Black Streaks on Your Roof: What They Are and How to Remove Them Safely

What the streaks are

The Asphalt Roofing Manufacturers Association identifies the algae Gloeocapsa magma as probably the most common cause of brown-to-black streaking on shingle roofs.¹ Owens Corning names the same organism.² It spreads by airborne spores and grows where shade and moisture hang around, which in North Carolina's humid climate⁴ is a lot of roofs.

The streaks usually start on the north side or under trees and run down the slope, because rain carries the growth with it.

Is it hurting the roof?

Mostly it hurts how the house looks and what buyers think of it. Moss and lichen are a different story: they hold moisture against the roof, and they need treating too.

How the manufacturers say to clean it

• Use a bleach-and-water solution. ARMA says household bleach (sodium hypochlorite) and water lightens algae staining, and to leave it on about 15–20 minutes. Mixing directions vary by manufacturer.¹ Owens Corning gives a 1:1 bleach-to-water mix with a 10–20 minute dwell.²
• Rinse at low pressure. Rinse the roof and the plants below it.²
• Do not pressure wash, brush or broom. ARMA says high-pressure systems are likely to damage asphalt roofing.¹ Owens Corning says high-pressure power washing loosens the protective granules.²

That is a soft wash. It is the method our roof cleaning service uses on shingle and metal roofs.

Why this isn't a great DIY job

Wet roofs are slippery, bleach solutions are hard on skin, eyes and unprotected plants, and the people most likely to get hurt are homeowners on ladders. A professional crew brings the right equipment, mixes the solution correctly, protects plants and carries insurance for the work.

What it typically costs

Fixr's 2025 national figures put roof cleaning around $450–$700 (about $612 on average), with soft washing at roughly $0.30–$0.75 per square foot.³ Pitch, height and how heavy the growth is drive the final number. We quote roofs free.