WARNING: This post gets dense!
Rainwater collection is another vital aspect of my farm operation. While a well is drilled, pumped and plumbed, I would prefer to save its operation for emergencies. I will be designing a system that uses as little energy as possible.
An aspect of large initial investment will be to instal gutters on the barn. Its 60 foot by 80 foot pitched roof serves as a perfect mechanism to harvest solar distilled water, aka precipitation. I want to do my own legwork in designing a system before I contact installers for quotes. GutterSupply.com has available a resource in this PDF Proper Gutter and Downspout Sizing.
An important piece of data to acquire is the rainfall intensity over a 5 minute period for 10 year and 100 year rainfall events. The National Oceanic and Atmospheric Administration (NOAA) provides a tool that allows you to retrieve data for a specific location either visually by panning and zooming in a map or by inputting coordinates.
Following the sample calculations found on page 6 of the Gutter Supply publication using my barn and location, the process is as follows:
Total roof area in two dimensions is 80 ft *60 ft = 4800 square feet
Roof pitch is vertical rise/horizontal distance from start to end of rise is 15 feet vertical over 30 ft horizontal * 12 inches/foot = 6 inches per foot (on my barn)
According to Table 1-1 in the Gutter Supply publication, the constant for my roof pitch adjustment = 1.10
Therefore roof area adjusted for pitch is 4800 square foot * 1.1 = 5280 square feet
Number of downspouts desired = 4 (my preference)
Roof Area covered by each downspout is 5280 square feet / 4 downspouts = 1320 square feet per downspout
Because of the poor amount of cities represented in the PDF, I compared my rainfall to a few cities using the NOAA tool. Knoxville had a 5 min rainfall total of .440 inches while my location has .437. Close enough.
Per the terribly formatted Table 1-2 in the PDF, 1 sq inch of gutter can drain 180 square feet of roof area. So 1320 square feet / 180 = 7.333 square inches as a minimum per downspout.
Finally, comparing the figure of 7.33 square inches to the Table 1-3 in the PDF shows that 4 inches of any shape gutters would perform adequately in a 10 year rain event.
Following the final 3 steps shows that I would need a minimum downspout size of 10.15 square inches in a 100 year event at .690 rainfall intensity. 4″ gutters would still serve in every shape except rectangular corrugated.
Wow Bravo if you stuck through that. None of the other posts in this series will be this technical!