According to Tim Carter, author of the nationally syndicated column, Ask the Builder, bleach will kill the moss growing on your roof. Chlorine bleach will do the job, but it can damage or discolor certain types of roofing, such as wooden shingles. Oxygen bleach, however, will kill the moss without harming your roof.
Causes
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Moss, fungus, and algae flourish on rooftops in areas of the country that receive high rainfall and have high humidity. But moss can grow in other areas as well. If tall trees keep your roof in almost constant shade, or if part of your roof faces north, it is not getting enough heat and light from the sun to evaporate morning dew and rainfall quickly. Such damp conditions are a perfect growing environment for moss.
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Risks
Left untreated, moss will shorten the lifespan of wooden shingles and even asphalt shingles. Moss is like a sponge that holds in dampness, rotting wood and corroding asphalt. In extreme cases, the moisture may seep beneath the shingles or tiles, rotting the wooden structure beneath the roofing.
Chlorine Bleach
Chlorine bleach will kill roof moss, but when you rinse if off the roof, it may kill the plants growing alongside your house, too. If your moss problem is extensive and you plan to use a large quantity of chlorine bleach, bear in mind that it will accelerate the corrosion of your metal gutters and downspouts. If you must use chlorine bleach, prepare a mixture of half bleach and half plain tap water. It will kill the moss while reducing the potential damage to your roof, plants, and gutters.
Oxygen Bleach
Oxygen bleach will not damage your roofing, garden plants, or gutter system, but it requires more effort than chlorine bleach. It must be applied when the roof surface is cool, such as a cloudy day in autumn. You must keep the moss damp with the solution for at least 20 minutes. Using a scrub brush mounted on a pole, gently brush off the moss using a downward stroke. It may require more than one application of oxygen bleach to remove all the roof moss.
Precaution
Working on the steep pitch of a roof is dangerous. To avoid an accident, consider hiring a professional roofer or handyman to do the job for you. If your roofing consists of fragile ceramic or terra cotta tiles, or slate shingles, consult a professional before applying chlorine bleach, oxygen bleach, or any kind of moss killer.
Prevention
Once your roof is free of moss, apply zinc strips to the peak. When it rains, the moistened strips release zinc carbonate, which runs down the roof slope and prevents moss from taking root on your roof.