Before and after of a stained roof with dark streaks vs. a new, cleaned roof

You’re walking around your neighborhood and you see those weird marks on top of roofs, usually darker in color and running down the shingles.  You happen to walk back to your own home and see them, too! These streaks look absolutely horrible, and for your home’s curb appeal, they could bring down the property value of your home and those in your neighborhood. When you see these unsightly streaks, the first thing to do is determine what they are and the steps to take to get rid of them. You and other homeowners alike are searching to find the answers every day, and A to Z Construction Inc. has compiled some frequently asked questions, as well as answers for these unsightly streaks.


Q: Why does my roof have black streaks? What makes these dark black streaks?

A: The dark black streaks running down roofs are actually an algae called “Gloeocapsa magma

  • As this blue-green algae begins to accumulate, they can develop a dark, hardened outer coating, which results in the dark black stains you see.


Q: How does the algae grow? What do they eat?

A: The algae feeds on limestone, found in most professionally manufactured shingles.


Q: How do I get black streaks off my roof?

A: Multiple roof cleaner products, as well as professional contractors.

  • There are multiple answers to this question. TSP, bleach, roof cleaner products, and water can fight Gloeocapsa magma spread. There are multiple roof cleaner products for the average DIY found online. Most bleaches can do wonders. Oxygen bleach and chlorine bleach have different outcomes. Oxygen bleach will mainly lighten stains and is less harmful to the environment. Chlorine bleach will have a more immediate effect but isn’t one you’d want to have run off into drains.


Q: So I have my bleach/TSP/water/product, now what? What are the next steps?

A: Start at the top and work your way down.

  • Starting at the top of the roof and spraying down is the best practice, as gravity works to your advantage, while spraying up into shingles can loosen them over time. If you purchase a product, make sure to follow its directions.


Q: Does cleaning the roof damage it?

A: Regular roof cleaning is an integral part of roof care.

  • Neglecting your roof can lead to expensive damage repair in the end. Doing at least an annual inspection can go a long way to prevent long-term issues with your roof.


Q: Does Zinc work against algae?

A: Yes

  • Zinc can act as a natural inhibitor to most algae growth, including Gloeocapsa magma. By installing certain zinc metal strips on the tops of your roof areas near the ridge caps, above the algae prone areas. When rain hits the zinc metal strip, it will carry the zinc down and over the shingles.


Q: I don’t want to get on my roof to clean. If I hire a professional roof cleaner, how much will the roof cleaning cost me?

A: Anywhere between $250 to $1,000.



We hope our list of frequently asked questions and answers regarding the dark streaks on your roof has given you clarity on the next steps to take to get your home in tip-top shape. Remember, A to Z Construction Inc. is here for you every step of the way, whether that means answering questions regarding your home or helping you start your next exterior and/or interior project. Also, if your home has recently been hit with storm damage, fill out our Free Roof Inspection Form.