Preventing and Removing Algae and Moss from Your Roof

If you've noticed dark streaks or green patches on your roof, you're not alone. Algae and moss are common problems in North Alabama's humid climate. While they won't destroy your roof overnight, left untreated they can accelerate deterioration and hurt your home's curb appeal. Here's what you need to know.

Understanding Roof Algae

Those dark streaks you see on many roofs are caused by a type of blue-green algae called Gloeocapsa magma. The algae itself is actually greenish; the dark color comes from a protective outer coating the algae produces to shield itself from UV rays.

Algae feeds on the limestone filler used in asphalt shingles. It spreads via airborne spores, which is why you often see the problem on multiple houses in a neighborhood. Shaded areas and north-facing slopes are most susceptible because they stay damp longer.

While algae won't immediately damage your shingles, it does:

Reduce curb appeal significantly, making your home look older and poorly maintained.

Hold moisture against the shingle surface, which over time can accelerate deterioration.

Reduce energy efficiency if dark algae streaks cause your roof to absorb more heat.

Understanding Roof Moss

Moss is a different problem. Unlike algae, which lies flat against shingles, moss grows in thick, spongy clumps. It thrives in shaded, damp conditions and actually lifts shingle edges as it grows, creating opportunities for water intrusion.

Moss is more damaging than algae because it physically affects the shingles. If you have significant moss growth, it needs to be addressed promptly.

Prevention Strategies

Preventing algae and moss is easier than removing them:

Trim overhanging branches to increase sunlight and reduce debris on your roof. Roofs that dry quickly after rain are less hospitable to growth.

Keep gutters clean so water drains properly rather than backing up and keeping roof edges damp.

Ensure proper ventilation to minimize moisture accumulation in and around your roof.

Consider algae-resistant shingles if you're replacing your roof. These shingles contain copper granules that inhibit algae growth. Major manufacturers offer algae-resistant versions of their popular shingle lines.

Install zinc or copper strips along the ridge of your roof. When rain water washes over these metals, trace amounts run down the roof and create an inhospitable environment for algae and moss. This is a cost-effective prevention strategy for existing roofs.

Cleaning Options

If algae or moss is already present, you have several options:

Professional cleaning is the safest approach. A roofing professional will use appropriate cleaning solutions and techniques that won't damage your shingles. Avoid any contractor who uses a pressure washer—high-pressure water damages shingles and voids warranties.

DIY chemical treatment is possible for algae (not recommended for heavy moss). Mix a solution of 50% water and 50% household bleach. Apply with a garden sprayer on a cloudy day, let sit for 15-20 minutes, then rinse gently with a garden hose. Never use a pressure washer.

Important: Bleach solutions can harm landscaping. Wet plants thoroughly before application, cover sensitive plantings with plastic, and rinse everything after treatment.

For moss, manual removal may be necessary before chemical treatment. Use a long-handled brush to gently remove moss clumps, brushing downward to avoid lifting shingles. Then treat with a moss-killing product.

What NOT to Do

Don't pressure wash your roof. Ever. High-pressure water strips away protective granules and can force water under shingles.

Don't scrub vigorously or use stiff brushes that can damage shingle surfaces.

Don't ignore the problem hoping it will go away. Algae and moss only get worse without intervention.

Don't use pressure washer chemical attachments—even with the right chemicals, the pressure causes damage.

When to Call a Professional

Consider professional cleaning if:

You're not comfortable working on or around your roof, the growth is heavy or widespread, moss has been present long enough to lift shingle edges, your roof is steep or otherwise hazardous, or you want the job done right the first time.

At River City Roofing Solutions, we assess algae and moss during our inspections and can recommend the appropriate treatment. Sometimes cleaning is sufficient; other times, the growth has caused enough damage that repair or replacement makes more sense. We'll give you an honest assessment and help you make the right decision.

Don't let algae and moss diminish your home's appearance or shorten your roof's lifespan. With proper prevention and prompt treatment, you can keep your roof looking great for years to come.