"The rain water collected from off the roof helps plants and flowers grow bigger, faster and better and has done wonders for our garden. Hose water contains chlorine and fluoride which would have hindered their growth." great from under a deck
A typical house has a roof area of 1,200 square feet and four downspouts that will each drain about 300 square feet of roof. That means a rainfall of 0.3 inches will fill a 55-gallon rain barrel placed under each downspout.