With limitedspace in urban areas, cement cisterns are being built beneath homesas a rainwater harvesting solution.

With the new “Green Storm Water Infrastructure” or GSI mandate, Seattle based RainBank Rainwater Systems is seeing increased interest in rainwater collection. But, with limited space on an urban building lot, the storage of rainwater in above ground tanks is not an option when required to mitigate thousands of gallons of roof runoff.

With many new residential construction projects building on small city lots, there is just not enough room for a rain garden. One way to solvethis issue is with a cement vault, constructed beneaththe house, which stores the captured rain water for potable use.

The average 2,000 square foot roof area will yield over 44,000 gallons of water. However, a cement vault cistern needs special engineering and considerations. Water weighs approximately 8 lbs per gallon and 1 cubic foot equals approximately 8 gallons. Calculations of weight and sheer need to be engineered for Seattle’s seismic zone. Additional ventilation should be considered along with aeration.

Cement water tanks are not naturally waterproof; cracks in concrete can resultyears after construction. The use of a liner, whether spray on epoxy, or a poly bag will prevent groundwater from entering the water tank as well as improve water quality.