Black Streaks and Roof Algae in Johns Creek

The dark streaks running down many North Fulton roofs are usually algae, most often a blue-green type that feeds on the limestone filler in asphalt shingles. It thrives in warm, humid weather and on north-facing or shaded slopes, which describes a lot of Johns Creek. The streaks are mainly cosmetic at first, but they hold moisture and can shorten shingle life over time.

Here is how to tell algae from damage, how to clean it safely, and how to keep it from coming back.

Algae, moss or damage?

Algae shows as dark streaks or stains that run with the slope. Moss is green and raised, often in clumps along shady edges, and can lift shingles. Damage looks different: missing granules leave bald patches, and cracked or missing shingles show as gaps. If you are unsure, a roof inspection can tell the difference.

Safe cleaning

Pressure washing can strip granules and shorten the roof's life. A soft wash, applying a low-pressure cleaning solution and letting it work, is the method generally recommended for asphalt shingles. Published 2026 estimates put soft washing at about $0.30–$0.60 per square foot. Protect plants below, keep runoff out of storm drains where possible, and leave steep roofs to professionals.

Prevention

Trim branches to let in sun and air, keep gutters and valleys clean, and consider zinc or copper strips near the ridge, which release metal ions when it rains. When replacing the roof, algae-resistant shingles with copper-treated granules are widely available and worth asking about.

When streaks signal something more

Heavy algae on an older roof often comes with granule loss and brittle shingles, which can mean the roof is near the end of its service life. Published estimates put architectural shingles at up to about 30 years, and in this climate 20 to 30 years is a realistic planning range. If the roof is in that window, an inspection before cleaning makes sense.

HOA notes

Some Johns Creek HOAs send violation letters for stained roofs. Cleaning is usually not regulated by the City, but replacing the roof may need HOA approval of color and style.

Published roofing cost estimates

Sources: Angi: Roof replacement cost (2026); Angi: Roof repair cost (2026); Angi: Cost to tarp a roof (2026); Angi: Roof pressure/soft wash cost (2026). Figures are published national or regional estimates compiled by those sites, labeled as estimates; they are not quotes and not prices from any provider on this site. Johns Creek's own permit fees are set in the City's Community Development fee schedule.

Local rules and code references

• City of Johns Creek, Do I Need a Permit?: roof replacement with no structural work (up to 32 square feet of decking) is listed as not needing a permit; new roofs requiring structural alterations do need one; non-structural minor repairs under $2,500 may proceed without a permit only with the Building Official's permission.
• City of Johns Creek Building Codes: the City enforces the Georgia State Minimum Codes, including the 2024 International Residential Code with Georgia amendments effective January 1, 2026.
• City of Johns Creek Building FAQs: permits must be obtained before work starts; working without a required permit can bring stop work orders and penalties; permitted work requires City inspections.
• Georgia DCA, State Minimum Standard Codes: Georgia's adopted construction codes and amendments that cities such as Johns Creek enforce.

Code text and department procedures change; confirm current requirements with the jurisdiction before work starts.

Related local guides

• Country Club of the South Slate Roof Repair
• Emergency Roof Tarping in Johns Creek
• Gutters With a Roof Replacement