Water Collecting Information

Water Collection information
On a 1-foot square flat surface area, one inch of rain will produce .62 gal. of water. Using this value, you can calculate the amount of water that will come down your downspout on a given rain.
This link will give you the yearly rainfall in your area. http://countrystudies.us/united-states/weather/ (Courtesy of countrystudies.us)

Using the drawing illustration below, if line A is 15 feet and line B is 50 feet you would have 750 sq. ft. of roof area and it will produce 465 gallons of water for every 1 inch of rainfall. If you use this figure to include the entire 30' x 50' building it is surprising at the amount of rainwater that a building or home will produce with a 1" rain. (930 gals)

 Star Symbol = Down Spout location
 Circle Symbol = High point of the gutter

Gutter Tips
The drawing below illustrates the amount of rain a different pitch roof will collect.  Assuming the rain comes straight down they are all the same, even though the roof area of a 12/12 pitch roof is more than that of a 4/12 roof the building area (footprint) is the same.

The difference will be the rate the roof will shed water, the roof material (metal, wood, asphalt) also regulates runoff.  Larger downspouts or more downspouts may want to be used for a 12/12 metal roof.  A standard rule for downspouts is that a 2 x 3 will drain 600 sq. ft. area, 3x4 1200 sq. ft. area.