Roofing receives constant exposure to the elements during all 4 seasons. Discoloration, mold and algae can quickly grow and consume parts of the roof, eventually causing damage. It is vital to clean mold sooner than later to prevent the problem from growing worse. There are more correct and less correct ways to manage mold growth with different cost depending on the method.
The distinction between algae and mold is an important one. Algae has a tendency to grow on the asphalt material from which singles are made. While mold thrives on debris, branches, leaves and moisture from which is foreign to the roof. While it can be difficult to prevent all together there are some actions which can minimize the risk. Keeping a roof clean of debris and accumulating leaves is one of the best ways. Where corners of the roof come together and along the gutters are the most common mold locations, but also where there is any form of standing water.
One of the easiest treatments for both algae and mold is a combination of water and chlorine based bleach. They can assist in removing any discoloration which is already there, with little scrubbing or special products. Ideally it should be allowed to sit for about 5 minutes to be most effective. There is risk of deterioration or damage if concentrated bleach is on for too long on the shingles. There are additional risks of using harsh chemicals like bleach which occur when it is rinsed off. It is a toxic substance which can damage flowers, plants and soil that lies beneath the roof or in proximity to it.
There are commercial products as well which are specially made to treat moldy roofing. These have various applications which can be applied using spray bottles or special attachments that go on the end of the hose. These should be used similarly to the bleach combination where they are allowed to let sit, then rinsed off. Some of these products advertise the ability to prevent future growth when used for an extended time.
For further long-term prevention there are zinc and copper metal strips which are installed at the top of the infected areas. They are easy to install with simple nails. They work on their own by shedding microscopic particles of the metals which are repellents to algae and mold. Each time It rains it will clean and prevent further growth in the areas installed and below.
When the time comes to replace shingles all together a final option might be to install mold resistant shingles. Similar to the metal strips mentioned above they used fine particles of metal mixed in with the normal materials of the shingles to prevent growth from day one. A proper roofing profession would be able to advise on which ones are on the market and appropriate for your roof. This would be one of the best options for a stress free good looking roof.