You can't just spray any cleaning agent on a substance and hope that it gets clean. You have to scrub it into the surface, which, for a roof, involves potential danger, vis-a-vis, slipping and falling off it, especially as it gets wet.

Simply brushing the moss away when the roof is completely dry is hard work and dangerous.

Presumably, your moss-invaded roof is in a shaded or wet area. As such, if you can trim overhanging branches from trees (if such is the case), so that more light hits the roof, which would, eventually, kill the moss. However, if the roof is particular thick or layered with moss (i.e., more than just on the immediate surface), then additional light won't help for a long time, and, worse, it could spread to the underside of the roof, causing severe damage to the baseboards and/or allowing water to seep in through the weakened materials. Furthermore, if removing light-blocking items doesn't help or isn't applicable, you may have no other choice but to call a landscaper, gardener, or professional roofer to have it cleaned or replaced, if you are unwilling or unable to do so on your own (n.b., if the contractor injures him/herself on your property, you might be liable for damages!).

P.S. Using any cleaning agent may cause roofing materials to discolor, so, whatever you decide to go with, do it on a small, out-of-sight area, first, and wait a month before applying it to the rest of the roof (you waited THIS long, right, so, be patient!).

REFERENCES:

http://ths.gardenweb.com/forums/load/cleaning/msg041239239153.html

http://www.clevermag.com/investigations/moss.htm

http://www.popularmechanics.com/home_journal/how_to/4217233.html

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