Quick Answer
Estimating runoff from sloped roofs for water cistern sizing involves understanding the roof's surface area, slope, and rainfall intensity. A common approach is to use the Uniform Building Code's (UBC) rainfall intensity values. Calculations include determining the roof's surface area, converting it to a square footage, and then applying the UBC's rainfall intensity values to estimate the volume of water that will run off.
Calculating Roof Surface Area
To estimate runoff, you first need to calculate the roof’s surface area. For a rectangular roof, multiply the length by the width. For example, a roof with a length of 40 feet and a width of 30 feet would have a surface area of 1200 square feet. You can also use a general formula: A = (L * W) + (2 * (L * overhang) + 2 * (W * overhang)), where overhang is the distance from the wall to the edge of the roof.
Applying UBC Rainfall Intensity Values
The Uniform Building Code (UBC) provides rainfall intensity values for different regions. For example, in regions with a moderate climate, the UBC recommends a rainfall intensity of 2.5 inches per hour. To estimate the amount of runoff, multiply the roof’s surface area by this rainfall intensity value. Using the example from above, a roof with a surface area of 1200 square feet would yield approximately 3000 square feet * 2.5 inches/hour = 7500 cubic feet of water per hour.
Adjusting for Roof Slope and Other Factors
The actual amount of runoff may vary depending on the roof’s slope and other factors such as gutter size and downspout capacity. A steeper roof slope will tend to shed water more quickly, reducing runoff. Conversely, a flat or shallowly sloped roof may collect water and increase runoff. To account for these factors, consider consulting local codes and regulations or consulting with a civil engineer or architect familiar with water harvesting systems.
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