How Much Rainwater Can I Collect From My Roof?



How Much Rainwater Can You Collect From Your Roof?

With only one inch of rainwater, you can collect around 600-gallons off of a 1,500-square-foot roof every time it rains. Using rainwater collection as a water source has been done for thousands of years, with many states have a resurgence of the sustainable practice. In some areas, there are financial incentives and rebates available.


    • In Texas, rainwater systems are tax-free. Learn More


    • California just passed an amendment to start excluding rainwater systems for property tax reassessment starting in January of 2019. Learn More


    • Homeowners in Tucson, Arizona are offered rebate incentives to implement rainwater systems. Learn More


    The basic equation of calculating how much rainwater you can collect off of your roof is the catchment area (ft 2 ) multiplied by the rainfall depth (inches of rain) multiplied by a 0.623 conversion factor.


    There are many factors that affect this, including what materials the roof is made of, the slope of the roof, and rate of the rainfall. The Texas AgriLife Extension Office has created an extensive spreadsheet calculation that breaks down rainwater collection by the month. Link

To Calculate the Rooftop Catchment Area


Calculating the square footage of the rooftop catchment area can provide you for an estimate how much rainwater you can actually collect, and how much you need to prepare to store. The size of the catchment roof area is based on the “footprint” of the roof, which can be calculated by finding the area of the building and adding the are of the roof’s overhang. The below image shows how the difference in roof slope doesn’t always change a building’s catchment area.


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