In a regenerative building, water does not come from a municipal supply. It comes from the sky. Rainwater harvested from the roof is the primary water source, stored in cisterns and filtered to potable standard for all household uses. This approach is not exotic: rainwater harvesting has been the primary water supply for billions of people throughout human history, and it continues to be practiced at scale in many parts of the world today. The design of a rainwater harvesting system begins with the site’s precipitation data and the roof’s catchment area. A simple formula relates annual rainfall, catchment area, and collection efficiency to determine how much water can be collected. Cistern size is then determined by how much storage is needed to bridge the longest dry period that can reasonably be expected. The Water Textbook, Book 5 of this series, covers the complete design and construction of a rainwater…