If you have black, unsightly stains on your asphlat roof, the most likely cuase is algae feeding on the limestone within your asphalt shingles. While there several ways to prevent further growth, and many companies that provide roof cleaning, it is a project you can do yourself, of course taking all the precautionary measures required when working at heights, on potentially slippery surfaces. In order to remove or limit future algae growth you will need to remove the existing algae stains. There are a number of commercial cleaners available, but oxygen bleach is just as effective, and not as hard on your lawn, shrubs and the environment in general.
Watch the weather closely and try to wait for a dry, overcast day. This cleaning solution works best out of direct sun, but you also need a dry spell so the cleaning solution isn’t just washed away!
Using a pump type sprayer (a backpack style unit is ideal as it will free you both your hands for other things, like holding onto the ladder, keeping your balance on the roof, etc). Mix up the oxygen bleach in the reservoir of the pump sprayer. Check the manufacturer’s instructions for the correct dilution rates as each product may recommend slightly different proportions. Wear your safety goggles and gloves when you are mixing the solution.
Spray down the roof with the oxygen bleach solution. It is best to spray the entire roof so that you do not have lighter coloured patches where the bleach was sprayed and darker areas where it was not. Allow this treatment sit for 20 minutes to half an hour, but if the day is warm or windy enough that the solution may dry, then you will need to re-apply the solution.
Agitate the roof gently with the stiff broom using caution not to scrub so hard as to remove granules from the shingles themselves. The bleach will kill the algae and lighten the stains and using the broom, you will be able to scrape existing algae off and remove the stains as well. Work in a systematic manner from the peak down to the gutter. Use caution for the last section you are cleaning as you will need to stand on the area you have just scrubbed and it may be slippery! Use a safety harness over the peak and secured properly.
Rinse off the roof thoroughly using the garden hose using a gentle sprinkle of water starting at the peak and working toward the gutter. Never use a pressure washer, or even a focused stream of water from a pressure type nozzle. This will either dislodge excessive granules from the shingles or force its way under the shingles! If it doesn’t actually cause your roof to leak it will reduce the lifespan of regular asphalt shingles each time it is done! Also use caution to not apply such significant amounts of water that it races over the gutter and into your yard. This just adds another step to the clean up process. The rinse will dilute the bleach and remove the algae so that you can determine if you need to repeat the treatment or just let the roof dry.
Use caution and remember that if you don’t feel up to performing this job yourself there are many contractors that professionally do this kind of work. Get references and ask to see before and after pictures – it CAN make a huge difference in the appearance of your home!