Black Streaks on Bluffton Roofs: Algae vs. Mold vs. Dirt (And When Cleaning Becomes a Repair)

Asphalt shingle roofs in the Bluffton, South Carolina, area often develop dark streaks because of warm, humid weather that creates ideal conditions for roof discoloration. Most commonly, those streaks are caused by algae, but mold or simple dirt and debris can also play a role. Understanding what’s causing the marks will help you decide whether simple roof cleaning is enough or if you need a professional roof inspection.

Algae: The Usual Culprit Behind Roof Streaks

The most common cause of black streaks on Bluffton roofs is algae growth. Blue-green algae thrives in warm, moist climates and spreads through airborne spores. On asphalt shingles, it feeds on limestone filler and leaves dark stains that run down your roof surface. These streaks are usually flat and follow the slope of the roof. Roof algae often shows up on shaded, north-facing roof surfaces where moisture lingers longer after rain.

Algae doesn’t usually penetrate roofing materials or pose a serious health risk, but it does affect appearance and, if left unchecked, can trap moisture and accelerate shingle wear.

Mold vs. Algae: How They Differ

Distinguishing between roof mold and algae can be tricky, but a few clues help:

• Texture and pattern – Algae typically shows up as long, dark streaks. Mold tends to form irregular patches that may be fuzzy or slimy.

• Location – Mold usually grows in consistently damp, shaded areas with poor ventilation, and it can sometimes be found under roofing materials or in attics. Algae stays on the surface.

• Health and damage – Mold is a fungus and can pose health concerns and structural risks if it spreads inside your home. Algae rarely poses health issues, but can still wear down shingles over time.

When you see black streaks on your roof that look patchy or textured, or if you smell musty odors in your attic, the issue might be mold rather than algae or ordinary dirt.

Dirt & Debris: A Surface Stain

Not all roof discoloration is biological. Dirt, pollen, leaves, or other debris can collect on shingles, especially after storms or heavy winds. These marks are usually localized and can often be removed with safe cleaning methods. However, cleaning isn’t always simple. High-pressure washing can damage shingles and shorten your roof’s service life. It’s best to use professional soft wash methods that remove stains without harming your roofing materials.

When Cleaning Becomes a Repair Issue

Simple surface cleaning may help if the streaks are algae or dirt on the shingle surface. But if you’re seeing signs like bare spots, curled or cracked shingles, or dark patches that don’t wash off, the discoloration may signal deeper problems. That’s when a cleaning project transitions into the need for a professional roof inspection and possible repairs. These issues can lead to leaks, rot, or even mold growth under your roofing system.

Professional Evaluation by Local Roofing Experts

JW Roofing is a licensed and insured roofing company serving Bluffton residents, and we have decades of experience with coastal weather and how it affects asphalt roofs. Through our roof inspection process, we’ll carefully assess your roof’s condition, identifying whether the discoloration is cosmetic or a sign of underlying damage.

Our team will evaluate your roof and check for discoloration tied to granule loss, moisture, or wear. We will then recommend the right next step, whether that’s safe soft cleaning, targeted repairs, or roof replacement.

When a new roof makes sense, we offer top-performing materials. As an A GAF Certified Roofing Contractor, we install GAF Timberline HDZ® Shingles featuring GAF Time-Release Algae-Fighting Technology and backed by a 25-year StainGuard Plus™ Algae Protection Limited Warranty against blue-green algae discoloration.  

Get Help With Black Streaks on Your Bluffton Roof  

If you’re worried about black streaks on the roof of your home in Bluffton, SC, don’t wait until a small stain turns into a costly problem. Call JW Roofing today to schedule a roof inspection. We’re ready to help you protect your roof and keep it looking great for years.