If you have cedar or asphalt shingles on your roof, then you may have a problem with moss. You might be willing to dismiss moss as a quaint addition, but this would be a mistake. The problem with moss is that it holds moisture against your roof and can, thus, promote decomposition. If you have moss on your roof, you need to remove it, but more importantly you need to ensure that it doesn't grow back.
If you have live moss on your roof—you'll know its alive if it is green and spongy to the touch—you need to kill it before you remove it. Spray the moss with a oxygen bleach solution. Don't use chlorine because any bleach that runs off your roof can be bad news for plants or animals below. Leave the moss for an hour or so, so that it has time to act. Then sweep off your roof with downward strokes of your broom. Don't sweep upward because you can crack shingles or even rip them off your roof.
Preventing Moss Growth: Step 1
Moss likes dark, damp places. Thus, if you have any tree limbs that grow out over your roof, you should cut them back. This will expose your roof to more sunshine, help your shingles to dry out faster, and deter moss growth. On the other hand, if you have a north-facing section of roof, and/or you live in a humid area, you will need to take a further step to deter moss.
Preventing Moss Growth: Step 2
To further deter moss growth, install zinc flashing along the ridge of your roof. Whenever it rains or snows, the moisture will react with the zinc to create zinc salts, and as these wash down your roof, they will coat the roof with a natural moss deterrent. You never have to worry about climbing up on your roof to spray it down with an herbicide because mother nature takes care of the problem for you.
If you want to get the most life from your roof, then you need to maintain it. If moss is a problem in your area, then dealing with moss should be part of your maintenance program. Rather than climb up to your roof and sweep away moss anytime it becomes a problem, you should take an active roll in preventing moss growth. A little zinc flashing is one of the best roof upgrades for long-term moss prevention. Contact a roofer in your area for more information.