Hi Carol,

Thanks for reaching out to OSU Extension to ask about moss control.

Without knowing more about the exact product that was applied to your roof (and potentially reading the label), I can't say for certain if the timing is critical for the effectiveness of the product. A few OSU Extension-based sources (such as this publication from 1981, and this brief Q&A with OSU extension on Oregon Live) corroborate your thinking that in general the best time to apply moss control agents is when the moss is actively growing, and also during a dry spell when it will not rain more than a 1/2 inch over the next few days. If your initial application was insufficient and the moss continues to grow through the winter, you might try waiting until a dry period in the spring to apply a product again.

While your contractor stated that the product they used is safe for the roof and drains, it's worth noting that this publication from the City of Eugene (2020) recommends applying moss killer during dry weather to avoid it running off into nearby water bodies. Some of the most common products found in roof-specific moss killers can be toxic to fish and invertebrates. A low-toxicity option is potassium salts of fatty acids. According to OSU Extension, "These soap-based biodegradable products kill moss by penetrating the cell walls and changing the membranes so that the cells leak and the moss dies. The product is noncorrosive and pose minimal risk to humans and animals."

I hope these resources help you decide what steps to take next. If you have any questions, don't hesitate to reach out. Thanks for contacting OSU Extension Service.

Regards, 

Sarah Bronstein, Master Gardener Trainee 
Reviewed by Elizabeth Records (she/hers)
Education Program Assistant 2
Community Horticulture & Master Gardener Volunteer Program
Oregon State University Extension Service | Linn & Benton County