I've heard that putting a length of copper tape along the top of the roof (if you can get there safely) is very effective.

any moss killer will turn it black then you have to rake it off. just rake it off alive and dispose of. same for walls really. moss is a sign that the area is damp/shady or both. solve that problem and then the moss wont take hold! its a problem we have with the garage but I like it and OH just has to put up with it.

Thanks a lot for you answers gona give them a try !!!!

Copper tape does work - its usually affixed just below the ridge tiles on apex roofs, so that when it rains, a small amount of copper is washed over the roof, which stops the moss growing. You could try a strip at the top of a wall, but it might not be that effective on a flat vertical surface.