Dark Streaks on Roof Shingles
Appearance only — no structural concern
Black streaks on your roof? It's likely Gloeocapsa magma algae. Learn removal methods ($250–$600), prevention, and whether your shingles need replacing. Estimated repair cost: $250–$600. For a precise estimate based on your actual roof, get a free satellite quote at RoofVista.
Likely Causes
Gloeocapsa magma — a blue-green algae that feeds on limestone filler in shingles, creating dark streaks
Excess moisture from shade trees, north-facing slopes, or poor ventilation accelerates growth
Zinc or copper strip erosion — if protective metal strips at the ridge are gone, algae colonizes freely
Humid climate with warm summers provides ideal growing conditions
How to Diagnose This Problem
- 1
Confirm the streaks are dark/black and run vertically down the roof — this pattern is classic algae
- 2
Check if the streaks wipe off with a damp cloth on an accessible shingle — algae will smear greenish-black
- 3
Compare north-facing vs south-facing slopes — algae is almost always worse on the shaded north side
- 4
Check for zinc or copper strips along the ridge line — their absence means no natural algae prevention
- 5
Inspect shingle granules beneath the streaks — if granules are intact, the shingles are structurally fine
Estimated Repair Cost
$250 – $600
Typical repair range (varies by location and severity)
When to Replace Instead of Repair
Algae alone never requires replacement. However, if shingles under the algae are curling, losing granules, or over 20 years old, the algae is cosmetic on top of an aging roof that needs replacement.
Materials Affected
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