Sewer Line Installations on Steep Slopes
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• How to install sewer or septic lines at steep sites
• Proper drain line slope is important in avoiding septic or sewer line clogging and backups

What is the proper slope or pitch for a drain waste pipe? When wastewater flows at the correct speed in a drainpipe, the water carries the solid waste, feces and toilet paper, for example, along with the water out to the septic tank or sewer mains. Plumbing codes and wastewater piping guides commonly specify that building drains should be pitched at 1/8" to 1/4" of slope per foot of linear run or distance.

Problems with steep drops between house and septic tank: But at a steep building site such as the property shown in our page top photograph, the drop in elevation between a building main drain and the septic tank inlet opening (or sewer main connection) may be so much that the slope of the waste piping will exceed the recommended limits for slope.

When installing a building drain line for wastewater, the wastewater flow rate we want is 2 feet per second. If waste flows in the sewer line faster than 2 fps, the risk is that water leaves the solids behind in the pipe where clogs may be recurrent.

If waste flows in a septic or sewage drain line more slowly than the desired speed, the risk is that is that listless flow leaves sewage which blocks the pipe.

ZigZagging Drain Line Piping Down a Slope

One approach to achieving the proper wastewater flow rate in a drain line down a steep slope would be to zigzag the pipe down the slope, making numerous turns. My OPINION is that the added turns and length of this approach may increase the chances of future sewer line blockages, the zigzag drain line approach will also make it more difficult to clean out blockages, or you will need to add sewer line cleanout access points at every run and turn in the installation.

In short, a zigzag pipe run down a slope costs more, has more turns that risk clogging, is harder to clean out, takes up a lot more space on the site, and is usually not installed by contractors.

Straight-run Drain Line Piping Down a Steep Drop Slope between House and Septic Tank or Sewer Main

My experience is that many waste line installers simply install a straight sewer line from house to septic tank or house to sewer main, regardless of the building slope, provided that we have at least 1/8" per foot, preferably 1/4" per foot, or more.

Lower slope or nearly-flat drain lines are more likely to clog. On that side topic, if you're installing a drain line that may be sloped too little and you cannot easily remedy the situation, be sure to install extra cleanout access ports.

Experience in Installing Steep Sewer Drain Line Piping

Steep Drain Slopes May be Tolerable

My OPINION is that if the total pipe run is steeper than what's normally specified, you might never see a blockage

 At the property shown in these photos and in the other photos in this series on sewer line replacement, the sewer line drops on a slope between 2” and 3” per foot over a 40 foot run between house exit line and septic tank inlet baffle. This house-to-septic tank drain piping should be in the “danger zone” of leaving solids behind liquids during a toilet flush

But having managed this property for more than twenty years, we can say with confidence that we never had a problem with too-fast drainage blocking the waste line. Blockages did not occur when the original waste pipe was constructed of jointed clay pipe. Sewage blockages have not occurred since we replaced the original clay with plastic piping

I suspect that plastic drain piping is more slippery inside and has fewer joints to serve as obstructions, further reducing the chance of a blockage. The only problem we had with the line was when the old clay line was crushed and then blocked by mud

SO if you use plastic piping, straight runs, good smooth joints, you will probably be ok

Other Steps to Avoid Problems with Septic or Sewer Drain Lines on Steep Sites

• Septic or Sewer Drain Line Cleanouts: For convenience I’d add outdoor cleanout access ports to the sewer line every 20’ or so
  
  
• Proper septic pipe connections: Make sure that the new piping connections are made correctly, lubricated and fully seated during assembly. In our photo at left the grease helps assemble the plastic sewer piping joint. The black line on the right hand pipe section tells the installer when the pipe sections have been fully joined.
  
  
• Proper sewer pipe directions: The receiving pipe hub or "female" end of the pipe is the next downhill section. Don't do this backwards or you invite leaks or clogging in your sewer line
  

• Use smooth drain line connections: the hub-less drain pipe connector shown in our photo at left was used to connect the new plastic waste line (running downhill to the septic tank) to the old cast iron waste line at its point of exit from the building.
  
  It was important that these pipes be properly aligned (to avoid leaks at the connector) and that the connection and pipe ends be filed smooth to minimize the chances of waste line blockages at this point in the system.
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You can see the entire drain replacement project, from house to septic tank, beginning at: GUIDE TO SEWER LINE REPLACEMENT

Installing Septic Drainfield Piping on Steep Slopes is a Different Matter Entirely

Just to be clear, in this article we've discussed the installation of solid piping between a building and its septic tank or sewer main and we've said that steeper slopes may be ok.

But the possible "OK" for steep drain piping does not in any way apply to the perforated piping installed in a septic drainfield gravel trench.

Readers concerned with septic installation advice for steeply sloped or rolling sites should see these articles:

• Sewer or Septic Line Installation at Steep Sites which describes sewer piping between a building and septic tank or sewer main at steep sloped sites which slope down significantly between the building and the septic tank.
  
  
• DOSING SYSTEMS PRESSURE which describes pressure dosing systems that may be useful for disposing of septic effluent at steep sites where the drainfield must be located either uphill or downhill from a septic tank or building.
  
  
• GRAVELLESS SEPTIC SYSTEMS- some gravelless systems can handle gentle curves necessary to follow rolling slope lines
  
  
• Pumps Septic pumps, sewage ejector pumps, grinder pumps, effluent pumps, sump pumps, & septic pumping stations, septic pump alarms - systems that will be needed if a building or septic tank are lower than the drainfield or sewer main.
  
  
• STEEP SLOPE SEPTIC DESIGNS which describes installation of septic drainfields on steep or rolling sites.

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SEWER LINE REPLACEMENT
  Diagnosing a Blocked Drain
  When to Call a Plumber
  How to Locate the Main Building Drain
  How to Use a Power Snake on Building Drains
  How to Find Distance to Drain Blockage
  Determining Need for Replacement
  Replacing the Sewer Line, Step by Step
  Making Other Septic Repairs
  Installing the new sewer line
  Bedding the sewer line in Sand
  Final Backfill of the Sewer Line Trench
  Documenting Buried Components
  Final seeding and soil restoration
  Sewer or Septic Line Installation at Steep Sites

• Thanks to Thomas Gleason, excavators, Poughkeepsie, NY 845-454-3730, for the excavating work at the project photo documented here in September 2006
• Thanks to Cleveland Plumbing, the prime plumbing contractor, Staatsburgh, NY 845-485-7700 for the plumbing work and drain clog diagnosis work documented here in September 2006

Contact Daniel Friedman for website content suggestions or for fee-paid consulting