Its a great idea for your normal garden but I wouldnt on CPs. They are too sensitive. A lot of people use moss killers and other chemicals to keep their roofs clean, or they use zinc strips or shingles with copper flakes in them to kill moss and whatever else grows up there. Remember the whole wash down effect.

Personally I use a 10 pnd bag of baking soda. Its so alkaline that the moss wont grow(not acidic enough to grow) and no chemicals! Not good for CPs since they grow in very acidic places.

I know someone who had issues after installing a new roof where zinc strips were used, and the runoff water was diverted toward his pond area and killed off his fish and frogs. Not a pretty story. Id be really afraid with my CPs. Now Im sure there are ways around this but???

Take it easy -Jeff