guttters / downspout drain line - what's the minimum slope?


Old 10-03-18, 05:42 PM
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guttters / downspout drain line - what's the minimum slope?

I had 4" triplewall pipe put in the ground to take water from downspouts to the woods.

At the farthest downspout, I can look down and see standing water. I dug up 35' of it as it runs along the front of the house. There were loads of big rocks in there (I remember the guy saying they hit lots of rocks when trenching and just put them back in the ground.)

Over time, the rocks pushed the pipe up a bit / created high spots?

People here gave me the great idea for getting the right slope to put a piece of wood on one end of a level. I got a 5' level and put a 1 1/4" piece of wood on 1 end.

(so 1.25 / 5 = 1/4" slope?)

but over 35', that's almost 9" rise. Because of the grade of the yard, etc, that makes it a few inches above the ground at the far end (and to think it was buried previously / the slope was never right). I tried lowering the low point to give me more margin to play with, but it goes under a sidewalk / can't lower it more.

Any suggestions?

What's the minimum slope you'd do on a downspout drain line?

Any problem with it coming out of the ground / me just covering it with mulch at the far end : ) ??
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Old 10-04-18, 04:52 AM
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Water flows downhill. How much slope determines how fast it moves. The general rule of thumb is 1/4" per foot but any downhill slope will move water. The gotcha is the shallower the slope the less margin for error. If you've only got 1/16 or 1/8" per foot slope it doesn't take much of an imperfection to create a high spot that will stop the flow. So, more slope is better.

Whether or not going above ground is OK depends on where you are located. If you get freezing and the pipe is near or above the surface the water in the pipe may freeze causing a clog. But, downspout drain pipes are often above ground. If they have enough slope water freezing inside isn't much an issue.
Old 10-04-18, 07:03 AM
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As always, thanks!

great point: less slope - less room for error ; )
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