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I have a 4" PVC drain from a downspout that runs parallel to the garage for about 25' before joining another drain. This run is about 12" deep and with a very slight grade. While the driveway is being repaved, I am replacing the thinwall PVC with schedule 40 (the pipe was clogged because the inlet was slightly below grade and had filled with mud and gravel over the years).

Is the 12" (approx) depth adequate? What should the pitch be (there is room to lower the downstream end as much as 12" if needed)?

  • Pitch should be between 1/8" and 1/16" per foot. 1 inch per 10 foot section of pipe is a handy number that works. Less steep, water does not flow well. More steep, any debris in the water tends to be left behind rather than carried along...and the pipe plugs eventually. – Ecnerwal Nov 2 '15 at 1:36
  • Do you live where it freezes? If so, have you considered how deep it freezes, in the worst winter on record? – Yehuda_NYC Nov 2 '15 at 1:36
  • Yes, here in Western CT, it can freeze 20-30 inches deep. However as Jack points out in his answer, it's not likely that there would be water running into the pipe at that temperature. I suspect its proximity to the slab (about 12" away) would help a bit; it's an unheated attached garage, but the interior rarely goes below 40. – TomG Nov 2 '15 at 1:40
  • FYI, as a commentor, I only get notified of your post if you put @name before your reply. – Yehuda_NYC Nov 2 '15 at 15:02
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In my opinion you should be good as it was for depth, moving to Sch 40 is a good upgrade, that is what the company I worked for always used under slabs and especially driveways.

The main thing is that there are no low areas for water to collect and freeze. Depending on your area, it would be a hard freeze to get ground to freeze at the 12" depth, there would be exceptions as there is always is, but in a hard freeze there should be nothing running off the roof to freeze in the drain.

Although not needed, to increase the fall would be good for heavy rains, but for freezing, the benefit would be marginal.

  • It's been re-buried, the original pitch was about 1" in 8', so I left it as it was. – TomG Nov 3 '15 at 0:40

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