Mix 5 percent bleach with 95 percent water. Use a common water can to sprinkle it liberally on your entire roof on the north side on a warm dry day. Noon time is a good time. Enter and exit your roof on the south side to keep from falling on your butt. The chlorine will kill the mold. You can use a stable broom to brush it off once it is dead.

How old is the roof.Might be a sighn its time to remove it along with the shingles in preperation for a new roof.

You can also install zinc strips at the top of the roof. Look up zinc strips on Google.

Chlorine is a good way, once a year is enough. I used to just spray on a weak solution from the ground using a pump sprayer.

As the others have suggested, this may be a clue that your shingles are due for replacement. It may just be due to overhanging trees.

Zinc strips along the top are a good solution if the roof is in good shape. The zinc leaches out slowly, inhibiting growth and keeping the roof clean.

Finally, some new shingles also come with a growth inhibitor built into the shingles themselves.

John

Take a look at the link. These guys sell cleaners and the zinc strips once you get it clean. A lot of places say that using the chlorine bleach is not a good idea for a number of reasons, but I don't have any personal experience with it. We put these strips on with our new roof, but it is not old enough to know if they have done anything yet.

Here is a link that might be useful: algae stuff

Twenty-eight bucks plus shipping for one gallon of "roof cleaner" which uses sodium hydroxide as the principle ingredient.

Of course, sodium hydroxide is just a fancy name for common every day lye or caustic soda which can be had for a fraction of the cost from many hardware stores. Caustic soda is also known as drain cleaner and that too, is readily available at low cost.

A five percent solution of ordinary bleach in water is very mild. It's strong enough to kill the algea and mold but someone should explain to me what negative effect it has on roofing shingles comprised of a layer of stone chips bonded to felt paper heavily impregnated with tar.

The 5% bleach and 95% water sounds like a good deal to me. Will it hurt the shingles? Can I hose off the mold once it's dead?

Here are a couple of links that might be of interest.

http://www.epowerwash.com/node/24

The one below is from IKO.... an asphalt shingle manufacturer.

Scroll down to "Shingle Performance" and click on the first choice below that title.

Here is a link that might be useful: IKO

Whatever chemical you use, try to control the runoff or you may end up with a dead spot in your lawn or garden.

It doesn't matter what type of roof you have, if you have 'black' streaks, then you will need a strong solution to get rid of it. If you are willing to climb up there yourself, then I would get a 2 gallon pump up sprayer, fill it with half and half with water and pool chlorine that you can buy in the yellow jugs. Household bleach won't even touch roof mold. Then add a half cup of liquid TSP from Wal-Mart as a 'wetting agent' so that your solution will penetrate all areas around each shingle or tile. Otherwise, you solution will just bead up and roll off your roof because of the surface tension. First, wet all plants and shrubs near the house as well as all the walls and windows. Optimally, you would want someone on the ground with a water hose constantly rinsing everything off to prevent streaks. After wetting everything down, cover your shrubs and plants with plastic sheeting from Wal-Mart. Then commence to spraying your roof. Spray until the black mold begins to change color and hopefully it should foam up a little but you don't want to soak it. You are not trying to rinse off your roof with this solution. You will have some runoff, but this is normal. As you work your way around, touch up areas that need more. Before you get off your roof, it should look like new again. If not, then touch up the areas that need it. If you can do this before a good rain, that's even better. Once your roof looks clean and you are back on the ground, use the water hose to finish rinsing the roof from the ground. This will dilute the remaining solution on the roof. Then finish watering your plants and shrubs. Being in the business, I've tried all those other cleaning solutions that you put up there and wait six months, but all they do is leave streaks on your roof. As a business I am not trying to sell anything since I don't know where these posters are from. Just trying to help those that need it. If you need any more help, just email me at jim@scrubbingbubbles.ws

Here is a link that might be useful: Scrubbing Bubbles Home Exterior Cleaning Systems

My advice is to forget the 95 percent water and 5 percent bleach.
It is far too weak to clean roofs with.
You need swimming pool chlorine mixed 40 percent to 60 percent water.
Spray on with a pump up sprayer, and keep plants wet.
The link below may be helpful.

Here is a link that might be useful: Roof Cleaning