I'm with the masses on this one, If you have so much run off that you are thinking about trying to reuse it, you are (1) sparying way too much, or (2) need more suffactant. I have a few roofs under my belt now, and even some that were unwalkable, the run off for me has been minimal. After suggestions from some of the more experienced roof guys, I am no longer rinsing. Results were awesome, customer was happy. Downside, you have to go back for the after pic. Good luck!