Quick Answer
To collect water off a green roof for cisterns, it's essential to use a rainwater harvesting system designed for green roofs, incorporating features like roof drains, overflow valves, and first flush devices to maintain water quality. This system should be sized according to the roof's surface area and annual rainfall. Regular maintenance and inspections are also crucial.
Roof Drain and Overflow Valve Selection
When collecting water off a green roof, it’s crucial to choose roof drains and overflow valves suitable for the system’s size and requirements. A minimum of 1/4 inch per foot roof slope is recommended for effective water flow. Roof drains with a 2-4 inch diameter are a good starting point for most green roofs, with overflow valves set to prevent overflow during heavy rainfall events. Proper sizing ensures the system can handle the roof’s water collection capacity while preventing water damage.
Cistern Sizing for Green Roofs
To determine the optimal cistern size for a green roof, calculate the roof’s surface area and annual rainfall. A general rule of thumb is to size the cistern to hold 1-2 inches of rainfall per square foot of roof area. For example, a 1,000 square foot roof might require a 10,000-20,000 gallon cistern, depending on the local rainfall patterns and desired storage capacity. This will ensure the cistern can collect and store sufficient water for non-potable uses like irrigation and toilet flushing.
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