How does roof moss killer powder work?
Short answer. Most roof moss killer powders are zinc sulfate. You sprinkle the powder or granules on the moss or along the ridge, and rain or dew dissolves it so the zinc soaks into the moss and kills it. It works slowly, usually over days to weeks, and the dead moss then loosens and washes off.
Why zinc works
Zinc is toxic to moss, algae and lichen in small amounts. As water moves down the roof, dissolved zinc spreads over the area below where it was applied, which is also why zinc strips near the ridge slow regrowth.
How to apply it
- Read the label for rate, timing and whether to apply to dry or damp moss.
- Sprinkle a thin, even layer directly on moss patches, or in a band along the ridge for slopes that are hard to reach.
- Use a handheld spreader or a gloved hand from a ladder; do not stand on mossy shingles.
- Let rain or dew activate it. In a dry spell, a light misting from a hose can help if the label allows.
What to expect
- Moss turns yellow, orange or brown over days to a few weeks.
- Dead moss loosens and washes off over following rains; push leftovers off gently downslope.
- Some regrowth protection lasts while zinc remains on the roof.
Cautions
- Zinc sulfate can stain painted surfaces, concrete and some metals, so keep it off siding and walkways and rinse spills.
- It harms plants and is toxic to fish and aquatic life; keep runoff out of ponds, creeks and storm drains.
- Wear gloves and eye protection; the dust irritates.
- Check that the label lists your roof material.
Powder versus spray
Powder is slower but longer lasting and easy to apply along a ridge. Sprays act faster on visible moss. See moss and lichen on roofs and the Georgia roof maintenance checklist.