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 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 DESIGNS which describes installation of septic drainfields on steep or rolling sites.
Use links just below or at the left of each page to navigate this document or to view other topics at this website. Green links show where you are in our document or website.
GUIDE TO 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
Technical Reviewers
Particular thanks are due to experts and also consumers who read these articles and suggest corrections, changes, and additions to the material.
- Daniel Friedman - principal author
- 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
- Technical reviewers are invited to comment or ask questions - contact us
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