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Black Streaks on Roof (Mold)

I have black streaks of mold on the shingles on the roof of my house. How can I clean it?

By Susan from TX

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September 8, 20100 found this helpful

I have been in the roof cleaning business for years now and I can tell you that Sodium Hypochlorite (Chlorine, bleach) is safe for roof washing. The Asphalt Roofing Manufactures Association (ARMA) recommends a solution that contains bleach to remove the black streaks. It will not harm your roof. I have videos and before & after pictures on my web site as evidence.

When looking to hire someone to wash your roof, ask them these questions:

1) Do they have the proper insurance

2) What process will they be using? (soft wash or low pressure washing is the only way to apply the proper mixture to the roof. High pressure will not only void warranties but it will damage the shingles)

3) How long will it take before the black streaks are gone? (depending on the severity of the algae infestation, it should not take more than 1 hour)

4) How long before the algae returns? (the algae needs three things to survive, moisture, shade and a food source. If you have overhanging trees or tall building nearby, it will return sooner than later. Two to four years is average with many lasting longer.)

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Anonymous Flag
March 17, 20110 found this helpful

A few of the response have the answers right ARMA is the shingle manufacturer recommendations is correct. JiminNJ is right on target with his response. Here is what it looks like up close just to get a better idea.

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August 19, 2010 Flag
0 found this helpful

I have black streaks on the roof of my house, I understand this is some type of mold. I would like to know if anyone has any ideas on how to safely remove these black streaks? I was told that you shouldn't use bleach, a powerwasher or anything involving scrubbing the shingles since all of these solutions will ruin my shingles.


Debbie from NJ


Black Streaks on Roof (Mold)

My husband and I use a garden sprayer with Greased Lightning and bleach and then pressure wash our house and this usually brings off all the mold and any other stains that are on there. (01/13/2005)

By Shannon in NC

Black Streaks on Roof (Mold)

Having lived in the Pacific Northwest where roofs tend to accumulate mold, I learned that there is a zinc strip which can be applied to the roof to prevent this. Of course this will be after you are able to get rid of the mold which is already there. I suggest phoning a roofer for both instances. Good luck! (01/13/2005)

By Enter Marian

Black Streaks on Roof (Mold)

I have used Tide powder laundry detergent. Mix one cup per gallon of water and pour it from the roof peaks. Do not rinse, the rain will eventually wash it out and the soap powder kills the mold. (01/13/2005)

By alobarbear

Black Streaks on Roof (Mold)

Yes, bleach will kill the mold, however there are 2 problems with it.

  1. Bleach that drains on your ground around the house can and probably will neutralize you termite protection and if your exterminator finds out will void your termite contract.

  2. If you use something (hopefully clear) to fill the areas where the mold "was" it will come back even stronger than before. (02/17/2005)

By Dennis

Roof Cleaners

There is a category of cleaners designed for your roof. Too many household cleaners can damage the roof (such as bleach). And you should never pressure wash a roof. The granules protect the roof, pressure washing destroys them.

We use Cleaner Today ( Roof QSE and Roof OX for stained, streaked algae covered roofs.

The cleaners are safe for the roof, and do a great job cleaning.


By Scott

Black Streaks on Roof (Mold)

The AMRA (American Manufactures Roofing Association) and GAF (the largest maker of shingles) recommends a mixture of bleach, TSP, and soap. Bleach is perfectly safe to use, unless you are trying to sell a product. Most companies that are trying to "scare" us not to use bleach are simply pushing their products. It's basic marketing. The products they are selling contain sodium hydroxide. For those of you who don't know, SH is a caustic lye and a "very" powerful degreaser. Oh yea, I should mention that your shingles hold the granules on with tar. Tar is a oil "grease" product.

I own a professional exterior cleaning business and use bleach safely and with instant success. Of course care should be used. You would never just spray bleach on your plants, take the time to water them before, during and after, and you will have no problems. The ions in the sodium hypoclorite begin to break down upon contact with an organic material (mold/fungus). I'm not pitching a product, I just want you to be well informed. (12/21/2007)

By Michael

RE: Black Streaks on Roof (Mold)

RE: Black Streaks on Roof (Mold)

Black Streaks on Roof (Mold)

Chlorine helps too. Spray it on, leave it on, and then let it rain on it, no rinsing. Use 1cup of chlorine in your garden sprayer with 1 gallon of water! (06/29/2009)

By lilkat1

Black Streaks on Roof (Mold)

It is not mold it is a cynabacteria (blue green algae) called gloeocapsa magma. It turns black from the sun killing a top layer of it that then becomes a protective shield helping it grow and spread down the roof with rain water. There is tons of info on this on the web and yes GAF and ARMA both say to use bleach. Oxy cleaners and sodium hypochlorite are both bleach. Bleach is a term for anything that can lighten color, (just like the term sun bleached). I contacted some AMRA members who assured me that they meant household bleach or a stronger equivalent which is sodium hypochlorite. The reason for this is that the sodium hypo will kill the algae spores were oxy cleaner bleaches will not. No manufacturer wants you to pressure wash a roof, so avoid that as well. (02/07/2010)

By howtoguy

Black Streaks on Roof (Mold)

The black streaks are definitely mold forming and will cause permanent damage if left alone. There is a local New Jersey company, Sonshine Window Washing, that specializes in roof cleaning. The chemical is 100% biodegradable and safe, because it is a live enzyme. It uses the rain to naturally rinse the mold away after application. Here is their number 609 261 2765 (03/05/2010)


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In This Article
Removing Moss from a Roof, Cleaning Moss off Roof
Removing Moss from a Roof
Home and Garden Cleaning MiscellaneousAugust 19, 2010
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