Jim had an interesting problem with his mother-in-law's house. The 25 year old roof had no moss, ever. Then he re-shingled and the north and east sides have been constantly covered in moss. Why and what to do about it?
For a bit more money you can buy shingles that have chemicals in the shingles themselves that prevent the growth of moss. That's probably what Mom had on her roof before Jim re-shingled. Regular shingles don't cause the moss to grow, but they do allow it to grow.
To get rid of moss, first carefully scrape off any thick moss, without scraping hard on the shingles. Watch out, this is slippery stuff. Then use either a product specifically designed as a roof shingle cleaner, or a deck cleaner. But only use cleaners that have no bleach in them -- the non-bleach "restorers" for decks work well. You may need to use a soft-bristle brush and a hard spray of water. But be careful with power washers -- you could easily strip the granules off of the shingles so keep the pressure low. Let the chemicals and the flow of water do most of the work.
Moss can be kept off of roofs by attaching zinc strips near the top of the roof. The rain water will take minute quantities of zinc off these strips and the flow of this zinc solution over the roof will prevent any moss from getting started. It won't remove moss that is already established -- you have to clean that off -- but it will keep a clean roof moss-free. Nail the strip under the shingle, but let it stick out a couple of inches onto the open roof. You need to add strips below any obstructions that stop the water flow, like below chimneys and dormers.
The strips I showed on the show are called Shingle Shield and they offer the convenience of fitting neatly and snugly up to the shingle tabs. You can also find rolls of zinc to do the same job. On the West coast you can buy zinc pellets to scatter under the railings of decks for the same purpose. Although available in BC hardware stores, they are very difficult to find anyplace else. Apparently the stores think this is only a coastal problem. In the Montreal area both strips and pellets are available from roofing supply stores like Materiaux Dajet (Lachine) or any Roofmat location in Quebec. Here are two web sites that sell rolls of zinc by the post that are less expensive than the specialized zinc strips: ZincShield at zincshield.com and Award Metals in BC at AwardMetals.com. Also you might want to read a very interesting article about avoiding herbicides at www.Pesticide.org.
Yet Bill from Red Dear Alberta says that using zinc against moss is a farce: "Just caught the tail end of your spot on the weather network regarding roof moss. You suggested zinc strips. I can tell you from experience that the only people who get satisfaction from that solution are the sellers of zinc strips. I had it done on my cedar shake roof three years ago and did not have any reduction in the development of moss in the shady parts of the roof until I sprayed the areas with a dilute solution of chlorine bleach. Now that works and after a few weeks the stains from the bleach were not noticeable. Please don't delude your watchers with the zinc myth!"
Well Bill, This is one of those building science things that are very climate dependent. Zinc strips do in fact work wonders but they require abundant rain. Red Deer is almost a desert. You apparently do have enough shading and tree cover to grow moss, but not enough to activate the zinc strips. To get them to work in your area you would probably have to put a sprinkler on your roof. I can't recommend using the bleach because not only does it cause stains, it actually will degrade the protective oils in the cedar shakes or the asphalt in asphalt shingles, loosening up the protective gravel topping and shortening the life of your shingles. You might want to look at cutting back the trees to get the drying winds of your climate to keep your roof dry enough to not permit the growth of moss in the first place.