Mold is a constant enemy because it is so difficult to remove.  Mold on the roof is caused by moisture and poor ventilation in the attic.  Mold is dangerous because it affects the structural integrity of the home and it is a serious health hazard.  Prevention is the key, but if you already have a mold problem, getting rid of it should be a high priority.

Bleach is a common cleaner for mold.  However, it is important to note that bleach merely discolours the mold, making it look as though it has vanished.  It does not reach the root of the problem and eventually the mold will return.  This is primarily because bleach has no anti-microbial properties.

The best types of products to use to remove mold from the roof or anywhere for that matter are biocides and anti-microbial cleaners.  There are green options available, which is advantageous because these products are not harmful and will not damage the landscape under the roof.  These cleaners will destroy the mold and any underlying slime that might remain.  It is best to speak to professionals to get the best possible product for your mold removal needs.

Source:  http://normipro.blogspot.com/2007/08/bleach-does-not-kill-mold-d.html