Heating and the formation of methane gas begin as the dry litter absorbs moisture. Anaerobic bacteria generate about 50 to 65 percent methane, about 30 percent carbon dioxide, and a smaller percentage of other gases. Therefore, if the moisture content of stored litter is more than 40 percent in a stack with little or no oxygen, conditions are right for anaerobic bacteria to grow, producing methane gas. Heating and methane gas formation can be avoided if litter is allowed to dry completely before it is applied to the stack. If the stack has adequate pore spaces, any methane that forms will escape harmlessly into the atmosphere.
Stacks exposed to wind-driven rain can accumulate moisture and become a fire hazard.
Litter should be protected from blowing rain. Caked litter cleaned out from underneath waterers will usually have a high moisture content and can also contribute to litter storage fires. Caked litter will be much higher in moisture than litter located throughout the rest of the house. It is a good idea to separate piles of high-moisture caked litter from drier litters. Layering new, moist litter on top of old, dry litter creates an insulated, heat-producing situation.