Um, spray the underside of your deck w/some Fluid Film.Treegal, the best description I can give is after cutting a lawn (high nitrogen fed), not bagged, blades damp you have accumulated a mass of grass adhered to the underside of the deck. Later,no growth due to any irrigation/rain. The grass starts to decompose under the deck. You scrape the deck later. This is the odor I am referring to (Ammonia, NH3).