What Causes Roof Stains?

What Is Staining My Roof And What Should I Do?

Your home may have a beautiful exterior, but stains on your roof can be an eyesore. What’s more, you might have concerns as a homeowner. What’s causing the stains? Can the discoloration be removed or prevented? Do I need the help of a roofing professional?

There are a few things that could be causing the unsightly marks on your roof. Figuring out which type of staining you have (as well as the overall condition of your roof) will help you plan the best course of action.

Black Streaks Probably Mean Algae

The most common cause of roof staining is gloeocapsa magma–a type of algae. These tiny growths cause the ugly black streaks that can be found on roofs across the country. Although warm coastal climates are more hospitable to algae growth, it’s still an issue in Wisconsin and other northern states because of our hot and humid summers. Algae feed on the limestone in asphalt shingles and spread over a roof, causing that signature black streaking. By the time you notice discoloration, the algae have probably been growing for a few months.

The good news is that these algae are largely harmless and easy to treat. An algae problem doesn’t mean your roof needs replacing. However, if allowed to grow over an extended period of time, algae can start to degrade the UV protection on shingles or turn into a worse problem (lichen), so it’s best to take care of the problem sooner rather than later.

Preventing Algae Growth

Algae love moist environments. That’s why black streaks appear most often on northern roof exposures, where there’s less sunlight to evaporate lingering moisture. In fact, algae growth is a good indicator of where your roof might have moisture retention issues. Roofs are designed to get wet, but they shouldn’t stay wet!

It’s tough to prevent algae, but keeping your roof dry can help. Trim branches that block sunlight. Clear any debris which traps moisture and can also cause discoloration. Make sure your gutters are clear and draining well. Some sources recommend installing copper or galvanized steel strips to kill algae, but we’ve found that this isn’t as effective in Wisconsin’s climate. Simple maintenance is a better way to keep algae at bay.

Treating Algae Stains

Getting rid of the black stains on your roof is actually pretty easy. You can even do it yourself with some basic materials, as long as this is does not void your shingle manufacturer’s warranty.

You’ll want to buy spray-on algaecide, which is widely available at big box stores. Wait for a dry, overcast day, which will give your cleaning agent the maximum time to go to work without washing away or drying up. Cover landscaping under your gutter line and remove any furniture or outdoor decorations. Put on some protective gear and nonstick shoes and enlist a friend–safety first!–before climbing up onto your roof. Avoid using a power washer, which can damage your shingles. Instead, use a regular pump-style gardening sprayer and use it to gently wet down the roof with the algaecide solution. After about 20 minutes, rinse with regular water from a hose.

You might not immediately notice a difference in the algae staining, but don’t panic. It can take up to a month for there to be a visible reduction in algae growth. Once you’ve successfully treated your roof, it will stay pristine and algae-free for 1-2 years.

When Algae Become Lichen, Call The Pros

The real danger of algae growth is when it combines with airborne fungus particles to create lichen, splotchy growths which can be blue-green, grey, or a variety of other colors. Lichen grow roots and feed off of your shingles, which causes a lot of damage. Once lichen is on a roof, it can be extremely difficult to remove. If you notice lichen growing on your roof, contact a roofing professional at L.H. Krueger and we can help you evaluate your options.

Other Possible Causes Of Roof Stains

Although the staining you’re seeing is probably algae, there are a few other less likely suspects. Brown or red stains could mean rust from flashing or other metal components. This can be cleaned, but it’s best to contact a pro to make sure your roof hasn’t sustained any damage. If you notice dark discoloration around a chimney, it might be soot, and you should consider having your chimney inspected and cleaned. Extreme weather fluctuations can cause the pigment in old or low-quality shingles to rise to the surface and “bleed” down your roof. If that’s the case, it’s probably time for a new roof.

When Staining Means It’s Time For A Roof Replacement

Eventually, all roofs need to be replaced, whether due to age or damage. Unsure about whether it’s time for a replacement? Check out our recent post about the life expectancy of a roof, or contact us to request a Free Service Estimate. If you are due for a replacement, there are new algae-resistant roofing materials that might be beneficial for your home. Our team would be happy to assess your roof and provide recommendations.

Do you have a worn or leaking roof? Are you looking for advice or assistance with your next roof repair or replacement job? Contact L.H. Krueger and Son, Milwaukee’s home exterior repair experts.

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