Depends on what kinda rain we're talking about. Remember, we water down plants, shrubs, decks, exterior walls etc...for what purpose? To repel the Roof mix. Trying to shoot a Roof in the rain is going to do what? Repel the mix from the Roof. It MAY work if it's not a hard rain but your going to go though a lot more Roof mix than usual. We do not spray in the rain. total waste of time and money IMO. Not to mention the fact that you will never get the full cleaning affect. The mix needs time to work before it gets rinsed off.