If you’ve noticed dark streaks running down your asphalt roof — especially on north-facing slopes or shaded sections — you might have assumed it’s just dirt or discoloration from aging. It’s not.
Those streaks are alive.
What’s Actually Growing on Your Roof
The dark staining is caused by Gloeocapsa Magma, a cyanobacteria (blue-green algae) that feeds on the limestone filler used in asphalt shingles. It thrives in moisture-rich, shaded environments — which makes Central Ohio roofs a prime target, especially through our humid summers.
Gloeocapsa Magma starts as a thin, invisible layer. Over time, it produces a dark pigmented shield to protect itself from UV exposure — that’s the dark streak you see. By the time it’s visible, the colony has been established for months or years.
Why This Is More Than Cosmetic
The streaking looks bad, but the real problem is structural. Gloeocapsa Magma feeds on limestone. Asphalt shingles use limestone as a filler to add weight and fire resistance. As the algae colony grows, it gradually consumes the limestone from your shingles.
The result:
- Reduced shingle mass and structural integrity
- Granule loss (granules protect shingles from UV)
- Shortened roof lifespan — potentially by years
- Voids some manufacturer warranties when left untreated
What looks like an aesthetic issue is actually accelerated deterioration.
Why Black Streaks Spread
Once established, Gloeocapsa Magma spreads via wind, birds, and rain runoff. You’ll notice it migrates down roof slopes following the path water takes. It also commonly spreads from one house to neighbors — which is why you often see multiple homes on a street developing the same problem around the same time.
If your roof currently has streaks on one section, expect it to spread across the full roof without treatment.
The Wrong Fix: Pressure Washing
The instinctive response is to blast the streaks off with a pressure washer. Don’t.
High-pressure cleaning (anything above 200 PSI on a roof) causes serious problems:
- Granule loss — granules are what protect your shingles from UV degradation; once they’re gone, aging accelerates
- Voided warranty — GAF, CertainTeed, and most major manufacturers specify low-pressure cleaning only; pressure washing voids the warranty
- Shingle cracking — older shingles become brittle; high pressure cracks them
- Water intrusion — high pressure forces water up under shingles and into the roof deck
Even if it removes the surface staining, pressure washing leaves the root algae colony alive and doesn’t prevent regrowth.
The Right Fix: Roof Soft Washing
Soft washing uses a biodegradable cleaning solution — typically sodium hypochlorite-based — applied at very low pressure (60–100 PSI, similar to a garden hose). The solution:
- Kills the algae colony at the root, not just the visible surface
- Removes the dark pigment and staining
- Slows regrowth — treated roofs resist recolonization significantly longer than untreated ones
The treatment is rinsed with low-pressure water. No granule loss. No warranty issues. No shingle damage.
Results are typically visible within a few days as the dead algae wash away with rain.
How Long Does It Take to Come Back?
Treated roofs typically see 2–4 years before significant regrowth requires another cleaning. Shaded roofs or those in heavily wooded areas may see faster return. Proactive treatment before streaks become heavy keeps the roof in better shape long-term and is typically less expensive than treating a heavily colonized roof.
What About Lichen and Moss?
Black streaks (algae) are the most common issue, but Ohio roofs can also develop:
- Lichen — algae + fungus symbiosis that bonds to shingles with “holdfasts” (root-like structures); requires targeted treatment and may leave surface marks even after removal
- Moss — especially in heavily shaded areas; holds moisture against shingles and causes deterioration
Both are treated with soft washing chemistry, though heavily established lichen may require longer dwell time and follow-up treatments.
Is Your Roof Ready for a Soft Wash?
If you can see dark streaks from the ground, the algae colony is well-established. The sooner you treat it, the less damage accumulates. We serve homeowners across Union, Franklin, Delaware, and Logan counties.
Get a free roof soft wash estimate and we’ll assess your roof’s condition and recommend the right approach.