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The Capture System
by Doug Pushard

You have decided to invest in a rainwater harvesting system, but where do you start? What questions do you need to answer before you can build the right system to harvest rainwater? Let’s start with learning how much rain you can collect.

Rainwater systems need not be complicated. They have been built and used for thousands of years. There are 5 major components to consider: Capture, Conveyance, Holding, Filtering/Purification and Distribution.

The Capture system is the place to start. For homeowners, it is the roof of your house. The size of your roof determines how much rain can be caught, which tells you how large the other components need to be.

Calculating the maximum amount of rain that can be caught on a roof is a straightforward equation:

Maximum Annual Gallons of Rain Capture = Annual Rainfall x Square Footage of Roof x .623 Gallons

The Annual Rainfall in your area is readily available on the Internet, in the local paper or from your local weather station (see related links for a listing of some US cities). The average annual rainfall in Santa Fe, New Mexico, for example, is 14 inches (i.e. .3556 meter).

The Square Footage of Roof is calculated by multiplying its length by its width. For example, the roof of a single story house that is 50 feet (i.e. 15.24 meter) long by 40 feet (i.e. 12.191 meter) wide has an area of 2,000 square feet (i.e. 185.8 square meters) on which rainfall can be captured and collected.

The last factor (i.e. .623) in the equation, is how many gallons are in an area of one square foot by one inch deep of rainwater.

So, putting these numbers together in one example, the Maximum Annual Gallons of Rain Capture on a 2,000 square foot roof in Santa Fe is calculated as follows: 14 inches x 2,000 square feet x .623 gallons = 17,444 gallons (i.e. 66,023 liters).

But the Maximum Annual Gallons of Rain Capture is an ideal. You need to reduce it slightly due to real world system inefficiencies.

The first reduction is due to the inefficiency of the roofing material. Roofing materials vary in their ability to deliver rain runoff. The least efficient roof type for capturing rainwater is a grass or "green" roof. If planned and installed correctly, almost no runoff would occur in light to moderate rains on a "green roof". These roofs are great for slowing down and eliminating rainwater runoff, but are terrible for rainwater catchment.

Other kinds of roofing material will allow most of the rain to be captured, but some will be lost due to splashing off the roof, water absorption, and other factors. Estimated efficiency ratings are:

  • For a tile/metal roof assume a 95% runoff efficiency
  • For a concrete/asphalt roof assume a 95% runoff efficiency
  • For a gravel roof assume a 70% runoff efficiency
  • For a bare soil roof assume a 75% runoff efficiency
  • For a grass roof assume a 17% runoff efficiency

Multiply the Maximum Annual Gallons of Rain Capture by the correct efficiency rating for your kind of roof. Using our working example and assuming a tile/metal roof, multiply 17,444 gallons by 95% (or .95) and you get 16,571 gallons (i.e. 62,728 liters) of Available Annual Harvestable Rainwater.

You now know approximately how much rain runoff you can harvest. Next start planing how to keep debris out of the system. Install screens on each of the downspouts or canales to prevent large contaminants from entering the system. Screens with small holes will clog quickly and force water over the gutters, causing water loss or, worse, leaks into the house. Choose a screen with holes that are larger than those of traditional window screens. Screening material of this kind is available from hardware stores. It will not keep everything out, but it will keep the large debris from entering the system and avoid unnecessary maintenance.

Next in line is your Conveyance system, which transports the rainwater from the Capture to the Holding system. However, before you can design the Conveyance system, you must think about how many tanks you may want or need and where they will be located. I will help you with that in the next article.

Related Links

Rainfall for US Cities (To add others, please send with source)
FREE Excel Spreadsheet for Calculating Maximum Capture
Supply and Demand Tank Sizing Calculations
Comparing Storage Alternatives
List of State and City Programs and Vendors

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Notes

1. Square footage of the roof of an "L" shaped house would be the sum of the two shapes (i.e. 20x30 + 20x20 = 900 square feet of roof area. On a two or more multi-story house, the calculation would be for only the nonoverlapping roof space.

2. For a round roof, the area calculation would be the Radius squared x π (i.e. 3.141592654). The Radius is the distance from the center of the roof to the outside roof line.

3. A sloped roof will be slightly larger than the base square footage of the house foot print due to the slope of the roof and the overhangs. However, these typically only add minimally to the area and are not usually included in the calculation.

4. If potable water is desired, than the roof material will be critical. Tile and non-painted metal are the preferred options for drinking water as the other materials will leach chemicals in the water that will be very expensive, but not impossible to remove.

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