# 4" drain for downspouts big enough?



## Nate926 (Apr 6, 2014)

I just had a 36'x72' barn built. That is 2' from a 28'x48' barn long sides running parallel to each other. I have a gutter on one Side of the 36'x72' with two down spouts and gutters on both sides of the 28'x48'. I plan to drain 5 down spouts into one 4' pvc pipe that will drain 150' from the barns. Is 4" pvc pipe big enough to hold that much water? Thanks!!


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## Bonfire (Oct 21, 2012)

What kind of slope is the 150' on? I think at least a 6" pipe would be better. Your looking at 2640 sq ft right? Half the new one and all the old one?


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## Nate926 (Apr 6, 2014)

The first 72' would be about a 3' drop because it falls of down to a flat area at the end of the barn 2 or more feet below the barn. I will go the rest of the 150' probably another 2' so 5' to 6' slope total. It would be half the 36'x72' and 3/4 of the 28'x48' total footage of 2304' not accounting for the extra roof footage cause by pitch.


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## rjmoses (Apr 4, 2010)

Assuming I've done my arithmetic correctly, the rough calculations give:

Your collection area is 2640 sq ft. So, different rainfall rates give:

1"/hour = 220 ft3/hr =1650 gal/hr = 28 GPM (Normal rain)

2"/hour = 440 ft3/hr = 3300 gal/hr = 55 GPM

3"/hour = 660 ft3/hr = 4950 gal/hr = 82 GPM

4"/hour = 880 ft3/hr = 6600 gal/hr = 110 GPM (Toad strangler)

etc.

PVC piping can handle (green is gravity to low pressure)

Pipe Size *GPM* *GPH*
3" 140 gpm 8400 gph

4" 240 gpm 14,400 gph

5" 380 gpm 22,800 gph

6" 550 gpm 33,000 gph

For a 4"/hour rainfall, you would be using slightly under 1/2 the capacity of a 4" pipe.

No problem so far, until.....

Here's the web sites I used:

http://www.calctool.org/CALC/other/default/rainfall

http://www.calctool.org/CALC/eng/civil/hazen-williams_g

http://flexpvc.com/WaterFlowBasedOnPipeSize.shtml

Hope this helps

Ralph


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## Nate926 (Apr 6, 2014)

Rjmoses I sure do appreciate it! I am installing the 4" now!!


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