Pitch of sewer line.

Old 04-21-05, 11:43 AM
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Pitch of sewer line.

I "know" that the preferred pitch (or downward slope) of the sewer line is 1/4 inch per foot (1" per 4'). But I also see flow capacity for sewer lines with pitches from 1/8" per foot up to 1/2" per foot.

I'm in a situation where if I maintain 1/4" per foot I'll have to put in a couple 45-degree bends to make the drop I need to connect with existing sewer at the bottom of the run. But if I run the line at 1/2" per foot I can do it all at a straight pitch without the 45-degree vertical jog (actually, I bet it will be closer to 3/8" per foot).

This will be on a 3" line carrying waste from toilet, lavatory, and shower. I just cannot find any discussion of when one can get away with either the 1/8"/1 foot or the 1/2"/1 foot drop. I'm actually doing this because the original line--as laid out by an expensive plumber--was too flat, didn't drain and caused toilet back up. He charged by the hours x number of fittings--and expected to be called back.

Thanks, W.
Old 04-22-05, 01:52 AM
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The trade accepted practice is described in minimums. 1/8"/12 for 4", 1/4"/12 for 3 ". A steaper pitch, over a long distance can create a siphoning effect wich will pull water out of the traps.
Old 04-22-05, 10:19 AM
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Thanks. I think I'll stick in the opposing 45s and keep my run to 1/4"/12". W.
Old 04-22-05, 08:21 PM
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How is a vented trap going to siphon? I have seen plenty of sewer lines with a lot more than 1/2" per foot of fall that have no problems. I have never seen a problem with too steep of a fall. I have heard people theorize that the water will flow too fast and leave the solids behind; I think this is an ol wives tail. Codes don't mind steep falls.
Old 04-23-05, 11:47 AM
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I could find nothing describing maximums for fall, only minimums and they indicate that it is too shallow a fall that creates the problem of leaving solids.

7-2. IPC Tables 704.1, Slope of horizontal drainage pipe, and 710.1(1), Building drains and sewers. These tables permit 3" horizontal drains to run at 1/8" per foot slope. All other model plumbing codes require that 3" and smaller drain piping be run at 1/4" per foot minimum slope. The 1/4" minimum slope assures sufficient flow velocity for the transport of solids. Two (2) feet per second velocity is the minimum recommended for soil and waste lines. A 3" drain at 1/8" per foot slope has a flow velocity of only 1.59 fps. A 3" drain at 1/4" per foot slope has a flow velocity of 2.25 fps. This is particularly important where 1.6 gpf water closets are involved due to the limited waste carry of some low flow water closets.

UPC Section 708.0 requires that horizontal drain piping be run at 1/4" per foot minimum slope where possible. It permits pipe 4" and larger to be run at 1/8" per foot slope when approved by the Administrative Authority.
Old 04-24-05, 06:24 AM
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For smaller sized piping, there are only minimum slopes. Only when you look at the extremely large piping in street sewers will you find limits on sloping. This is to slow down the flowing of water with suspended matter so it does not erode the bottoms of the mains. Usually a flow of 2 to 8 feet a second is the design norm

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