After five years, our new shingle roof was showing signs of mildew. I have been warned plenty about the dangers of power spraying and loosing too much stone off of the shingle. My roofer suggested 'Wet & Forget' and that it worked wonders just by wetting down the roof letting the sun and rain do the rest. The trick was how to apply! The thought of using my pump sprayer resulted in two bottles of 'Wet & Forget' sitting on my workbench for two months. Then, on my last trip to Home Depot, I passed by the Ryobi Chemical Sprayer. More than I wanted to spend but decided to go for it!