When the rain barrel reaches maximum capacity, the overflow kicks in, and the rain gutters from the barrel as normal. However, it's important to position your rain barrel correctly. A full barrel 50-gallon can weigh up to 450-lbs. Therefore, you need a stable base around the barrel to ensure it doesn't topple over under the weight. Look for models with release valves. During heavy downpours, this valve assists with the overflow of excess water.