How to Safely Remove Algae and Black Streaks
Those ugly black streaks on your roof aren't dirt—they're a living organism. Here is how to safely remove Gloeocapsa magma from your Alabama roof without destroying your shingles.
Executive Summary
- It is Algae, Not Dirt: The black streaks are caused by Gloeocapsa magma, a type of algae that feeds on the limestone filler in asphalt shingles.
- Soft Wash Only: Never use a high-pressure washer on your roof. It will strip the protective granules and instantly void your warranty.
- Prevention works: Zinc or copper strips installed at the ridge can prevent future algae growth.
- Safety First: Roof cleaning is dangerous. If you aren't comfortable walking on a wet, slippery roof, hire a professional who specializes in soft washing.
What Are Those Ugly Black Streaks?
If you live in Central Alabama, you have undoubtedly seen roofs plagued by long, dark streaks that look like soot or dirt washing down the shingles. This is actually a hardy strain of algae known as Gloeocapsa magma.
This algae thrives in hot, humid environments—making Alabama its perfect home. It feeds on the crushed limestone filler used in the manufacturing of modern architectural shingles. As the algae grows and dies, it leaves behind a dark, protective sheath to shield itself from UV rays, which is what causes the black staining.
Does Algae Actually Damage Your Roof?
While primarily an aesthetic issue that destroys your home's curb appeal, severe algae growth can cause minor damage over time. Because it feeds on the limestone, it can slowly degrade the shingles. More importantly, the dark stains absorb more heat from the sun, which can increase your attic temperatures and force your HVAC system to work harder. Proper attic ventilation can help mitigate the heat, but removing the algae is the best solution.
The Number One Mistake: Pressure Washing
Never, under any circumstances, use a pressure washer on an asphalt shingle roof.
High-pressure water will blast the protective ceramic granules right off the fiberglass mat of the shingle. This instantly reduces the lifespan of your roof by years, exposes the asphalt to damaging UV rays, and will absolutely void your manufacturer's warranty. If a contractor offers to "power wash" your roof, politely decline and send them away.
The Safe Method: Soft Washing
The only manufacturer-approved method for cleaning algae off an asphalt roof is called "soft washing." This involves applying a specialized chemical solution at very low pressure (similar to a garden hose) to kill the algae down to its roots, followed by a gentle rinse.
The ARMA Recommended Solution
The Asphalt Roofing Manufacturers Association (ARMA) recommends a simple, DIY-friendly solution for removing algae:
- 50% regular household bleach (sodium hypochlorite)
- 50% water
- A squirt of liquid dish soap (acts as a surfactant to help the mixture cling to the roof)
How to Apply It Safely
Protect Your Landscaping
Bleach will kill your plants. Thoroughly soak all bushes, grass, and flowers around your house with fresh water before you begin, and cover delicate plants with plastic tarps.
Apply the Solution
Using a pump sprayer, apply the mixture to the dry roof. Spray from the bottom up to avoid streaks. Do this on a cloudy day to prevent the solution from evaporating too quickly.
Let it Dwell
Leave the solution on the roof for 15 to 20 minutes. You should literally see the black streaks turn brown or clear as the algae dies.
Rinse Gently
Using a standard garden hose, rinse the roof thoroughly from the top down. Again, water your landscaping heavily afterward to dilute any runoff.
How to Prevent Algae from Returning
Cleaning the roof is only half the battle. In Alabama's humid climate, the algae will eventually return unless you take preventative measures.
- Install Metal Strips: Installing zinc or copper strips along the ridge of your roof is highly effective. When it rains, metallic ions wash down the roof, creating an environment where algae cannot survive.
- Trim Overhanging Trees: Algae thrives in the shade. Trimming back trees to allow more sunlight to hit the roof will keep it drier and less hospitable to growth. Learn more in our Roof Maintenance Guide.
- Upgrade to Algae-Resistant Shingles: If you are due for a roof replacement, ask about shingles treated with copper granules (often marketed as AR or Algae Resistant). These typically come with a 10 to 15-year warranty against algae growth.
When to Call a Professional
Walking on a roof is inherently dangerous. Walking on a roof covered in soapy bleach water is incredibly hazardous. If your roof is steep, higher than one story, or you simply aren't comfortable on a ladder, hire a professional exterior cleaning company that specializes in soft washing. The cost is minor compared to the risk of a serious fall or accidentally ruining your roof with a pressure washer.