Got Black Streaks On Your Roof? Here’s How To Remove Black Algae & Roof Mold

roof-black-streaks.jpgHave you ever noticed how some roofs have black streaks streaming down the asphalt shingles?

Sometimes, you see a green mossy growth on the roof.

Both of those are forms of roof mold or algae.

Here’s why it accumulates in the first place, plus how to remove algae from the roof of your house…


Why Algae Grows On Roofs

Believe it or not, those black streaks only appear on roofs in certain areas of the country. (Those that tend to be the most humid.) Furthermore, they will only appear on the north side of the roof.

The black streaks are also known as black algae or roof mold. Technically, the black streaks on roofs are a form of algae known as Gloeocapsa Magma.

Here’s why algae grows on roofs:

If you live in a humid area of the country, you’ve probably seen unsightly dark streaks on asphalt shingle roofs. Though often attributed to an accumulation of dirt, defective shingles, mold, or mildew; the most common culprit is actually a blue-green algae known as Gloeocapsa Magma that is spread by airborne spores. While algae growth does little harm to roofing, the stains don’t help the overall appearance or resale value of your home.  Source


The algae grow on the north side of the roof that gets the least amount of sun and stays damp the longest.  They feed on limestone filler in the shingles and grow and darken until removed. Airborne algae spores are carried from roof to roof and can infest a whole neighborhood.  Source


How To Remove Algae From Asphalt Shingles

roof-mold-roof-algae-by-ell_brown.jpgTo remove the algae from asphalt shingles, you have a few options.

You could use regular household bleach to remove the algae from your roof.

Spray wash the roof with a 50% mix of water and bleach to get rid of the algae. (No pressure washers, please. They’re likely to damage the shingles.) Just be sure to wet your foundation plantings first, and rinse everything in clean water when you’re done. Plants don’t like bleach, and wetting them with plain water first protects them.  Source


A bleach or chlorine-based solution applied with non-pressure methods will kill lichen and moderate moss growth almost instantly.  It should turn a whitish color within minutes, which tells you that it’s dead.  Unlike black algae stains which come off immediately with the rinse phase of the cleaning, dead lichen and moss will require a few heavy rains to finish rinsing off the roof.  It could take awhile but rest assured that lichen and most moss situations, once dead, will eventually dislodge and rinse off the roof thanks to mother nature.   Source


Some people prefer not to use chlorine-based products on their asphalt roofs. 

The fear is that the bleach will damage the asphalt shingles (due to its high sodium content).  The use of bleach on asphalt shingles could reduce the elasticity of the asphalt — resulting in stiff, brittle shingles. They may curl eventually as well. Source


Instead of straight household bleach, you could use oxygen bleach.

The way to clean off roof algae is to go ahead and mix up some oxygen bleach with water. It is a powder. It is safe. It does not take the color out of the shingles and it does not hurt the shingles or vegetation or anything else near your home. What you do is put it into a pump sprayer, spray it on the roof and you need to keep the roof wet with the solution for about 10 or 15 minutes. The oxygen bleach starts to go ahead and attack the roof algae and it does its best to kill it and take away the stains. Take a scrub brush. Lightly scrub the shingles. Apply more solution if you have to, to get a nice workable solution. And then once the shingles are cleaned, you go ahead and take a hose and just rinse the roof off, it is really that simple. Okay, this roof is going to need as second application, maybe even a third when it is extra dirty.  Source


homeowners-guide-to-mold-book.jpg Finally, there are lots of special products made just for removing algae from asphalt shingles that work really well. Here are a few:


How To Prevent Black Streaks (Roof Mold)

If you’ve already removed the algae from your roof once, and you want to make sure you don’t get it again, or if you’re building a new house and you want to make sure that your roof doesn’t get those black streaks from algae, there are 2 things you can do to prevent the algae from forming:

If exposed to sunlight, the ultraviolet rays will readily break down asphalt shingles. That’s why the shingles you see are covered with the small colored ceramic granules. These create a powerful sunscreen and offer you different colors to match your exterior decorating scheme. Many shingle manufacturers make shingles that have special colored granules that contain copper. These help to prevent the ugly black roof-algae stains you might see on your existing roof or that of a neighbor.  Source



To keep the algae from coming back, insert 6-inch-wide strips of zinc or copper under the row of shingling closest to the roof peak, leaving an inch or two of the lower edge exposed to the weather. That way whenever it rains, some of the metal molecules will wash down the roof and kill any algae trying to regain a foothold on your shingles. Source



Lynnette Walczak

Lynnette Walczak

I like to help people find unique ways to do things in order to save time & money -- so I frequently write about "outside the box" ideas that most wouldn't think of. As a lifelong dog owner, I often share my best tips for living with and training dogs. I worked in Higher Ed several years until switching gears to pursue things I was more passionate about. I've worked at a vet, in a photo lab, and at a zoo -- to name a few. I enjoy the outdoors via bicycle, motorcycle, Jeep, or RV. You can always find me at the corner of Good News & Fun Times as publisher of The Fun Times Guide (32 fun websites).

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Fun From Around the Web

  • Ponchoaurelio

    if you do what he’s telling you your gonna take forever, just low pressure wash with nothing but water, if you buy that cleaner he;s talking about your gonna spend a fortune. in S. Texas a 3000 sq. ft. home can be cleaned for 300 dollars and look like new. believe me i’ve cleaned my home twice in 10 years and the granules are in place. you just have to know how to do it. if you wanna do it your self and save money, yet risk slipping and hurting yourself, you can use a #40 tip and it will work as good as the profesionals. the web helps in a lot of things but it can also confuse you because all the opinions, also the guys who try and sell you the roof reviver oxygen cleaner are full of baloney, it’s just regular oxy clean which you can buy at any home depot or lowes for 10 bucks. believe me i’ve been cleaning roofs for over 10 years. they do it on purpose so you can buy their product.

  • Nuri Rossignol

    Ugh, that black goop can be really gross. You really have a lot of work to do with the roofing, so it’s best to know what you’re doing. Otherwise keeping the goop there isn’t going to do your roof any favors.