Roof Algae Removal: Getting Rid of Black Streaks on Shingles
Those black streaks are algae, and they thrive in humid North Carolina summers. Here is how to remove them without damaging the shingles, what a professional charges and how to stop them coming back.
What it is
Those black streaks are algae
The dark streaks running down many Triangle roofs are not dirt or soot. According to the Asphalt Roofing Manufacturers Association (ARMA), they are algae, and Gloeocapsa magma is probably the most common species behind them. The spores travel through the air and settle on roofs.
ARMA notes algae grows in all regions of the US, especially those with warm, humid conditions, which describes a North Carolina summer. Almost every roofing type can discolor. It just shows most on light-colored shingles.
Algae is mainly cosmetic. Moss is different: it holds water and lifts shingles. See roof moss removal if you have green clumps rather than streaks.
Streaks or something worse?
We will tell you whether it is algae, moss or damage, free.
How to remove it
How to remove roof algae without damaging shingles
The method ARMA describes, in order.
1. Use a bleach and water solution. ARMA says discoloration can be lightened with liquid household bleach (sodium hypochlorite) and water. Mixing directions vary by shingle manufacturer, so check yours. Do not use other chemicals without the manufacturer’s approval.
2. Apply it gently. Disperse the solution gently on the surface, protect plants and the rest of the house, and wear eye protection and rubber gloves.
3. Leave it 15 to 20 minutes. At least 15 minutes, no more than 20, then rinse gently with water.
4. Never pressure wash or scrub. ARMA warns against power washers, brushes and brooms. Friction and pressure strip the granules that protect the shingle.
5. Stay off the roof if you can. The solution makes shingles slippery. ARMA encourages using professionals and working from a ladder or walkboards.
ARMA is clear that bleach cleaning is only temporary and the streaks may come back. The lasting fix is algae-resistant shingles when you reroof, which ARMA says are designed to inhibit algae growth for extended periods. HomeGuide also notes zinc strips at $30 to $60 per 50-foot roll help against moss and mildew.
Trimming overhanging branches so the roof dries faster helps too. If you are comparing products, see the best roof cleaners.
Questions
Roof algae questions
What causes black streaks on a roof?
Algae. ARMA says Gloeocapsa magma is probably the most common species, spread through the air and thriving in warm, humid climates.
Does roof algae damage shingles?
It is mainly cosmetic, but the wrong cleaning method damages shingles. Pressure washing and scrubbing remove protective granules.
How do you kill algae on a roof?
ARMA describes a household bleach and water solution, applied gently, left 15 to 20 minutes and rinsed with low pressure. Check your shingle maker’s mixing directions.
How much does roof algae removal cost?
A soft wash costs $0.15 to $0.60 per square foot, and the average roof cleaning is $440, per HomeGuide.
Will the algae come back?
Usually. ARMA says bleach cleaning is temporary. Algae-resistant shingles are the long-term fix when you reroof.
We get on the roof, photograph what is actually there, and hand you a written report with a price. No charge, and no obligation to book the work with us. If the roof has years left in it, that is what the report will say.
