When it comes to adding water to a roaster oven, different roaster oven models have different requirements. Some manufacturers provide a removable roasting pan where liquids, including water, broth or wine, can be added. Other brands are designed to cook more efficiently without adding water.
For roaster ovens that feature a buffet tray, add 5 to 6 quarts of water, and preheat the oven at 350 F until it is very hot. Place food in the serving dishes inside the roaster oven, and lower the temperature to 250 F to keep food warm and moist. Check the water level hourly, and add more hot water as needed.