Runoff Potential from Various Roof Types: What Should You Know?

Runoff potential varies by roof type, with asphalt shingles averaging 0.04-0.07 inches of rainfall per square foot, while metal roofs can range from 0.03-0.10 inches. Understanding these values is crucial for accurate water cistern sizing. This knowledge can help you capture and store sufficient rainwater for off-grid applications.

Roof Types and Runoff Potential

Asphalt shingles are the most common roof type, accounting for over 80% of residential roofs. They have a moderate to high runoff potential, with an average of 0.05 inches of rainfall per square foot. This translates to approximately 0.5 gallons of water per square foot per inch of rainfall, or 1.25 gallons per square foot per 1/4 inch of rainfall.

Metal Roofs and High-Flow Potential

Metal roofs, on the other hand, have a higher runoff potential, with some styles reaching up to 0.10 inches of rainfall per square foot. This is due to their sleek design and smooth surface, which allows water to flow easily over the roof. For example, standing-seam metal roofs can experience runoff rates of up to 0.09 inches per square foot, while corrugated metal roofs may reach 0.08 inches per square foot.

Sizing Water Cisterns with Runoff Potential in Mind

When sizing water cisterns for off-grid applications, it’s essential to account for the runoff potential of the roof. A general rule of thumb is to increase the cistern size by 10-20% to capture the additional runoff. This ensures that you have sufficient water storage to meet your needs during periods of prolonged rainfall. For example, if you have a 10,000-gallon cistern and a 1,000-square-foot roof with an average runoff potential of 0.06 inches per square foot, you may need to increase the cistern size to 12,000 gallons to capture the additional 1,200 gallons of runoff.