Dark discolorations on asphalt shingles are common, and they can appear in blotches or in streaks. That’s algae, a particular kind of algae that spreads through airborne spores and often takes hold on shingles because of the moisture and calcium found there. Such algae won’t compromise your shingles physically, but it does ruin curbside appeal and often require the attention of a roofing professional.
Professional Shingle Cleaning
Most roofing companies do provide a shingle cleaning service. Many even offer regular cleaning plans or include it as part of seasonal maintenance packages. If your roof is dulled by algae, it doesn’t necessarily require professional attention, however. This is a job that many homeowners can handle themselves, but if the growth has been rapid or you’ve procrastinated, then calling in the pros is probably in order.
So if you do decide to do it yourself, let’s look at how you should go about it. You may be surprised to learn that you don’t need a lot of expensive equipment and supplies. Generally, a mixture of tap water, oxygen bleach and a household detergent is good enough. If the growth is particularly sharp, then you can mix in an algaecide as well. Mix it all in a garden sprayer. Apply to the affected areas. Wait several hours, and then, rinse with a hose. Note that professional roofing contractors generally prefer oxygen bleach over chlorine bleach because it doesn’t harm plants or pose a danger to animals.
Never Use a Pressure Washer
Never use a power washer yourself or hire a professional who wants to power wash your roof. Even at low settings, pressure washing is too powerful and will strip away the granules that protect the asphalt. Once those granules are gone, shingles can degrade quickly.
If your shingles seem prone to this type of discoloration, then you can pre-treat the shingles with the same mixture outlined above. The best time to treat your shingles is when it’s reasonably cool out and shingle aren’t in direct sunlight. If heat and sunlight are unavoidable, covering the treatment in plastic is an option.
Note Your Neighbors’ Homes
Algae on shingles is generally a neighborhood-wide problem. If one home has it, others will as well. If you have a neighbor who has these stains and does nothing to eliminate them, then the algae will likely be a recurring problem for you because that colony will continue to generate new spores.
More Permanent Solutions
An alternative to regular treatment is to hire your roofing company to install flashing that’s made of zinc, copper or a similar metal at the ridgeline. When it rains, the rainwater reacts with this metal to create a natural algaecide that then cascades over your shingles and cleans them.