What is the best way to remove moss/black from a roof and keep it off?

Use a product called Wet and Forget

The black mold-like stains and streaks that appear on roofs, particularly light-colored asphalt shingles, is actually a blue-green algae (Gloeocapsa magma). Commonly found in climates with warm, humid summers, it does no damage to the roofing, but it certainly does looks bad.

The less expensive solution is to spray wash the roof with a 50 percent mix of water and bleach to get rid of the algae. (No pressure washers, please. They're likely to damage the shingles.) Just be sure to wet your foundation plantings first, and rinse everything in clean water when you're done. Plants don't like bleach, and wetting them with plain water first protects them. You can also cover them with tarps or plastic sheeting, which will protect the soil around your plants as well. Make sure that you create a wide radius around your home to be safe.

Allow the solution to soak into the shingles for at least 15 minutes. If they are severely molded, you may want to give it a little longer. Next, rinse the shingles thoroughly with a garden hose at low pressure. Start from the top shingle and rinse downward to prevent water seepage.

To keep the algae from coming back, insert 6-inch-wide strips of zinc or copper under the row of shingling closest to the roof peak, leaving an inch or two of the lower edge exposed to the weather. Use a putty knife to break the shingle seal, slip the strips under the shingles ( about 2" of the strip should show), nail the strip with a galvanized nail under the shingle every two feet ( so they don't slip out), caulk the nail heads with clear silicone caulk, press the shingle down. Do this the full length of the roof.That way whenever it rains, some of the metal molecules will wash down the roof and kill any algae trying to regain a foothold on your shingles. Any tree branches or trees close to the house shade the roof and prevent it from properly drying allowing the moss to grow.

We live on the Oregon coast and get lots of moss on the roof Head down to the $Tree and buy 3 boxes of their dry Sun laundry soap in a box. Toss it around the moss on your roof. And, bye bye moss But you will need to sweep or blow it off in the spring. The dry moss needs to be removed or it becomes the bedding for new moss.

You can also buy Moss Out, either in the shaker can and getup on the roof and sprinkle all over, or you can buy in liquid container that fastens on the hose, and spray it on....my option.

We had this problem on a North facing porch roof that has a tree that always shades it. We put Wet It and Forget It on it two summers ago, and it never came back. It takes a little time, but, it does take care of it. I have also used the same product on a concrete statue with excellent results. If your local hardware store doesn't have it, you can order it online. It is also safe and biodegradable. Just follow the directions. Ours was a mos