You should never apply chlorine bleach to a roof in its full strength. Chemical bleach is an extremely corrosive chemical compound and requires heavy dilution for safe application. This Old House recommends that you mix 50 percent bleach and 50 percent water in a spray bottle or garden sprayer. Do not put bleach into a power washer under any circumstances. It may damage the power washer as well as your roof shingles.
Spray the bleach solution over the moss of your roof, as well as over any other dirty areas. Scrub with a firm brush to loosen the moss, algae and dirt particles. After removing all of the moss, rinse your roof to remove the bleach. Do not bleach your roof as a routine procedure, but reserve it only for emergencies, in order to maintain the color and longevity of your roof shingles. After rinsing, allow your roof to air dry.
If you object to using chlorine bleach, due to personal sensitivities, environmental concerns or worries about the corrosiveness of the compound, you may use oxygen bleach instead. Home improvement expert Tim Carter recommends that you spray oxygen bleach onto your roof, wait 20 minutes and then scrub. Rinse the bleach away and allow the roof to air dry. Oxygen bleach does not contain the harsh chemicals found in chlorine bleach, and is eco-friendly.
If using chlorine bleach to remove moss from your roof, first water down any nearby plants. This will prevent any bleach from destroying them as it drips from the roof. Wear rubber gloves and a breathing mask when handling chlorine bleach. If you have any concerns at all about the use of chlorine bleach, try the oxygen bleach alternative.