Why Are There Dark Streaks on My Roof?

The dark streaks on your roof are not mold, mildew, or dirt; they are blue-green algae (technically called gloeocapsa magma). These growths begin as tiny spots and after a few months, they become visible unsightly streaks that are easy to spot, diminishing your home’s curb appeal. This problem usually occurs on asphalt shingles, and often on the north-facing part of your roof.

The problem is that when algae grow on your roof, they form a protective dark-colored coating over them to shield them from UV rays—causing the ugly-looking black streaks on your roof.

Let’s dive into what causes black streaks, what to do about them, and how you can get a more algae-resistant roof in the future!

What Caused the Dark Streaks on Your Roof?

Here is some background info about the dark streaks on your roof.

How algae end up on your roof

If there are rooftops with algae in your neighborhood, airborne algae can be carried from roof to roof, wreaking havoc on your home’s looks and compromising your roof shingle integrity.

Asphalt shingles are the most popular roofing shingles today. While they were traditionally made of (oil-based) asphalt and felt material, they now often contain fillers such as crushed limestone—which unfortunately promote unwanted algae growth. In fact, algae use this filler as their primary food source.

What affects algae growth on shingles?

Although algae growth is a problem in all climates, warm, humid weather impacts algae growth on your asphalt shingles. Even if winters get frigid, if they are followed by hot, humid summers, algae thrive.

When the sun travels from the east to the west each day, the north side of your roof is shaded most of the day. The shade creates ideal conditions for algae growth.

Large trees that overhang your roof keep the sun from drying your roof quickly after it rains, which also contributes to algae growth.

Should I Be Concerned about the Dangers of Roof Algae?

Having algae on your roof isn’t dangerous, but over time if left unaddressed, it can lead to other problems:

How algae impact your roof shingles over time

Roof algae eventually causes your asphalt shingles’ protective granules to dry out and come off. Have you ever noticed a pile of granules just outside of your downspout after a storm? The moisture is washing off loose granules that may have been compromised by algae growth.

How do black streaks on your roof affect your home’s value?

Curb appeal is vital to the sale of your home. If you don’t deal with the algae problem on your roof, it can hinder you from getting all you should out of your home sale.

What makes the algae situation worse?

The algae situation can get worse. An airborne fungus (lichen) can attach itself to your roof, feed off the algae and roof shingle filler (crushed limestone), and develop roots, making it tougher to remove than algae alone.

How to Remove Algae Growth from Your Roof

It’s possible to remove or significantly reduce algae growth from your roof by cleaning your roof. There are commercial remedies and home remedies to the algae growth problem. Most solutions contain chlorine bleach, trisodium phosphate, and water. The challenging part is the danger involved in cleaning your roof.

DIY roof cleaning

Before you decide if you want to clean your roof or hire someone to clean it, be aware of the potential dangers of climbing a ladder and working on your (slippery) roof. If you choose to clean your roof, be sure to use a garden hose. Avoid using a power washer as this tool can damage shingles and void your roof warranty. Choose an overcast day, and follow the manufacturer’s guidelines for the specific roofing shingles you have. Cover your landscaping to protect it from being damaged.

Wear a protective eye covering, slip-resistant shoes, and rubber gloves. Work as a team with another individual, and take every possible safety precaution to ensure your safety (like a fall-protection harness). Start at the top of your roof and work down. Walk on the dry part of your roof to avoid slipping on wet, algae-covered roof shingles.

Professional roof cleaning

If you don’t want to take the risks involved in cleaning your roof yourself, or if your roof shingle manufacturer will not honor your warranty if you clean your roof yourself, contact a professional roof cleaning service to remove the black algae for you. These teams often have biodegradable, non-corrosive products that take care of the problem while protecting you, your landscaping, and your property. And that is worth the investment.

How to Prevent Black Algae from Discoloring Your Roof

The good news is that there are things you can do to keep the airborne algae from feasting on your shingles. Trim any large trees that come into contact with your roof, keep gutters clean and free of debris, and consider installing algae-resistant shingles the next time you replace your roof.

Algae-resistant shingles from Owens Corning are a more permanent solution to dealing with airborne algae. Owens Corning’s StreakGuard inhibits algae growth to keep your home looking beautiful!

Safeguard Your Dayton Home with a New Roof from American Way

Faulty roofing can leave your home vulnerable to costly damage. Don’t allow your biggest investment to be compromised. A new roof protects your home from water intrusion, maximizes your curb appeal, and increases energy efficiency.

At American Way Exteriors, we offer roof replacements, roof inspections, and roof repairs from storm damage to homes in the Dayton area. We’re an Owens Corning Preferred Contractor, ensuring your roofing, ventilation, and insulation maximize your home’s protection, energy efficiency, and comfort.
Find out more about our roofing services.