Moss, mold, and streaking algae on your roof can be caused by a number of circumstances surrounding any roof. Lack of ventilation in the attic or closed ceiling porches creates moisture and condensation with changes in ambiant temperature(s); the north facing surfaces are the last to dry, especially with overhanging vegetation that keep surfaces wetter-longer, no matter which direction they're facing, and the shingles manufactured since the 1970's. Oh yeah! Here we go again. Yet another innovation to a product that was working well until the cost of oil enters the equation or until someone gets creative and fixes it (whichever comes first). Seems the old asphalt shingles that Grampa or Great-Grampa installed on your Aunt Mavis' unvented roof were impervious to the a-forementioned. MM&SA because they were just that; solid asphalt impregnated with mineral stones, and not much of anything would grow directly onto or out of them. Grampa said they were rather weighty, as well; something akin to shoveling cow manure uphill.
Given the oil shortage(s)/prices of 1973 and beyond, our well-meaning roofing material industry decided they should use less asphalt along with fibreglass to deter weatherwear, and limestone for filler and weight. "Good. We're back in business". Only, the word is that just about all roof fungi live quite well on this limestone filler, particularly the streaking algae, probably because it looks the worst; itty-bitty specks growing to the lenghth and breadth of U.S. Rte 2 and 7, oozing its smug and merry way to the dripedge, gutters, and adding odd colors to Aunt Mavis' zinnias. But that leafy, gray-green moss, usually in round or off-round patches, called Lichen (pronounced likun) that does the most damage. The roots grow between the shingle granules, then loosen and wash away, beginning the shingle deterioration process. Depending on the severity of the conditions described above, you may notice adverse goings-on as soon as two years after new shingles are installed.
So here we sit in Chittenden county looking @ our hideous roofs, wondering whether to live with them, watch the roof deteriorate 25 to 50% faster than they ought to and replace them, or try and clean it before snow flies... [This can be done down to 40 degrees F minus the snow]. Enter the guy with the funny hat and Sofsprayer.