cycle. In Fort Wayne's climate, treated roofs stay clean for years with a light maintenance rhythm. For the full picture on safe roof cleaning methods, see our guide on whether roofs can be pressure washed.