Spring is the best time for this treatment, but just about any time of the year will help. I do not recommend this in the heaviest rainfall, freezing temperatures, or in the dry heat of the Summer. The treatment is applied to high points of the roof, as it does require some rain to spread it down the roof and activate it. It should be applied to all ridges, hips, peeks, etc. and again down the roof panels in horizontal lines every 6-8 feet.

All of the moss treatments available on the market are zinc-sulfate monohydrate based and all are similarly effective at killing moss. I usually use the powder form. I also offer a baking soda moss treatment for customers more concerned with their environmental footprints. In my opinion, the baking soda is not quite effective as the zinc mix, but it does work. I do not recommend using laundry detergent with bleach. Over time, the roof shingles will disintegrate from the continued use of the bleach. 

Note: Moss treatment is guaranteed to kill all existing moss on the roof, but is not the same as roof detailing.