So I finally got around to spraying some 10% bleach on an inconspicuous section of the roof (*) to see whether it would have any effect on that black fungus / mildew / crud. It’s too soon to tell, but in the process I discovered that the sprayer nozzle didn’t produce the nice, round pattern it used to. I completed the mission and took the nozzle to the basement. The problem was obvious:
Soaking it in vinegar didn’t have any effect; whatever made those deposits wasn’t soluble in water or mild acid. A few minutes with an awl and a (manually turned!) Dremel grinding point restored it to good condition:
You’d be more careful cleaning the orifice of a fine spray nozzle, but this is for a hand-pumped garden sprayer: Good Enough.
As soon as the weather clears, we’ll see if the situation up on the roof has improved. If so, I get to spray the rest of it.
(*) The whole north slope over the garage, in case you’re in the market…