Don Vandervort, Head Homeboy, has written more than 30 DIY home improvement books, been a segment host on HGTV, served as MSN.com's home improvement expert and written countless magazine articles.
The amount of rain you can collect in a rainwater collection system is determined primarily by the rainfall in your region and the size of your roof. The accepted rule of measurement is to determine the cubic feet of water that will fall
on your collection surface and then convert the cubic feet to gallons by dividing the monthly average of rainfall by 12.
Once you have that number, simply multiply it by the square feet (width x length) of the roof. The equation looks like this:
Inches of rain ÷ 12 x square feet of collecting space = gallons of water
So, by measuring the size of the collection area (that part of your roof that will drain into your storage container) and finding the local rainfall (monthly averages should suffice) you will be able to see how fast your storage containers will fill.
For example, Los Angeles receives, on average, .72 inches of rain in the month of April. With a 60-by-30-foot collection surface (1,800 square feet), a Los Angeles homeowner can expect to collect about 108 gallons in April. However, during the month of July, when rainfall averages only .01 inches, the same system will collect less than 1 gallon of water. Having a storage system that can hold several hundred gallons of water can help that homeowner make it through the dry season without having to use municipal water.
