Roof water downspouts are perfectly designed to collect rain water before it hits the ground and deposit it directly into a storm drain. They are also perfect for filling rain barrels and water storage tanks. Consider if a half inch of rain fell on a 1,000-square-foot roof, it would yield 300 gallons of free water. Most roofs are 2,000-3,000 square feet, so over the course of an entire season that’s a lot of free water that can be stored and reused.