You pull into the driveway, look up at your roof, and notice dark streaks running down the shingles.
Is it dirt? Mold? Is something wrong with the roof?
Most of the time, those black streaks are actually algae.
And while they can definitely make a roof look older or dirtier than it really is, they don't automatically mean your roof is damaged or needs to be replaced.
What Causes Black Streaks on a Roof?
The dark streaking commonly seen on asphalt shingle roofs is often caused by a type of blue-green algae called Gloeocapsa magma.
The algae travels through the air and can settle on roofing surfaces. Once it finds the right conditions - especially moisture and humidity - it can begin to grow and create the dark discoloration you see from the ground.
That's one reason black streaks are so common here in the Southeast.
Why Is It Worse on Certain Parts of My Roof?
You may notice that one side of your roof looks much worse than another.
That's normal.
Algae tends to do well in areas that stay damp longer, which can include:
Shaded sections of the roof
Areas surrounded by large trees
Roof slopes that receive less direct sunlight
Areas where moisture tends to linger
So it's completely possible for one side of a house to have noticeable streaks while another side looks relatively clean.
Are Black Streaks the Same as Mold?
Usually, no.
Homeowners often describe dark roof staining as mold or mildew, but the streaking commonly seen across asphalt shingles is typically algae.
Moss and lichen are different as well. Those tend to create more noticeable growth on the roof rather than the flat, streaky discoloration associated with algae.
If you're unsure what you're seeing, a roof inspection can help identify it.
Do Black Streaks Mean My Roof Is Damaged?
Black streaks alone do not necessarily mean your roof is failing.
In many cases, algae staining is primarily an appearance issue.
That means you shouldn't assume that a stained roof needs to be replaced simply because it looks old from the driveway.
However, if the roof is already showing other signs of wear - such as missing shingles, cracking, curling, excessive granule loss, or leaks - those issues should be evaluated separately.
The appearance of the roof is only one part of the picture.
Can I Pressure Wash the Black Streaks Off?
This is one thing we wouldn't recommend doing.
A pressure washer may make the shingles look cleaner temporarily, but high-pressure cleaning will loosen or remove the protective granules on asphalt shingles.
Roof manufacturers have specific recommendations for cleaning algae staining, and those instructions can vary depending on the roofing product.
If you want the roof cleaned, it's a good idea to first find out what type of shingles you have and follow the manufacturer's recommended cleaning method.
And because a wet roof becomes extremely slippery, roof cleaning also comes with a very real safety concern.
Can Black Streaks Be Prevented?
Some newer shingles are manufactured with algae-resistant technology designed to help reduce discoloration over time.
That doesn't necessarily mean you should replace a perfectly good roof just because it has algae staining.
But when the time eventually does come for a roof replacement, algae resistance can be worth asking about - especially in a warm, humid climate like ours.
Don't Judge a Roof by Its Color
A roof covered in black streaks can look like it's in terrible condition from the ground.
Sometimes it isn't.
The opposite can also be true: a roof can look perfectly fine from the driveway while having issues that aren't visible from below.
That's why it's important to separate appearance from condition.
If you're seeing dark streaks and aren't sure whether you're looking at algae, normal aging, or something that deserves a closer look, Beemer KangaRoof can inspect the roof and help you understand what's actually happening.
