Part of designing a rainwater catchment system is determining how many rain barrels will be needed to catch the majority of run-off from your roof. To do this, you first need to know the following:
*How many gallons of water are produced by one inch of rain falling over one square foot? Answer: .6234 gallons
*What is the square footage of your roof or catchment area?
*How much rainfall is produced by a typical rainstorm in your area?
*What size are your rain barrels?
SIZING YOUR RAINWATER CATCHMENT SYSTEM: STEP BY STEP
1. Multiply the square footage of your catchment area by the number of gallons produced by one inch of rain falling over one square foot
2. Multiply the number of gallons collected form a one inch rainstorm by the amount of rainfall produced by a typical rainstorm in your area.
3. Divide the total number of gallons collected by the (gallon) size of rain barrel you will be using in the system. This will give you the total number of rain barrels (of a particular size) you will need.
*To place rain barrels, you will need to visually break the roof into potentially guttered areas, measure these areas individually, and use the above calculation to determine how many barrels are needed for each area.
Click here for a step by step example of how to size a water catchment system.