Illinois Roofing License IDFPR #19HC00108700 · GAF Certified Contractor
The dark streaks running down asphalt roofs are everywhere in Illinois – especially on north-facing slopes and shaded sections. They’re not dirt, not dirt stains, and not always harmless. This guide explains what’s actually growing up there, when it’s purely cosmetic vs. when it’s shortening shingle life, how to remove it safely, and how to prevent it from coming back when you re-roof.
What’s actually growing on your shingles
The dark streaks are caused by Gloeocapsa magma – a type of blue-green algae (cyanobacteria) that feeds on the limestone filler in standard asphalt shingles. The algae thrives in moist, shaded conditions and produces a dark pigment that becomes visible as the colony grows.
The streaks typically run vertically down the roof slope, are darker at the top (where moisture lingers near the ridge), and most pronounced on:
- North-facing slopes (less direct sun, slower drying)
- Slopes under tree cover or in shade from neighboring buildings
- Older roofs (5+ years) where granule wear has exposed more limestone filler
- Coastal or humid regions – and Illinois summers easily qualify
Moss is a separate organism – true plants that root into the shingle surface and grow as green cushions, typically in heavily shaded areas. Moss is much more damaging than algae because it physically lifts shingles and traps moisture against the deck.
Cosmetic vs damaging
Algae staining is mostly cosmetic. The algae feeds on filler material, not on the asphalt itself, so it doesn’t directly shorten shingle life. However:
- The dark color absorbs more heat, increasing thermal stress on the shingles below – speeding asphalt aging
- The algae layer traps moisture against the shingle, accelerating granule loss over years
- The visual impact reduces home curb appeal and resale value
Moss is actively damaging:
- Roots can penetrate the shingle surface and grow under shingle edges, physically lifting them
- Trapped moisture rots the asphalt mat and the underlying deck
- Heavy moss colonies (1-2 inches thick) hold significant water weight and can degrade the entire shingle field
Bottom line: streaks are mostly cosmetic; moss is a real problem that needs addressing before it spreads.
Safe removal methods
The wrong way to clean algae or moss off shingles: pressure washing. Pressure washing strips granules, dramatically shortening shingle life. We’ve seen homes lose 30-40% of their remaining shingle life from one aggressive cleaning.
The right way:
Soft-wash method (low-pressure chemical):
- Apply a 50/50 mix of household bleach and water to the affected area (or a proprietary algae-removal product like Wet & Forget)
- Let the chemical sit for 15-30 minutes
- Rinse gently with a low-pressure garden hose (no pressure washer)
- Algae dies and washes off over the next several weeks of rain
Protections to take:
- Wet plants and shrubs below before applying the chemical (bleach kills plants)
- Wear protective gear – bleach mist is a respiratory irritant
- Use a roof safety harness or hire a professional for steep pitches (over 6/12)
- Cover gutter outlets to prevent bleach runoff from killing landscaping below
For moss specifically, the chemical kills the moss; you’ll need to physically remove the dead moss with a soft brush or use a moss-killer product designed for roofs (like sulfate or phosphate-based products).
Algae-resistant shingles for prevention
Modern algae-resistant (AR) shingles include copper or zinc granules embedded in the shingle surface. When rainwater interacts with the copper or zinc, trace amounts dissolve and create a microenvironment that inhibits algae growth. The roof stays clean for 10-15+ years.
Most major shingle manufacturers now offer AR versions of their architectural lines:
- GAF Timberline HDZ with StainGuard Plus (10-year algae warranty)
- CertainTeed Landmark with StreakFighter
- Owens Corning Duration with StreakGuard
- Atlas Pinnacle Pristine with Scotchgard Protector (extended warranty)
The cost premium for AR over standard architectural is small – typically $0.10-$0.30 per square foot. For a 2,000 sq ft roof, that’s $200-$600 – a small fraction of total project cost for a roof that won’t streak for 15+ years.
If your home has shade or moisture conditions that lead to algae, AR shingles are worth the upgrade. If you’re in full sun on south-facing slopes, you may not need them.
Other prevention strategies
If you’re not planning to replace your roof soon, two other prevention approaches:
Copper or zinc strip installation: a 4-6 inch wide strip of copper or zinc sheet metal installed under the top course of shingles, just below the ridge. The strip releases trace metals when it rains, inhibiting algae growth on the slope below. Cost: $200-$500 installed. Effective for 10-20 years.
Tree trimming and gutter cleaning: reducing moisture exposure helps. Trim back tree branches within 6 feet of the roof to increase sun exposure and reduce debris accumulation. Keep gutters clean so water doesn’t pool at the eaves.
When to call a pro
DIY cleaning is feasible for low-pitch roofs (4/12 or lower) and accessible sections. For:
- Roofs with pitch over 6/12
- Multi-story homes
- Areas with heavy moss (more than a few patches)
- Roofs over 10 years old where you want a professional inspection alongside cleaning
…hire a professional. Trill Roofing offers algae removal as a service paired with roof inspections. We use soft-wash methods, protect landscaping, and identify any other issues we find while we’re up there.
Schedule a free inspection at /free-inspection/ and we can discuss whether your roof’s condition warrants cleaning, replacement with algae-resistant shingles, or just monitoring.
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