First consider laying some copper strips on the roof. I have seen them placed underneath stone chippings and it has worked very well at keeping moss
growth at bay. This page has more information on how to best use copper as a moss killer. This
option is great for flat roofs that are not seen, such as dormers.
Copper is very much a safe product and is used quite often on roofs, such as a replacement for lead or as a material from which guttering is made from. Drinking water
is also supplied to many homes via copper pipes.
Another option is to use chemicals directly onto the roof. Because you cannot be so rigorous at removing existing moss you may need to apply more
of the chemical than you would onto a patio or other surface.