Roof basics
Moss and Algae on Roof Shingles: Harmless or a Problem?
Dark streaks and green patches make a roof look old. Here is what they are, what the manufacturers say about them, and how to deal with them safely.
The short answer: If your roof has years left and the damage is in one area, repair it. If shingles are failing across the roof or the same leaks keep returning, a replacement saves money over time. We tell you which one applies, with photos.
Algae streaks vs moss
Not every stain on a roof is the same.
- Algae shows up as brown-to-black staining, often in streaks. The Asphalt Roofing Manufacturers Association (ARMA) names *Gloeocapsa magma* as the most common cause. Airborne algae settle on the roof and grow, especially in warm, humid conditions, and the stains can look like soot, dirt or fungus.
- Moss is a green, fuzzy plant growth that tends to collect in shaded, damp spots. Algae tends to spread more evenly across a roof, while moss and tree droppings usually cause localized spots.
Indiana has humid summers and shaded lots, so you may see both. ARMA says algae is most widespread in the Gulf States and along the Northwest and Eastern Seaboards, but it occurs to some degree across the country, especially in warm, humid areas.
Is it damaging your roof?
ARMA treats algae discoloration as mainly a cosmetic problem, more visible on lighter roofs. The real risk to the shingles often comes from how you clean it, because rough cleaning can loosen the protective granules.
Moss is different in practice. It can hold moisture against the roof and may lift or loosen shingle edges, according to many roofers. If you see thick moss, loose shingles, or granules in the gutters, have the roof inspected. See our roof inspection page and our guide to the signs you need a new roof.
How to clean it safely
ARMA’s guidance for algae stains:
- Use a diluted household bleach solution (sodium hypochlorite). Mixing directions vary by manufacturer, so check them.
- Apply it gently, leave it on for roughly 15 to 20 minutes, then rinse gently with water.
- Do not scrub. Friction can strip granules.
- Avoid power washers, brushes and brooms. High-pressure washing is likely to damage asphalt shingles.
- Do not use other chemicals or methods without the shingle manufacturer’s approval.
- Protect the landscaping and wear eye protection and gloves.
- The roof gets slippery during and after treatment, and ARMA recommends hiring professionals.
Results are often temporary, and the stains may return. Because climbing a roof is dangerous, we recommend leaving this to a professional.
How to prevent it
- Algae-resistant shingles. ARMA’s main recommendation when you reroof is to choose algae-resistant asphalt shingles, which are designed to resist algae growth for extended periods. Ask your roofer about the product line and its warranty. See our asphalt shingle roofing page.
- Trim overhanging branches. More sun and airflow help dry the roof and reduce moss.
- Keep gutters clean. Debris and standing water keep the roof damp.
- Schedule inspections. See our roof maintenance page.
When it means something more
If the roof is covered in moss, you see missing or lifting shingles, or the roof is near the end of its life, a cleaning is not the answer. Our repair vs replacement guide can help you decide, and you can contact us for an inspection.
Sources
Moss and algae questions
Are black streaks on my roof dangerous?
The streaks are usually algae, which ARMA describes as mainly cosmetic. The danger comes from rough cleaning or from moss and moisture issues, so have a roofer inspect if you are unsure.
Can I pressure wash my roof?
No. ARMA warns that high-pressure washing is likely to damage asphalt shingles and strip granules.
Related:
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Roofing contractor serving Indianapolis, Indiana and the surrounding suburbs.
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123 Main Street, Indianapolis, IN 46204
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