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We are currently connecting our small weekend vacation house—which includes a WC, shower, sink, and washbasin—to the main sewage line on the street. The construction company will install the sewage pipe from the street to our house. The pipeline will be fairly long and will include one inspection (revision) pit roughly in the middle. Since the pit on the street is already made, we have to go this long with the pipe. The final 6 meters of the line will be quite steep, as our house is located on a hillside. The minimum pipe burial depth is 0.8 meters (see drawing).

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We are responsible for installing the main sewage pipe out of our house. My question is: Is there a maximum slope that needs to be respected at the start of the line?

The company plans to lay the pipe at a depth of 0.8 meters, but due to several considerations (because of close located foundations we are not able to dig too deep), I would prefer to install our main pipe at a depth of 0.4 meters. Would it be possible to bridge this height difference with a steeper slope over a short distance? Or could this cause long-term issues with flow or system performance?

isherwood
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user15391
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3 Answers3

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In order for stuff to flow downhill, as opposed to water flowing downhill and leaving stuff behind to clog the pipe, sewage pipes have both a maximum and minimum slope. This is generally enshrined in the applicable plumbing code for your Local Area Having Jurisdiction. You should consult that if you are installing your own sewage pipe. It may also vary somewhat with pipe size (diameter), but usually is around 2%

If you have what seems to be a need to go at a steeper slope, what you actually need is to go vertical between sections of the correct slope, or dig a deeper trench. Stuff will fall down a vertical pipe, and then flow along with the water in a correctly sloped pipe.

If the local code requires a minimum depth of 0.8 meters, then burial at 0.4 meters would be both a violation of code and a reason to fail inspection, and a bad idea because such code minimum depths are generally based on good reasons, such as frost depth. You can go deeper, you can't go shallower.

Ecnerwal
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The minimum depth is usually controlled by the frost line.

You should contact the authorities for your area to make sure you do it correctly.

Having a frozen pipe will definitely mess up any winter getaway...

Solar Mike
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Where I live, sewage and drain pipes have required minimum slope and recommended maximum slope. There is no hard upper limit.

This stems from the way the water streams carry solids that are denser than the water itself. Both Archimedes force and hydrodynamic pressure effects are involved.

Too little slope and you allow water to flow over the solid, the stream is too slow to pick it up. There is no much of a "hydrodynamics" part.

Too much slope and a small amount of water can run around the solid, never engulfing it to exert much of an Archimedes force and not really exerting hydrodynamic force upwards over the steady object.

The particular slope limits vary from place to place so your best course of action is to look up the local regulations or ask a local expert.

fraxinus
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