How-to Avoid Black Streaks on Your Roof

17 June 2020

I have taken many videos and pictures of our work on roofs (and all around the house) to showcase what a difference it makes to pressure- or soft wash any outdoor area. Today I would like to present to you two before-and-after shots of a big roof in Tampa that we recently cleaned to introduce the subject of black-streaking of your roof.


Those black streaks are easily visible from the ground and readily catch s.b.’s eye before they notice anything else about the house. Besides the fact that they are an undeniable eyesore they also ruin the curb appeal of your home and even pose a high risk for the other homes around to get contaminated with the spores of these algae. (More on this later in this article.)


pressure-washing-land-o-lakes-roof-before-cleaning

Why are Your Roof Shingles under Attack?


The greenish-black streaking that you see on the shingles is not just dirt but roof algae that took foothold on the shingles. Against some beliefs, this blue-green algae with the Latin name Gloeocapsa magma is indeed harmful to your roof.


I am not just saying this. It is a fact that these algae feed off an integral part of your roof: The limestone filler and the nutrients trapped in your moisture rich shingles. The crushed limestone, which is used as a filler in roof shingles for support and strength, is a superb source of key nutrients for colony growth. When blue-green algae start spreading on your roof, they also produce a secretion that allows it to stick well to the roof surface - even after cell death. These bacteria prepare grounds for further growth of other organisms like fungi and lichen.



Algae form a symbiotic relationship with fungi. Then create new organisms: Say ‘Hello’ to lichen!


How so? The algae produce organic compound that offers food for many forms of fungi. In doing so, they form a symbiotic relationship with the fungi. Once this is happening, bacteria and fungi intertwine very closely and often even form new organisms called lichen. This is why you find not only black streaking on your roof but also, over time, the growth of lichen.


Blue-green algae grows best in regions with warm, humid weather and thus loves our subtropical Florida climate. The colony growth typically on the northernmost side of houses because of its limited sun exposure. This is where growth and reproduction start. Then it slowly takes over the entire roof surface. But how does it survive the sunbeams beating down when it generally loves more shaded or less exposed areas? Well, you see, its clever bacteria: When sunbeams hit your rooftop, the bacteria protect themselves from ultra-violet rays by forming a black/dark green sheath around itself. This pigmented covering is what we see as black streaking. Over time, when the bacteria die off, this black covering forms a stain which gets increasingly worse over time.


And, to make things even worse, we learned, that blue-green algae colonies are airborne.



Is Algae Growth “Contagious”?


Imagine you live in a gated community or any neighborhood for that matter, and you start seeing black streaks on one roof. You can expect other roofs to get contaminated over time because these bacteria are airborne: To continue their life-cycle, colonies of Gloeocapsa magma dissociate into small groups of cells on your roof and are then spread by the wind or by animals and relocated to a new site where growth and division will now begin.


This means, that you have to handle that one roof quickly, to avoid contaminating all the other roofs. Naturally, the spreading of the bacteria is not visible overnight. It’s happening over time but will become visible sooner than later. A thorough soft wash can stop it from spreading. If you want to know more about the life-cycle of the blue-green algae please check out this page.



Prevention of Algae Growth on my Roof


There are a few proven ways to prevent the spread of the blue-green algae from gaining foothold on your roof. This involves some handyman work or a licensed contractor to do it for you.


  1. If your roof has reached its expiration date and needs replacing you can have the roofing company install new shingles that are dark enough to disguise the inevitable staining.

    - or -

  2. Use shingles laced with copper granules, which are lethal to algae and prevent their growth.

    But both of these solutions only make sense if the shingles were worn out and needed replacement. Most people have roofs that are still intact and thus need a different solution:

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

This solution also prevents moss from growing. Here is a link to a video where general contractor Tom Silva of This Old House shows how to install zinc strips on a roof.


how-to-install-zinc-strips-on-a-roof

Softwashing Roofs Kills Algae, Lichen and Mold


Prevention is of course key but like I mentioned above, once you own a house that has not been prepared with zinc or copper strips on the roof, you are left with handling the black streaking on a regular basis.


Softwashing is the way to go. We offer a careful and thorough cleaning of your roof top. The procedure eliminates algae, lichen, fungi and mold in one fell swoop. Whether it’s for your commercial property or residential home, we work fast, effective and cost-efficient.


We also offer a 15 percent discount for senior citizens living in the Tampa Bay area for all of our services.


Contact us for any type of exterior cleaning. We have successfully cleaned surfaces from countless roofs and driveways, pool cages and patios, to tennis courts, playgrounds, walls and other structures. If it's a hard surface, we can clean it! Just ask our happy customers.


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Please call me at 1-813-841-5024 to get a Free Quote for your roof.