What Are Those Stains on My Roof and How Did They Get There?

The appearance of your roof significantly affects your home’s curb appeal. The last thing you’d want is to have unsightly stains on your roof, making your home look older and unkempt. What exactly are these stains and how did they get there? 

There are many possible causes, but we’ll focus on the most common cause, which is algae, in this article.

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Causes of Roof Stains

Roof stains can come from a combination of things, such as defective shingles, mold, vegetation, rust from the chimney, or mildew too. However, the most common cause is algae, which goes by the name of Gloeocapsa magma.

Because this type of algae grows slowly, you won’t immediately know it’s there until you see it in visible dark streaks. By that time, it would have been on the roof for around at least two months. Algae has a dark outer coating to protect it from the UV rays, which is why it appears black. 

Rest assured that the algae are not like black mold, but it will lead to those dark stains on your roof.

The streaking happens mostly because of gravity and weather. As the algae grow, the gravity and rain pull them down to create streaks. This will be especially visible on lighter roofs. 

What Causes Algae?

You’ll likely see algae most often on asphalt roofs or on any roof using a form of cement—which algae like. Developing from spores, it’ll also grow well in places with high humidity and warmth, and once it’s there, it stays there. 

Algae can also be caused by tree branches that are hanging over your roof, pooled water, gutters that are clogged, or even inadequate roof ventilation.

How to Prevent Roof Stains

You can prevent these roof stains, especially if you’re redoing your roof. In this case, construct your roof with darker shingles with copper, and ask your roofer about installing your shingles with lead or zinc. The point is to use materials that algae doesn’t like, but it’s vital to note that you can’t ever fully prevent algae.

Maintain Your Roof

One thing you can do to keep the algae at bay is to maintain your roof regularly. Trim your tree branches, do your best to prevent debris from falling atop your roof, and clean your roof as regularly as possible.

Keep your gutters clear too and make sure that water can drain properly off of the entirety of your roof. You don’t want water to pool anywhere.


This might seem like a simple fix. Algae don’t like sunlight, so all you need to do is trim the trees around your roof to allow sunlight to get on your roof. This will slow the growth of more algae, and prevent the growth of other things like moss.

Regular Cleanings

Beyond keeping your roof maintained and your trees trimmed, it’s a good idea to have your roof professionally cleaned at least every year and a half. This will do more than check for algae, and will actually ensure that you catch any other potential problems on your roof before they can become expensive issues.


Don’t panic if you see stains or dark streaks on your roof. It may not look great, but it’s not exactly dangerous. You’ll still need to get it cleaned up though because these algae can cause you problems in the future, such as retaining moisture and slowly eroding your shingle base.

Give us a call and let us help you give your roof a thorough and professional clean to remove these stains.