I get it from you old school guys…but there is new data from all sorts of scientific sources. They started upping the limestone content in shingles about 15-20 years ago, which makes for more potential fodder for the bacteria, and more potential damage as it eats away at it.
" In order to keep down the cost of composition shingles, manufacturers have begun to include more “fillers” in their “recipes”. These fillers replace some of the oils in the shingles. Of course, those oils are normally the most costly component of the shingles. Removing the oils often seems to shorten the lives of the shingles, making them less resilient and supple. But, an additional problem is that the gloeocapsa magma feeds off of those fillers, leading to the unsightly streaks and stains.
A predominate filler used in composition shingles, because it is so plentiful and inexpensive, is limestone. It is the calcium carbonate in limestone that gloeocapsa magma really likes to feed off of, along with moisture that naturally collects on a roof’s surface. Once the algae growth becomes noticeable as those dark streaks on the roof, gravity and rain water flowing down the roof carry the spores down the roof, causing the commonly seen pattern of streaks extending downward on the roof. And, as the algae feeds off of the limestone over time, it invades the structure of the shingles and shortens their lives." citation Classic Metal Roofing
Only in recent years has it been determined to have they decided it should be cleaned due to damages it causes…
“These black stains are the bacteria themselves in mass amounts covering the surface of the roof shingles. Gloeocapsa magma has been around historically and up through the present, except only in the recent decades has it been considered to have detrimental effects worthy of prevention. This cyanobacteria causes substantial destruction revolving around shingle decay and loss of reflective power. Over time, Gloeocapsa magma begins to break down the contents of the shingles by feeding off of the limestone granules embedded in the shingles and this decreases the roof’s ability to reflect ultra-violet rays of light. Also, the quality of the shingles slowly diminishes. They have been found to curl up at the corners as well as form raised humps on the surface. This ultimately can result is broken or disintegrated shingles. In the long run, this has been known to increase air conditioning bills and lead to more frequent roof repair and reconstruction. The main preventative strategy known to date is frequent (annual) roof cleanings using roof algae cleaners. Installing copper or zinc strips along the roof boarder also inhibits bacterial growth by killing off new colony formation.” citation MicrobeWiki (from the Microbiology Dept. @ Kenyon Univ.)
There’s plenty more out there, granted all in the last decade or so (after the change in composition of composition shingles)…been researching a lot lately. Of course there’s a ton from wash contractors as well, but I dismiss that as worthy of citing due to the obvious potential for conflict of interest.
EDIT: I didn’t mean to post the internet, but I definitely do my research before I’m willing to make any sort of statement of value to a client. I won’t do business like that.