The white stuff on my garage roof will kill the moss that had taken hold on it, but it needs a little rain to dissolve in.
The crew that had put it on (it’s environmentally friendly, they assured me), will be back in 4 to 6 weeks to brush off the moss and clean out the gutters.
I will have to be patient: July and August are the driest months in Seattle, each with about 1 in. of total rainfall, on average.
