When it comes to moss or mold on a roof, the culprit is actually a blue-green algae which is most commonly found in warmer, humid climates. Although it does not damage the roofing the black stains it leaves do not look good.
One solution if there are only a few shingles involved, would be to replace the discolored ones with new shingles to match. This means that you would either have to have some extra shingles lying around or that you could find them at a local supplier. However, if you shingles are pretty old, this may not be possible.
Your options then would be to try to find the closest match available or take shingles from another area of the home that where the shingles are less visible and use the old shingles in the spots that you want to replace the bad ones and then install the new shingles that don't match as well in the spots where you took the old shingles from. Please see my earlier response to the topic "Shingles" for more on this procedure.
Of course, this problem is going to show up more on lighter shingles, so replacing them might be your best option. However there is a method of cleaning that might just do the trick.
One of the suggestions out there that has been used for years is to get a garden sprayer and mix a 50/50 solution of water and bleach and spray wash the roof to get rid of the algae. Keep in mind that a garden sprayer is not a pressure washer. Do not use a pressure washer because you can blow the surface of the shingles right off the roof and thus damage the shingles.
Due to the very delicate bond between the composition shingle and its protective layer of marble chip granules, the use of a pressure washer is NOT RECOMMENDED AT ALL. If one insists on the use of a pressure washer, the spray tip nozzle MUST BE KEPT far enough away from the shingles so that the pressure does not exceed that of an ordinary garden hose.
Once even a small percentage of the granules are gone from the shingles, the effect of direct sunlight will accelerate the degradation of the underlying shingle and destruction of the shingle will occur long before the normal life expectancy of the shingle
A better solution would be to use one of the deck cleaners out there that are safe for the environment. Products containing oxygen bleach clean well and will not harm the plants. Anytime you can use an product that is good for the environment, the better off you are.
Input from a roof cleaning professional:
There are a few schools of thought on how to properly clean a roof. Based upon my research, years in business and tried and true method, nothing is more effective than using sodium hypochlorite, more commonly known as bleach. With this chemical (applied at approximately a 6% solution with other surfactants) you do not have to rinse the roof at all. This mitigates all danger of damage from the use of pressure. Any pressure is too much. The difference will be instantaneous. After the first good rain, dead lichen and any remaining haze will be washed away leaving your roof looking nearly new.
Here is a common sense thought to add. Hire a professional. I am all for DIY and saving money but sodium hypochlorite will burn your skin and it can wreak havoc on lungs. It will also kill plant life and surrounding vegetation on contact. Professionals will use protective gear (respirators, safety harnesses, roof protectors on ladders, special climbing shoes, etc) as well as protect your landscaping from damage.
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