How To Clean Asphalt Shingles

Illustration of two garden pump-style sprayers

If you’ve ever noticed dark streaks or areas on roofs around your neighborhood, you know it isn’t a good look. Luckily, removing and preventing those stains is a fairly simple process (once you get yourself up on your roof, that is).


What Are Those Stains On My Shingles, Anyway?

The dark stains on your roof are a form of algae called Gloeocapsa magma, and it is actually blue-green in color, though it looks black on your shingles. Like mold, this algae needs a lot of moisture to spread, and roofs are a great place to find a steady water supply.

North-facing roofs and areas with shade are the most common places to find these algae. The lower level of sun exposure means that those areas stay wet for a longer period, which allows the algae to gain a foothold on your shingles.


How Do I Remove These Shingle Stains?

Luckily, algae have nowhere near the tenacity of mold and are easily removed with a 50/50 solution of laundry bleach and water. There’s no need to spend your hard-earned money on purpose-made bottles of harsh chemicals. Sure, they’ll kill the algae, but they also hurt your roof.

In fact, you don’t even have to scrub the roof unless you have moss. Just follow these simple steps to get your roof looking great once more:

  1. Pretreat all the vegetation around your home with water. This will help protect your plants from the cleaning solution you are about to make.
  2. Create a batch of 50% laundry bleach and 50% water and put it into a standard garden pump-style sprayer. Make enough to treat your entire roof, not just the areas that have darkened.
  3. Get to your roof, using all the proper safety protocols (ladder, harness, etc.)
  4. Work your way across the entire roof surface while spreading the bleach solution with the sprayer. Make sure to always leave yourself dry areas to walk on.
  5. If you have moss, use a soft-bristled brush to gently scrub it away as you go along while being careful not to scrub away the granules on your shingles.
  6. Let the solution sit for about 20 minutes.
  7. Take a standard garden hose and clear the solution from your roof, using a top-down method. DO NOT USE A POWER WASHER! Power washers can damage your shingles by removing the protective granules.
  8. Make sure to give your plants a thorough rinsing and soak the ground around them to help dilute the bleach solution once you are finished cleaning your roof.

That’s it. Your roof is now clean, and the algae are dead and gone.


How Do I Prevent Shingle Stains In The Future?

While looking at your neighborhood’s darkened roofs, you might have noticed that the areas directly below any metal flashing were free of algae stains. That’s because the galvanized steel and copper in the flashing is deadly to algae, and rain washes enough particles of the metal onto the roof to kill the algae.

You can take advantage of this fact and install zinc panels underneath the topmost shingles on your roof. Leave two inches of the metal exposed, and it will release molecules of the metal every time it rains and kill any algae on your roof.

If you are ready to replace your roof and are in a high humidity area, you may want to use copper-infused shingles. The traces of copper in the granules will kill any algae that visit your roof.

If you have any concerns about the roof on your New Hampshire, Massachusetts, or Vermont home, contact us at Jancewicz & Son for a free estimate.

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