is it a shingle roof or another material like shakes?
here is an article I wrote that deals with moss, some of these same idea will apply here.
The most important thing to remember is that over growth of plants etc that shade the roof oftentimes is the real culprit here. The roof needs sun to dry out. Overgrown tree branches shade the roof allowing this moss to develop. Cut the tree branches back and get some sun on the roof. Then follow KMS link and have someone install copper or zink strips along the top edges of the roof. This should help prevent the re-occurrence of the moss.
The roof however will need to be properly cleaned free of the moss first. There are companies that do just that type of work. Just remember that if your roof is getting up there in age, you may end up needing a new roof as well.
There are products available which contain zinc. In fact, the newer asphalt shingles have zinc in them to help eliminate mold. Once clean, you can get zinc strips and put them on your roof. There may be professional roof cleaners in your area. We used one for $250 and $90 annual touch up. It was well worth it.
We have a company in Vermont called Vermont Roof Cleaners.... The guy and his wife use a bio-chemical, and power wash it on, then off. It's cheaper than a new roof, and lasts a long time. I agree with the guy above who said, the roof needs sun.....
Its getting to much shade and no sun...have the same problem, tree guy scheduled to remove tree next week.
I used an excellent product on our side roof (northeast side of the house) that just never gets enough sun. It's called "Wet and Forget". I applied it last summer after we discovered how much crud was growing on that roof. I bought an inexpensive sprayer at the hardware store along with a gallon of the solution (it goes a long way, follow the directions). We went out on the same roof this spring, to find that the big areas of moss were all brown and dried up and easily removed. We cleaned the upper gutters above the lower roof, too, now that it is easier to walk out there. I had enough left in the sprayer that I also treated a concrete statue in our yard that was growing green areas along some of the details and a small concrete bench next to it. Previously I had scrubbed, used bleach in soapy water and even a tooth brush to scrub the stuff out, but, it always came back. The bench has stayed clean since last year, and only needed regular spring cleaning, and only the shaded side of the angel needed a reapplication. I purchased mine at a local hardware store but, I've seen it online also. After filling the applicator/sprayer and using it for all of the above, I still had quite a bit left over, so don't mix more than you can use. It's a concentrate and dilutes with water (follow the directions but I think it was a couple of cups of concentrate to then fill the sprayer to one gallon with water).
just about any copper based cleaner will kill the moss & mold.
and like the man said, trim back the trees so they do not over hang the roof.
lastly on every peak in the roof, install a copper ridge cap & yu will no longer have any moss or mold problems
Wet and forget works wonders and is not labour intensive