Many people assume that unsightly buildups on their roof are caused by mold or mildew. More often than not, however, they are actually caused by a type of algae growth. The reason this algae growth on roofs is so common is that most shingles contain a type of liquid asphalt made from limestone, and that limestone is a perfect food for this type of algae. Fortunately, although the algae buildup is definitely not attractive, it will not actually damage your roof. Even better, it is not particularly difficult to remove and prevent if you know the right method to use.
Blasting the algae off the roof with a pressure washer is the quickest and easiest fix; however, it is also one that is likely to damage the roof, which is why using a pressure washer voids most warranties on shingles. If you decide to use a pressure washer anyway, make sure you test it out on a loose shingle or two first; hopefully you will be able to find a pressure setting that is strong enough to remove the algae, but not so strong that it removes the shingles themselves.
Scrubbing the roof may not be a glamorous solution to your algae problem, but it is usually the most effective. Most of the time all you will need is regular dish soap and water, but you might need to use bleach instead of dish soap for especially heavy buildup. If you do need to use a bleach solution, use oxygen bleach if possible; unlike chlorine bleach, it will not harm any plants near the house if it drips on them.
If your roof is accessible and not excessively steep, you may be able to climb on top of it and clean it with an ordinary scrub brush, as long as you are careful not to fall off. If this is not an option, however, you will need to purchase a specially designed brush with an extendable handle that will allow you to clean the roof while standing on a ladder leaning against the house. Of course, you will only be able to clean a relatively narrow strip of the roof before you will need to reposition the ladder, but thankfully each strip will not take very long.
Conditions and Preparation
You should avoid cleaning algae off your roof on a sunny day, since the roof will dry faster than you can scrub it. Of course, a rainy day will wash away the cleaner too quickly, and will make climbing ladders and walking around on roofs quite a bit more dangerous. Ideally, you should wait for a cloudy day with moderate temperatures to deal with your roof algae problem. Whatever day you choose to get the job done, make sure you clean from the top of the roof toward the bottom to make sure you always have a dry surface on which to stand.
Thankfully, removing algae buildup from a roof is a fairly straightforward process. Of course, that does not mean that preventing the problem from arising in the first place is still not preferable. The easiest option is to place a copper crown on top of your roof. This crown will then allow trace amounts of copper to run over the roof whenever it rains, which prevents algae growth. Should you need to replace your roof entirely, you should have the contractor use copper-laced shingles, which will accomplish the same purpose. But, as was mentioned earlier, these precautions are simply for cosmetic reasons; the algae buildup will not actually cause any damage to a roof on its own.
Today’s Homeowner: “How to Remove and Prevent Black Algae Stains on Asphalt Shingle Roofs”
This Old House: “Getting Algae and Moss Off the Roof”
Ask The Builder: “Removing Moss and Algae From Roofs”
Chicago Tribune: “How to Remove Algae From Shingles and Keep It Away”
Washington Post: “Safely Remove Algae From Roof Shingles”
What other advice do you have for removing or preventing algae buildup on a roof?
Mike Teddleton from Midwest USA on April 06, 2015:
Thanks for the roof cleaning tips.