What is the white powder people put on roofs?

The white powder people put on roofs is typically one of several different products depending on the purpose, but most commonly it is zinc sulfate or a moss killer treatment. Many homeowners in damp, shady climates apply zinc sulfate powder or products like Moss B Ware, which contains zinc sulfate monohydrate, to kill and prevent moss, algae, and lichen growth on asphalt shingles. When it rains, the powder dissolves and washes down the roof, creating an environment that inhibits moss growth. Another white powder sometimes seen on roofs is oxygen bleach, sodium percarbonate, used as a gentler alternative to chlorine bleach for cleaning algae stains without damaging shingles. In some cases, the white powder is actually roofing chalk or marking chalk used by contractors to snap lines for shingle alignment. Less commonly, people apply hydrated lime, but this is not recommended as it can damage shingles. In commercial flat roofing, white powder might be a reflective roof coating or elastomeric coating applied to reduce heat absorption. If you see white powder or white streaks applied in lines near the ridge, it is almost certainly a zinc-based moss prevention treatment, which is a common professional recommendation for long-term moss control after cleaning.