How to Remove Black Streaks From Your Roof (Without Damage)

Those ugly black streaks running down North Carolina roofs aren't dirt — they're a living algae called gloeocapsa magma. It thrives in humid climates, feeds on the limestone filler in asphalt shingles, and if it's ignored it will shorten your roof's usable life by years. Here's how to remove it safely, and why pressure washing is the worst thing you can do.

What's actually growing on your roof

Gloeocapsa magma is airborne. It lands on the north-facing slopes of shingle roofs (the shaded side that dries slowest), grabs a foothold, and forms a dark biofilm. Over 3 to 5 years it can cover an entire roof and reduce reflectivity enough to raise summer cooling loads. Untreated colonies also invite lichen and moss — both of which physically lift shingles.

Why pressure washing a roof is a mistake

• Blasts off the granules that protect shingles from UV damage
• Voids most asphalt shingle manufacturer warranties
• Can force water under shingles and into the attic
• Only removes surface staining — the algae grows back in months

The manufacturer-approved fix: soft washing

The Asphalt Roofing Manufacturers Association (ARMA) recommends a low-pressure application of a diluted sodium hypochlorite solution — the exact method professional soft washing crews use. The chemistry kills the algae at the root, so it doesn't grow back for 4 to 7 years. There is no scrubbing, no walking on brittle shingles, and no risk to granules.

What Factors Affect This Service?

• Total roof square footage and pitch
• Story height and access for ladders and hoses
• Property condition — heavy algae, lichen, or moss coverage
• Materials — asphalt shingle, metal, cedar shake
• Landscaping around the drip edge that needs pre-rinsing
• Service scope — full roof vs. specific streaked elevations

Safety considerations

Working on a roof safely requires proper anchoring, chemistry handling, and careful control of runoff onto landscaping and neighboring properties. This is the single service where DIY attempts most often lead to falls and warranty voids — it's worth hiring a professional.

Common mistakes homeowners make

• Renting a pressure washer and blasting the shingles
• Scrubbing algae with stiff brushes (loosens granules)
• Using undiluted household bleach that damages plantings
• Ignoring the north slope while cleaning only the visible side