How Do I Build an Underground Rainwater Storage Facility?

For every inch of rain that falls, a rainwater collection system operating at 100 percent efficiency can be expected to gather about 0.

How to Build an Underground Rainwater Collection System

Rainwater collection systems provide additional water in and around the home.Rainwater collection systems provide additional water in and around the home.
62 inches of water per square foot of catchment surface. Most collection systems, however, only operate at 85 percent efficiency. When calculating the amount of water your collection system will supply, allow for a 15 percent loss due to evaporation, water run-off and overflow. Determine the appropriate size of your collection system based on the amount of anticipated rainfall in your area, as well as the anticipated demand for water in and around the home. Rainwater used for irrigation purposes will require less filtering than water that is to be used in the home. To use rainwater for drinking, you must ensure that it is properly treated prior to consumption.

Determine the catchment surface. The roof on an existing home is the most convenient surface to use. However, asphalt, composite and wood surfaces should not be used for collecting rainwater. If the surface of your roof is not appropriate for a collection system, a simple rain-barn or other open-sided structure can be built.

Attach gutters along the edge of the catchment surface to catch run-off water. Use PVC, vinyl or galvanized steel gutters to ensure that water quality is not affected by contaminates in the gutter system. Install gutters with the outside face slightly lower than the inside. Positioning the gutters in this way will ensure that if the water ever overflows the gutters, it will run outward and away from the building.

Select a location for the storage tank that is at least 50 feet from animal stables or septic fields. Bury the storage tank as close to the catchment surface as possible to minimize the need for long lengths of pipes. The substrate in the selected location should be stable enough to ensure that the tank will not sink over time. Compact the soil in the excavated hole and place a layer of gravel or compacted sand over the area.

Attach downspouts to the walls of the building using appropriate size fasteners. The downspout tube should run from the gutter to the storage tank. Insert screens and filters at both the downspout and tank levels. Filters and screens prevent debris from entering the storage tank. There is a wide variety of options for filtration. A simple metal screen will prevent large debris from contaminating the system. If you are planning to use rainwater collected for drinking purposes, more extensive filtration devices will be needed to ensure water quality and safety. Screens and filters will require regular maintenance and cleaning to keep the collection system working properly.

Attach an on-demand water pump as close to the tank as possible. As opposed to a standard pressure tank pump system, an on-demand water pump moves water directly from the storage tank. These types of pumps also incorporate the motor, pump, check valve, controller and pressure tank into one unit, making the job of installation easier and less time consuming. Use PVC pipe to carry water from the pump to the area of use.

Things You Will Need

  • Slate or metal roof
  • Gutters
  • Downspouts
  • Hardware, straps, fasteners
  • Screens or filters
  • Storage tank
  • On-demand water pump


  • Do not use this system in areas where winter temperatures are below freezing.

About the Author

Randa Morris began her freelance career in 1994 as staff reporter for the "Ogemaw County Herald." She works as a full-time content producer for online and print publications. Her writing is often motivated by her work with adult and child trauma survivors. Morris received level two trauma certification from The National Institute of Trauma and Loss in Children.