Calculating Roof Runoff: Which Materials Work Best?

Choosing the Right Roofing Materials

When it comes to calculating roof runoff for a water cistern, selecting the right roofing materials is crucial. A combination of materials that allow water to flow freely, without compromising structural integrity, is ideal. For example, a metal roof with a 6-inch overhang on all sides can collect up to 0.7 inches of rainfall per hour, which translates to approximately 1,800 gallons of water per year for a 1,000 square foot roof.

The Importance of Roof Angle and Orientation

The angle and orientation of the roof significantly impact roof runoff calculations. A roof with a higher angle (30-40 degrees) can collect more rainfall, while a steeper roof (45-60 degrees) can lead to increased runoff velocity. When calculating roof runoff, consider the roof’s orientation relative to the prevailing wind direction to ensure accurate water collection estimates. For instance, a roof facing north will collect less rainfall than one facing south.

Measuring and Calculating Roof Runoff

To accurately measure and calculate roof runoff, consider the following factors: roof size, material, angle, and orientation. Use a rainfall intensity map to determine the average annual rainfall in your area, and then apply the National Roofing Contractors Association’s (NRCA) rainfall coefficient to estimate the total annual rainfall. For example, if your area receives 40 inches of rainfall per year, and the NRCA’s rainfall coefficient is 0.6, then your roof can collect approximately 24 inches of rainfall per year. This information will help you determine the ideal size of your water cistern to meet your household’s water needs.