Roof Algae & Black Streaks: Causes & Cleaning Guide

Black streaks on your roof are caused by Gloeocapsa magma, a blue-green algae that feeds on the limestone filler in asphalt shingles. It is not dirt, mold, or a sign that your roof is failing. The algae is primarily cosmetic but reduces curb appeal, increases cooling costs, and worsens over time. Professional soft-wash cleaning costs $300 to $600 and removes the streaks without damaging shingles. Zinc or copper strips installed along the ridge prevent regrowth for 10 to 20 years.

In Myrtle Beach and coastal South Carolina, roof algae is nearly universal. Our combination of year-round humidity above 70 percent, warm temperatures, and frequent rain creates ideal growing conditions. Drive through any neighborhood on the Grand Strand and you will see black streaks on the majority of roofs that are more than 5 years old. The north-facing slopes are typically darkest because they receive the least direct sunlight.

This guide covers exactly what causes the black streaks, whether they are harmful, how to clean them safely, how to prevent them from coming back, and the full cost breakdown. If you are seeing fuzzy or raised growth rather than flat streaks, you may have roof mold or moss, which require different treatment.

What Are Black Streaks on a Roof?

The black streaks are colonies of Gloeocapsa magma, a species of cyanobacteria (blue-green algae) that has spread across North America over the past several decades. The algae itself is actually blue-green in color, but it produces a dark pigmented outer sheath to protect itself from UV radiation. This dark sheath is what you see as the black streaks.

Gloeocapsa magma feeds on the calcium carbonate (limestone) filler that is a standard ingredient in asphalt shingles. The limestone makes up 20 to 40 percent of the shingle weight and gives the algae an unlimited food source. The algae spreads by airborne spores, which is why once it appears on one roof in a neighborhood, it eventually spreads to nearby homes.

The streaking pattern follows the path of rainwater down the roof surface. Algae colonies are densest where water flows and where shade persists longest. This is why streaks typically run vertically from the ridge toward the eaves and are darkest on north-facing slopes and under overhanging trees.

Why Algae Thrives in Coastal South Carolina

Myrtle Beach provides near-perfect conditions for Gloeocapsa magma:

• Year-round humidity — Average relative humidity of 70 to 80 percent provides constant moisture for algae growth, even during dry periods.
• Warm temperatures — The algae is most active at temperatures between 70 and 100 degrees, which Myrtle Beach experiences for approximately 7 to 8 months per year.
• Frequent rainfall — 50 inches of rain per year keeps the roof surface regularly wet, providing the moisture the algae needs to grow and spread.
• Tree canopy — Live oaks and other mature trees throughout the Grand Strand provide shade that protects the algae from UV light.
• Mild winters — Unlike northern states where freezing temperatures kill surface algae annually, Myrtle Beach rarely sees extended freezes, allowing colonies to grow year-round.

Is Roof Algae Harmful?

Roof algae is not a structural emergency, but it is not purely cosmetic either. Here is what it actually does:

• Reduces roof reflectivity — The dark algae coating absorbs more heat than clean shingles, raising attic temperatures by 10 to 15 degrees. This increases cooling costs, especially significant in Myrtle Beach where AC runs 7 or more months per year.
• Lowers property value — A roof covered in black streaks reduces curb appeal and can lower property value by an estimated 5 to 10 percent based on real estate industry assessments.
• Accelerates aging — While the effect is slow, algae feeding on limestone filler can contribute to slight granule loosening over years.
• Attracts moss — Algae creates a biofilm that makes it easier for moss to take hold. Moss is more damaging because it lifts shingle edges and traps moisture against the surface.
• HOA violations — Many Myrtle Beach homeowners associations have exterior maintenance requirements that can result in violation notices for heavily stained roofs.

Soft Wash Cleaning (Recommended Method)

Professional soft washing is the manufacturer-approved method for removing algae from asphalt shingles. GAF, Owens Corning, and CertainTeed all recommend this approach and caution against pressure washing.

How Soft Washing Works

1. The cleaning crew protects landscaping below the eaves with plastic sheeting and pre-wets plants with water
2. A sodium hypochlorite (bleach) solution diluted to 1 to 3 percent concentration is applied to the roof surface using a low-pressure pump sprayer (under 100 PSI)
3. The solution is allowed to dwell for 15 to 20 minutes, during which it kills the algae at the cellular level
4. The roof is rinsed with low-pressure water to remove the dead algae and cleaning solution
5. Plants are rinsed thoroughly to remove any chemical residue

Results are typically visible immediately, though some residual staining may continue to fade over the following weeks as dead algae washes off during rain. The entire process takes 2 to 4 hours for a standard residential roof.

DIY Roof Algae Cleaning

If you choose to clean algae yourself, follow these guidelines:

• Mix one part household bleach (6 percent sodium hypochlorite) with three parts water in a garden pump sprayer
• Pre-wet all plants below the roof with plain water
• Apply the solution to the roof surface starting from the top and working down
• Let it sit for 15 minutes (do not let it dry)
• Rinse with a regular garden hose — no pressure nozzle
• Rinse plants thoroughly when finished

Why You Should Never Pressure Wash a Roof

Pressure washing (using more than 1,000 PSI) is the most common mistake homeowners make when trying to clean their roof. Here is why it is destructive:

• Strips granules — High-pressure water blasts protective granules off the shingle surface, removing years of UV protection in minutes.
• Voids warranty — All major shingle manufacturers (GAF, Owens Corning, CertainTeed) specify that pressure washing voids the warranty.
• Forces water under shingles — High-pressure water can lift shingle edges and force water underneath, causing leaks.
• Damages underlayment — Water forced under shingles can tear or displace the underlayment, creating future leak paths.
• Shortens roof life — A single pressure washing session can remove 2 to 5 years of effective shingle life.

Prevention: Zinc & Copper Strips

Zinc and copper strips are the most effective long-term prevention for roof algae. They work by a simple mechanism: rain washes over the metal strip and carries metal ions down the roof surface. These ions create a hostile environment for algae and moss growth.

Zinc Strips

• Cost: $50 to $100 per 50-foot roll
• Installation: Along the ridge under the cap shingles, with 2 to 3 inches exposed
• Lifespan: 10 to 20 years
• Effectiveness: Excellent in the first 10 years, gradually diminishes
• Best for: Prevention on new or recently cleaned roofs

Copper Strips

• Cost: $100 to $200 per 50-foot roll
• Installation: Same as zinc
• Lifespan: 20 to 40 years
• Effectiveness: Stronger algae-inhibiting properties than zinc
• Best for: Long-term prevention, especially on premium homes

The strips should be installed immediately after cleaning the roof, or during a new roof installation. Install along the ridge and, for wider roofs, add an additional row midway down each slope for maximum coverage. Professional installation costs $200 to $500 including materials.

Algae-Resistant (AR) Shingles

When it is time for a roof replacement, specifying algae-resistant shingles is one of the best investments you can make in Myrtle Beach. AR shingles contain copper granules mixed into the surface layer that continuously release copper ions, inhibiting algae growth for the life of the shingle.

• GAF offers AR technology in their Timberline HDZ and most other product lines
• Owens Corning includes AR in their Duration and TruDefinition lines
• CertainTeed offers StreakFighter AR technology
• The price premium for AR shingles is typically $2 to $5 per bundle over standard shingles
• AR shingles are warranted against algae staining for 10 to 15 years by most manufacturers

For Myrtle Beach, we recommend AR shingles on every roof replacement. The small additional cost pays for itself by eliminating or significantly reducing cleaning costs over the life of the roof. Compare shingle options in our GAF vs Owens Corning comparison.

Cost Guide

Over a 20-year period, a Myrtle Beach homeowner without prevention might spend $1,800 to $3,600 on soft-wash cleanings (6 to 8 cleanings at $300 to $450 each). Adding zinc strips for $300 to $500 reduces the cleaning frequency to every 4 to 5 years, cutting the total cleaning cost roughly in half.

Frequently Asked Questions

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