How to Tell the Difference Between Algae and Mold on Roofs

Bry'Ana Arvie
Written by Bry'Ana Arvie
Updated December 29, 2021
A house exterior with brick and siding trim
Photo: Michael Shake / Adobe Stock


  • Mold is a health hazard, while algae is a cosmetic issue.

  • Algae grow in areas with access to sunlight and moisture. 

  • Mold thrives in humid areas with access to wood.

  • Remove them both with a soft-wash and bleach and water solution.

  • A soft-wash for a roof costs $0.20 to $0.60 per square foot.

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Ever looked at your roof when coming home and thought those black streaks were mold? If you did, you’re not alone. It turns out it’s the first impression for many homeowners. They frequently confuse roof algae and mold, but while they’re both an eyesore, they’re not the same. 

This guide will explain how you can spot the difference between algae and mold on your roof, how to remove and prevent them, and when you should call a pro. 

How to Tell If It’s Algae or Mold on My Roof

Here are some ways to tell if what you see on your roof is algae or mold. 


Roof algae spores travel through the air and land on your roof. While you can find roof algae in most climates, you can mainly find them in humid environments where they have easy access to the sun and moisture. 

They’re able to continue growing and spreading, thanks to the limestone filler in your asphalt shingles since limestone is soluble, alkaline, with a pH environment encouraging algae growth. The dark color you see on the roof is the black residue after the algae eat the filling. 

You’ll typically find algae on the north side of your roof since they’re shaded by the sun for the majority of the day, allowing moisture for growth. Leaking and clogged gutters and overhanging trees also provide the perfect condition for algae to grow.


A moldy roof happens because of an internal problem, typically because of an attic with poor ventilation, which allows heat and moisture to create a damp environment for mold growth. So, to tell what that funky-looking stain on your roof is, check for black mold in your home—specifically the attic—by searching for any wood rot, discolored insulation, condensation, damaged roof, and musty odors. 

Another sign to look for is the appearance of mold itself. If it’s a slimy fungus that looks like black, brown, or green blotches, mold might be your culprit.

Algae vs. Mold: Which Is Worse?

A close up of roof with moss
Photo: joef / Adobe Stock

Roof algae, also known as Gloeocapsa magma, is an airborne, moss-like plant with a blue-green hue. While it’s without a doubt unattractive, it doesn’t pose a health risk. On the other hand, mold is far more problematic. According to the U.S. Environmental Protection Agency, mold exposure can irritate the eyes, skin, nose, throat, and lungs. 

How to Remove Algae and Mold

Even if algae don’t pose a risk to your health, they can eventually damage your roof if left alone. Plus, both algae and mold aren’t helping your curb appeal. Of course, you’ll have to fix any internal problems causing the roof mold growth so it doesn’t return, but one method to clean roof stains from mold and algae is with a bleach and water solution. 

Here are the steps below:

  • Step 1: Protect yourself with safety goggles, a respiratory mask, and heavy-duty rubber gloves. 

  • Step 2: Protect the environment around your home with drop cloths, or spray the surrounding environment with water as a barrier from the solution. 

  • Step 3: Mix 1 part bleach, 3 parts water, and 1 tablespoon of trisodium phosphate (you can find it at a local home improvement store) in a bucket.

  • Step 4: Place the solution in a pump sprayer.

  • Step 5: Cover all the affected areas of your roof thoroughly. 

  • Step 6: Allow 15–30 minutes for the solution to penetrate the algae or mold. 

  • Step 7: Rinse the solution off with a soft-spray pressure washer.

  • Step 8: Repeat steps four through seven if any algae or mold is left.

How to Prevent Algae and Mold on Roofs

A man cleaning gutter blocked with autumn leaves
Photo: Alena / Adobe Stock

Once you’ve removed algae and mold from your roof, prevention is key to keep it looking spick and span. Here are a few ways you can prevent future mold growth and algae growth (or at least catch it in the process):

  • Get your roof pressure-washed annually.

  • Keep your gutters clean and leak-free.

  • Schedule an annual or bi-annual roof inspection, depending on your location.

  • Trim overhanging tree branches.

  • Install ridge vents.

  • Get a mold inspection.

  • Change your roofing material to include algae-resistant roof shingles made of copper or zinc.

When to Call a Pro

Call a roofing contractor near you if removing algae or mold isn’t on your DIY project list or you’re not comfortable with the task. They’ll know how to clean your roof, see if you have any roof damage (and fix it), and help you develop an action plan to avoid this problem in the future. Getting your roof soft-washed costs $0.20 to $0.60 per square foot.

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