Q: How do I calculate the amount of water coming off of a roof?
A: The amount of water collected by a roof is easy to calculate. Multiply the length times the width to get the area. Multiply this result by the number of inches of rain that falls. Multiply that by .6. The answer is the number of gallons of water you could recover. A 1000 sq. ft. roof would yield 600 gallons. Atlanta gets an average of 50 inches of rain a year. However, the rain is heaviest in spring and fall, not in summer when your plants need it. Get a good guide to rainwater harvesting and storage at http://xrl.us/RAINHARVEST.