The best powder for killing roof moss is zinc sulfate monohydrate, which is the active ingredient in many professional-grade moss killers specifically formulated for asphalt shingle roofs. Products like Moss B Ware, Moss Out, and Lilly Miller Moss Out for Roofs contain 99 percent zinc sulfate monohydrate and are considered highly effective because zinc is toxic to moss, algae, and lichen but relatively safe for asphalt shingles when used as directed. The powder is typically sprinkled or broadcast onto a dry roof, often along the ridge, and rain gradually dissolves it and washes zinc ions down the roof, killing moss over weeks without scrubbing or pressure washing. Another effective powder is potassium salts of fatty acids in powder form, though most are liquid. Powder application has advantages as it stays in place longer and provides extended contact. When using powder, apply on a calm, dry day when no wind will blow powder onto landscaping, and lightly mist the roof first if directions call for it. Avoid using powders containing iron sulfate or ferrous sulfate on asphalt shingles, as they can cause rust staining. Also avoid using excessive amounts, as concentrated zinc sulfate can harm plants and grass below if heavy runoff occurs. After moss dies and turns brown, gently sweep dead moss off with a soft broom from ridge downward. Installing zinc strips after cleaning provides continuous powder-like protection long-term.
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