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You are right about considering this for attempting to sell the house and using a fine mist of bleach spray.  This would be a good temporary solution, but only a bandage- not a cure.  Not sure what the harm is in applying bleach to the roof as long as there's no pressure while applying it.  I guess it could accelarate the denaturing of the asphalt backing? 

The most common occurrence of the mildew is due to large trees that block out the wind and sunlight on sections of the roof.  A fix to the mildew problem would be to hack the trees.

I have seen a few zink strips on both side of the ridge that seem to work. Some of the mfg. anti fungi cost built in the price work better than others. You can not elimate it in the SE USA just live with it. Never never power wash any surface that you do not intend to paint several new coats, vinyl steel, aluminum included. Low pressure on bricks OK, but be careful with some bricks.

Does ever state allow you to cut tree limbs that hang over your property and more specific your house ?

Inadequate attic ventilation is often the culprit when you see those ugly streaks on light colored shingles, and there are no trees overhanging.   A lot of people mistakenly assume the streaks are from the sealant around roof vents or plumbing stacks.  If you see one of these zebra roofs and have occasion to go inside the home, you will likely find a correlation with the streaks being above bathrooms, laundry rooms and the kitchen where moist heat hits the attic.  At least that's the case in colder climates... I hadn't heard about the zinc strips before.  Is that pretty common in the south?

This problem started getting worse a couple of decades ago as glass fibers were substituted for organic fibers.  This coincided with the shortages in rag markets  and organic fibers due to heavy use of synthetic clothing.  The algae is fond of limestone which is used as a filler along with the glass fibers in the manufacture of the shingles.  Roofers and others noticed the stain free streaks under the metal accessories.  3M corp. who makes  granules capitalized by impregnating the granule base with copper and other metal compounds then coating them with the ceramic coating.  The metals leach through leaving an Algae resistant residue over the entire roof similar to those areas found under metals Trader was talking about.

ARMA and CASMA have issued suggestions on how to deal with the stain problems. 

RandyC