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The drawing to the right 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 (foot print) is the same. The difference is the rate the roof sheds water. The roofing material (metal, wood, asphalt, tile) also regulates run off. Larger or additional downspouts may be needed for a 12/12 metal roof. A standard rule is one 2" x 3" downspout per 600 sq. ft. area, and one 3" x 4" per 1200 sq. ft. area. -Technical Information- -One foot of 5" K-Style gutter holds 1.2 gallons of water. -One foot of 6" K-style gutter holds 2.0 gallons of water. One inch of rain on a 1 foot square flat surface area will produce .62 gallons of water. Using this value, you can calculate the amount of water that will come down your downspout on a given rain. Using the illustration to the right, if line A is 15 feet and line B is 50 feet you would have 750 sq. ft. of roof area and will produce 465 gallons of water for every inch of rainfall. If you use this figure to include the entire 30' x 50' building, it is surprising that the amount of rain water produced is 930 gallons with a 1" rain! (30 x 50 x .62) |
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