Most home buyers (or inspectors) will judge a roof on appearances. If they see stains or moss they will assume that a new roof is needed now or will be in the near future. They will subtract the roughly $15,000 (for a 2000 sq ft ) roof.
For $35 and 2 hours of work you can remove this impediment from buyers motivation. In other words this is an incredibly high return on your investment. Most realtors tell you a new kitchen will return 80 cents of price vs $1 of investment. A clean roof will give you $428 return per dollar invested. The trick is how to do it safely. Safe for you and safe for your plants
First of all, you should be aware that the black stains you're seeing all over your roof do not consist of dirt, grime, tar, or oil. They actually consist of a hardy type of blue-green algae called Gloeocapsa Magma. If you have any familiarity with algae at all then you know it's a living and feeding organism that only gets worse with time. Ignoring it will not solve the problem. This algae uses the limestone filler in your shingles to feed off of and expand, and eventually your entire roof will be covered with the black stuff unless it is cleaned. Not only are the streaks unsightly, but they will ultimately shorten the lifespan of your roof by hastening the process of granule deterioration.
To learn more about how to clean your roof safely go to http://www.wash-safe-canada.com/