You might want to look into a product called Wet and Forget (see link). I am a homeowner and have no connections whatsoever with any commercial products. I just spent several weeks on my concrete tile roof spraying the accumulated moss with a bleach solution, then scrubbing off the dead moss with a wire brush. Looking for an easier way to attack the problem, I went online and found W&F and bought some at Costco. Since my roof is now clean, I still need to figure out when to apply the W&F (as a preventative before the moss, or as a treatment after the moss comes). Anyway, it seems to entail a lot less physical labor and more environmentally friendly. My plants didn't like the bleach solution being sprayed or dripped on them.

Costco sells it as 4 jugs in a box for $30 or so (I don't remember exactly, but it was about that). Each jug is 3/4 gallons of concentrate. Depending on the mix ratio, which will vary according to the porosity of the surface and the degree of staining, each jug is supposed to cover between 550 and 1700 square feet.

Perhaps others who have actually tried W&F will chime in as I, too, would be interested in hearing from folks who have tried it. Good luck!

Here is a link that might be useful: Wet and Forget

taxonomist, What kind of roof do you have?

Why do you think crawling on the roof with bleach or TSP would fix the stains? Smarter is figuring out what is causing the stains.

If your neighborhood isn't new and the trees are tall, have you thought of tree sap?

Thank You Seattlemike for the very valuable tip on the Wet and Forget cleaner. The link you offered led to a deadend in this area. Costco is my next bet. The website relating to the product certainly escalates my hopes.

Those roof stains are caused by moss. It will grow on the side of the house that gets the least amount of sun, such as the north side. Shingles used to be made with asbestos in them but for obvious reasons, no more. Ground limestone is used instead and the moss feeds on it. There are products, such as "Moss Away" that is designed to kill the moss and in a few months time, clear the stains. There is a zinc strip you can purchase that gets installed under a row of shingles above the staining. The zinc kills the moss, clears the stain (over a few months period) and prevents the moss from growing again and thus prevents future staining. You can also get the same effect with a strip of copper designed to do the same thing.