Roof Water Runoff Calculator for Rainwater Harvesting
Every time it rains, a valuable resource flows freely off your roof and away into the storm drains. By harvesting this rainwater, you can create a free, sustainable water source for your garden, lawn, and landscaping, especially during dry summer months. The first step is to understand the amazing potential of your own roof. This calculator will show you just how many gallons of water you can collect from a single rainstorm, empowering you to plan your rainwater harvesting system.
How to Determine Your “Catchment Area”
For calculating rainwater runoff, we don’t use the roof’s actual sloped surface area. Instead, we use the horizontal footprint of the roof section you want to collect from. This is the simplest and most accurate method.
- Step 1: Identify the downspout where you plan to install a rain barrel or diverter.
- Step 2: Look at the roof area that slopes down to feed that specific downspout.
- Step 3: Measure the length and width of this section of your house at ground level. Multiply these two numbers to get your catchment area. For example, if the back half of your 25 ft. wide house feeds one downspout, and that section is 40 ft. long, your catchment area is 25 x 40 = 1000 sq. ft.
The Science Behind the Calculation
The formula for calculating runoff is simple and reliable. It’s a direct conversion from the volume of rain falling on a flat area into U.S. gallons.
The Formula: Runoff (Gallons) = Area (sq. ft.) × Rainfall (in) × 0.623
- The Conversion Factor (0.623): This magic number is the constant used to convert the volume from square feet of area and inches of rainfall depth directly into U.S. gallons.
- Runoff Efficiency: We then apply a 90% efficiency factor. In the real world, you can’t capture every drop. About 10% is typically lost to factors like wind, evaporation, splash, and water adhering to the roof surface (especially the first few drops). A 90% capture rate is a realistic estimate for a standard, clean roof.
Essential Gear for Rainwater Harvesting
Once you see how much water you can collect, the next step is setting up a system. Here are some of the most popular and effective products available on Amazon.
Good Ideas Impressions 50 Gallon Rain Barrel
This rain barrel combines function with style, mimicking the look of an oak barrel. It features a brass spigot, a debris screen, and an overflow port for a complete setup.
Check Price on AmazonOatey Mystic Rainwater Collection System
A diverter is the key component. It attaches to your downspout, channeling water into your barrel until it’s full, then automatically sending excess water back down the spout.
Check Price on AmazonSuperior Pump Submersible Utility Pump
A rain barrel spigot is gravity-fed. To get real water pressure for a hose or sprinkler, drop a small utility pump like this one into your barrel for easy water on demand.
Check Price on AmazonFrequently Asked Questions (FAQ)
Generally, yes, but with caution. Water from asphalt shingle roofs is fine for watering the soil around your plants, but it’s best to avoid spraying it directly on the leaves or edible parts of vegetables. Water from metal, slate, or tile roofs is considered cleaner. Always avoid collecting water from roofs with asbestos shingles or those treated with chemicals.
A 1,000 square foot catchment area will generate approximately 623 gallons of total runoff from 1 inch of rain. After accounting for a 10% efficiency loss, you can expect to collect about 560 gallons.
A first-flush diverter is a device that directs the initial flow of water from a storm away from your barrel. This “first flush” is important because it washes away the dust, pollen, bird droppings, and other debris that has accumulated on your roof, resulting in cleaner water in your storage tank.