What is the white powder people put on their roofs?

The white powder people put on their roofs is most commonly a zinc sulfate or sodium-based moss killer and prevention treatment. Homeowners in damp, shaded climates often apply products like Moss B Ware, which contains zinc sulfate monohydrate, or similar zinc-based powders to kill existing moss and prevent regrowth. When the powder is sprinkled on the roof ridge and rain washes it down, it creates a mild toxic environment for moss, algae, and lichen that inhibits their growth without harming asphalt shingles when used as directed. Another white powder sometimes used is oxygen bleach, sodium percarbonate, which is a shingle-safe cleaner that kills algae and moss through oxidation rather than harsh chlorine. People also use hydrated lime in some regions, though this is not recommended by manufacturers as it can damage shingles. In professional roofing, white powder may also refer to roofing chalk used for snapping lines, or reflective elastomeric coating in powder form. If you see white powder applied in lines or strips near the ridge of a roof, it is almost certainly a zinc-based moss prevention treatment, which is a common recommendation after cleaning a mossy roof. Zinc strips installed near the ridge provide the same long-term protection as rain washes zinc ions down the roof surface.