Reuben's Home Inspection Blog


Removing Black Stains on Roofs

If you have nasty black stains on your asphalt shingle roof, don't worry.  The black staining won't damage the shingles, and it won't make your roof leak.  This black staining is typically caused by an algae, and usually shows up on areas of the roof that don't get much sunlight, such as the north side.

Black roof stains

Garden SprayerIf you don't like the look of these stains, you can clean your roof with some basic supplies that you probably already have at home.  Simply mix up a solution of two parts water to one part bleach in a garden sprayer.  Spray down the affected areas of your roof with this solution, let the solution soak for about ten minutes, and then  spray the cleaning solution off your roof with a garden hose.

For this blog, I started by cleaning a small section of the roof and letting the shingles dry, just to get a good photo showing how effective this is.

Black roof stains cleaned

A few precautions:

  • Protect your lungs.  Bleach is nasty stuff, and you'll get it in the air when you spray it with the garden sprayer.  Wear a respirator.
  • Watch where you spray the cleaning solution.  The solution won't harm your shingles, but you could certainly do damage to other surfaces or people if you're not careful.
  • Wear grubby clothes.  If you get this solution on your clothes, you'll ruin 'em.
  • Protect your vegetation.  If the cleaning solution will run off the roof on to your plants, you should cover them up with tarps or something similar.
  • Be careful on a ladder, and don't walk on the roof.  This is 'no-duh' advice, but I'm saying it anyways.  Even if you're used to walking on your roof, it will get very slippery once the bleach interacts with the algae, and you could easily slide off the roof.
Some people get whipped up over the topic of bleach.  If you don't like the idea of using bleach, you could buy a product made for cleaning roofs, such as RoofOX 3000.  I've heard good reviews, but never tried this stuff personally.

Reuben Saltzman, Structure Tech Home Inspections


Comment balloon 19 commentsReuben Saltzman • November 01 2011 06:19AM


Reuben: This is great advice for do-it-yourselfers. In Bucks County, we have companies that will do this for several hundred dollars, depending on the size of the roof. I have always been concerned about damage to the roof and you have allayed my fears. Cleaning the roof does make it more saleable. Have a great day!

Posted by Anne M. Costello (Weidel Realtors) over 7 years ago

Anne - thanks.  I know there are companies here in Minnesota that offer this service, but I've never heard of someone actually having it done.

Posted by Reuben Saltzman, Delivering the Unbiased Truth. (Structure Tech Home Inspections) over 7 years ago

Hi Reuben.   I appreciate the info.  Those black stains can create doubt for some buyers and this is a good way to handle that objection before it arrives!

Posted by Carol-Ann Palmieri, "Cal" the Real Estate Gal (RE/MAX Executive Realty, Al and Cal Realty Group) over 7 years ago

Love it!  My recipe is virtually the same as yours with the additon of tri-sodium phosphate.

I found that it takes the algae about two years to come back.

However, the last time I used it my grass took a hit for a year!

Posted by Jay Markanich, Home Inspector - servicing all Northern Virginia (Jay Markanich Real Estate Inspections, LLC) over 7 years ago

Reuben....every time I read your blog I learn something new.  As Carol Ann mentioned above, it is good information for buyers.  Also good for Sellers who may be preparing a home for sale.

Posted by Christine Smith, Exclusive Buyer Agent & Attorney, Canton, MA (Buyers Brokers Only LLC - over 7 years ago

That looks like a dryer vent through the roof.  Is that what it is?  This practice really, really dismays me.  You saw my blog on it, I'm sure.

And that looks like a really high cricket behind your chimney!  Do you get a lot of snow there...?

Posted by Jay Markanich, Home Inspector - servicing all Northern Virginia (Jay Markanich Real Estate Inspections, LLC) over 7 years ago

Oh this is great advice. There are so many black roofs out there and it makes the home look so bad. We had this issue in our other home but then we had a huge storm and we got a new roof. ;)

Posted by Shar Sitter, Home Staging and Redesign Minneapolis/ St. Paul, M (Rooms With Style) over 7 years ago

Reuben, Dang you missed out on a opportunity to use the old "Pressure Washer" on the roof and blowing the shingles apart, but you can write you name in the shingles ; )

Great advice though for roof maintenance. I really do not see this very often in my neck of the woods but you go west a hundred miles or so and it is everywhere (along with the algae cousin Moss).

Posted by Donald Hester, NCW Home Inspections, LLC (NCW Home Inspections, LLC) over 7 years ago

Reuben, That's great advice! I've always heard that it was algae that caused the darkening of the roof, but never realized how easy it was to clean.

Posted by Michael S. Bolton, MN Appraiser (Michael S. Bolton,Inc.) over 7 years ago

Boy that roof cleaning solution ain't cheap! Bleach is a great cleaner, but you for sure need to be careful. 

Posted by James Quarello, Connecticut Home Inspector (JRV Home Inspection Services, LLC) over 7 years ago

We can prevent the need for cleaning

Posted by John over 7 years ago

Reuben, great post.  I have seen this discoloration, it normally is much easier to see on light coloured roofs.  I never knew it was so easy to remove.

Posted by Chris Smith, South Simcoe, Caledon, King, Orangeville Real Esta (Re/Max Chay Realty Inc., Brokerage) over 7 years ago

Carol Ann - you're right - a lot of my clients ask me about those stains too.

Jay - I remember that blog!  That's actually the first floor bathroom exhaust fan that comes through the roof.  The other thing you're referring to is just part of the wall; it's a nasty detail though.  Not much snow accumulation, but I definitely get ice there.  Good eye :-)

Christine - thanks!  If I were going to sell my house, I would definitely clean the roof before putting my house on the market if it had those black stains.

Shar - I'm still waiting for a good hail storm to hit my roof...

Donald - you're not kidding about the pressure washer.  Those things can really tear a roof up.

Posted by Reuben Saltzman, Delivering the Unbiased Truth. (Structure Tech Home Inspections) over 7 years ago

Michael - I've also heard it might be caused by 'bleed-throug', but whatever the case, it cleans up the same way.  Bleach is powerful stuff!

James - can you believe what they want for that stuff?  You're not kidding about the bleach either - I made sure not to pump up the garden sprayer until I was ready to use it.  My three-year-old son headed straight for the garden sprayer when I set it down on the sidewalk.

John - excellent.

Chris - yeah, that black staining looks really bad on white roofs, doesn't it?


Posted by Reuben Saltzman, Delivering the Unbiased Truth. (Structure Tech Home Inspections) over 7 years ago

Great information.  I have cleaned several clients roofs for them just in the last couple of years.  Makes a difference.

Posted by William Feela, Realtor, Whispering Pines Realty 651-674-5999 No. (WHISPERING PINES REALTY) over 7 years ago

William - you clean your clients roofs?  Wow!

Posted by Reuben Saltzman, Delivering the Unbiased Truth. (Structure Tech Home Inspections) over 7 years ago

Sounds fairly simple..thanks for the tip.

Posted by Ronald DiLalla, No. Orange Cty Real Estate (Century 21 Discovery DRE 01813824) over 7 years ago

I use the same recipe for cleaning the black off the roof shingles. Make sure its not a windy day. Wear long sleeves and long pants and glasses to protect your eyes.

Posted by Carl Winters over 7 years ago

Amazing, Reuben - I see black roofs all the time and they look awful.  This is a great solution, although I do try to avoid bleach if at all possible.

Posted by Peggy Chirico, REALTOR® 860-748-8900, Hartford & Tolland County Real Estate (Prudential CT Realty) over 7 years ago