Sewer line installation or replacement on steep slopes:

This article describes the installation of drain lines on steep slopes between a house and the septic tank.

Guide to installing the replacement sewer pipe line at Steep Sites

We present an actual case study, illustrated with photos of each step in the diagnosis and replacement of a blocked sewer line.

Proper drain line slope is important in avoiding septic or sewer line clogging and backups. Here we discuss

• Zig-Zag septic piping on steep hillsides
• U-turn septic piping on steep hillsides
• Septic piping along the fall line of a slope
• Steep septic designs used for sewer or septic piping replacement or new installations

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.

In our photo above 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.

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.


• 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 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.
  

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:

• See AEROBIC SEPTIC SYSTEMS, ATUs - home - some of these designs can work on steep slope sites
  
  Or See HOOT Aerobic Systems Drip Disposal Design & Installation Guide [PDF]
  
  Or
  
  See "Guidance For Design, Installation And Operation Of Subsurface Drip Distribution Systems As A Replacement For Conventional Title 5 Soil Absorption Systems For Disposal Of Septic Tank Effluent", [PDF] Massachusetts Department of Environmental Protection (2006) referencing MA regulations 310 CMR 15.240, 15.242, 15.247, and 15.280-15.289


• 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 gravel-less systems can handle gentle curves necessary to follow rolling slope lines


• SEPTIC 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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Reader Q&A - also see RECOMMENDED ARTICLES & FAQs

Issues with tank contents being stirred up by water shooting down slope and into tank at high speed?

I have a septic tank that’s 400 ft down a steep hill from my house and our effluent outlet filter gets clogged every 3 months. Only two adults and 5 year old in the house. No garbage disposal. Roughly 70 gallons water used a day.

Have you seen issues with tank contents being stirred up by water shooting down slope and into tank at high speed? I’m thinking my tank might be getting stirred up and stuff floating up into filter. I’m thinking of installing a second tee on inlet baffle so water coming in doesn’t shoot straight down. Thanks for any thoughts! On 2024-04-03 by Adam -

by InspectApedia Publisher

@Adam,

Thanks that's an interesting and helpful question.

The septic tank inlet baffle is intended to avoid both unwanted agitation from high velocity waste entering the tank and also to prevent floating scum/solids from clogging the tank inlet. But with very high wastewater entry velocity it might not be quite doing the job.

I am not sure just what is the arrangement of your current inlet baffle: is it a rectangular baffle or an actual PVC tee?

A tee should be delivering the wastewater downwards in the tank where its velocity is dampened. What we really don't want is the waste to flow directly, horizontally into the tank where it may break up the floating scum layer.

A review of our septic guides and manuals hints at what could be a design problem with your system: the size of the septic tank. You report a very small daily wastewater flow (I'd take another look at that calculation) - but beyond that, if a tank is too small for its usage, the result is insufficient settlement time - the tank remains agitated - regardless of the steep slope high speed waste entry.

So, what kind of tank and shape and volume do we have here?

At inspectapedia.com/septic/Septic_Tank_Size_Tables.php SEPTIC TANK SIZE you'll see typical tank sizing guidelines.

Let's double check the wastewater usage, tank baffle type, tank size, etc. before we start "fixing" the clogging effluent filter problem.

by Adam

@InspectApedia Publisher,

It’s a 1500 gallon concrete tank installed 10 years ago. We bought the house 6 years ago and I learned that no one was really living in house after the septic tank was installed down the mountain (old one was by house and failed but no room around house for new tank).

It’s rectangular box shape (not sure if there is better name for that. It’s rated for 660 gallons per day. I have a water softener system (was going to septic but 2 years ago routed it out back in woods) which has a water meter that shows daily and average daily water usage.

The tank was pumped in August 2023 and when I checked filter January 2024 filter was completely clogged. We do eat a lot of fiber so maybe more “food” making its way into toilet. The inlet baffle is pvc tee. I know that is slowing down water a bit.

I just can’t think of what is causing such frequent clogging. Added a lint filter on washing machine last time it was clogged (January) but didn’t seem to help because it was clogged again last week. Any other questions or thoughts? I appreciate you reading and taking time to think about it and respond!

by InspectApedia Publisher

@Adam,

Let's look more closely at the specifications for the effluent filter on your system. Effluent filters used at septic tanks are typically rated in capacity as some volume of gallons of wastewater flow per day.

There is an enormous range of filter price, design, and in wastewater handling capacity, from a low of 800 gpd to 10,000 gpd. or more.

In my OPINION rather than speculating about how to calculate the exactly correct effluent filter we need, or how much wastewater we're actually sending into a septic tank, we might recognize that these capacities are probably theoretical and that, as your own note suggests, conditions in septic tanks vary widely across quite a few factors that affect the cleaning frequency needed for the effluent filter, such as

in incoming wastewater volume and its constituents (solids, grease, etc.) velocity and agitation or septic tank shape, depth, volume, and inlet tee design - etc: factors that we've both mentioned.

SO: why not experiment by installing a much higher capacity effluent filter than the one you have already installed?

Below is a photo of the Sim/Tech STF-110 triple brush type septic effluent filter - detailed at https://www.rex-bac-t.com/Content/Documents/sim-tech-stf-110-effluent-filter.pdf - Rex-Bac-T Technologies, P.O. Box 661, Cumming, GA 30028 USA, Tel: (888) 886-1461 Web: rex-bac-t.com

I'm not saying that I know this is exactly the right product for you, but it does illustrate how easy it would be to triple the septic effluent filtering capacity at your existing system.

Useful reference:

Eliasson, John, Rule Development Committee Issue Research Report Draft: Septic Tank Effluent Values [PDF] Washington State Department of Health, retrieved 2024/04/03, original source: https://doh.wa.gov/sites/default/files/legacy/Documents/Pubs//337-105.pdf

Excerpt: Effluent leaving a conventional septic tank (one not equipped with an effluent filter) typically has concentration of 150 to 250 mg/L for BOD5, 40 to 140 mg/L for TSS and 20-50 mg/L for
FOG (Crites and Tchobanoglous, 1998).

Septic tank effluent from a tank with an effluent filter has different characteristics from unfiltered effluent. Typical effluent concentrations from septic tanks
equipped with effluent filters range from 100 to 140 mg/L for BOD5, 20 to 55 mg/L for TSS, and 10 to 20 mg/L for FOG (Crites and Tchobanoglous, 1998; Stuth, 2004).

Adding for emphasis: if you see a septic effluent filter clogging in just a week (when typically we expect to clean the filter once a year (or in some installations as often as every 6-8 months) - I have to suspect, as you do, that the tank is being agitated by either a design issue (undersized tank) or by high velocity entering wastewater, or by an under-sized effluent filter.

Sometimes the design issue can be subtle - at least to a novice. I

've inspected septic tanks installed in an area of shallow soil: the designer specified a 1500 gallon tank but had it built to be just about 30 inches deep - so shallow that there was never sufficient net free clear area between the settled sludge and floating scum - so suspended solids flowed from the tank and clogged the drainfield almost immediately.

Experts have commented on this and even given velocity calculations for water in steep pipes

• Montana DEQ, Flow in a rectangular shaped channel can be calculated by multiplying the velocity of the flowing stream by the channel area (calculated as width x depth of flow). " - Onsite Wastewater Treatment Manual [PDF] Montana DEQ, - retrieved 2024/04/03 original source: https://deq.mt.gov/files/Water/TFAB/Documents/SOPs/On-SiteWastewaterManual.pdf
  
  (Note: most wastewater velocity discussions are focused on the opposite problem: inadequate velocity of wastewater movement through a piping system leads to clogging).
  
  The slowing of velocity of wastewater entering the septic is essential to allow solids to settle, floating scum to rise and coagulate at the surface, and for suspended solids to be reduced before the effluent flows out of the septic tank. Otherwise the soakaway or absorption field will have a short life.

Uphill septic needs an effluent pump-up system

I have plans to purchase new construction with the slope uphill from the house. The drain field will be higher than the property because of the slope. What should I be concerned about? On 2023-04-03 by Ken -

Reply by (mod) -

@Ken,

You'll need an effluent pump-up system; if you're in a freezing climate be sure that it's protected from freezing.

The advice above on this page has some key suggestions - like being sure that the drainfield trenches run across the slope, not up-and-down slope.

The usual septic design concerns remain key: soil perc tests, depth of cover, available area, clearance distances, including from any nearby waterway, and what local codes permit.

Where is the property located? Country and nearest city are sufficient.

If you have a specific question I'll be glad to pursue this topic. I don't think an effort to re-type the whole set of instructions for steep slope systems again here would be the best approach.

Thanks for the question - DF

by Ken

The property is located in Lake Stevens, WA. Thanks with providing your knowledge and unbiased comments. Any additional advice you have is welcomed. I am more than nervous and on the fence with purchasing this property because of the slope going uphill from the 1st stage of the septic leading to the leach field.

by InspectApedia Editor

@Ken,

Sure, we're glad to discuss this with you. Working together helps us both.

Be sure to read our article on this topic

STEEP SLOPE SEPTIC DESIGNS which describes installation of septic drainfields on steep or rolling sites, and post any follow-up questions there.

Do I need to worry about where the steep slope changes to flatter slope?

A house is at the top of a hill, septic tank is located nearby also on the top of the hill. The drain field is 350' away down the hill. The pipe exits the septic tank and runs down a steep slope for 100' along a driveway, so not a drop-off, but a rather steep driveway nonetheless.

The pipe then flattens out to a normal fairly flat slope for the rest of the run to the drain field.

The pipe in the flat area has been freezing. It has insufficient cover.

So does the steeply sloping part, but the steep slope hasn't been freezing, presumably because of the swift speed of the water. The pros speculate that there is a belly in the flatter portion of the pipe due to settling, but this hasn't yet been confirmed by inspection.

Soon, the flat area will be uncovered, inspected and replaced or otherwise corrected, including removing any belly and either burying it deeper or insulating it. Insulation or cover will also be added to the area where the pipe changes from steep slope to flat slope.

My question is: given that the pipe contains effluent only, and assuming that a proper slope and cover are achieved in the flatter portion of the run, how much concern should there be about the location where the steep slope changes to flatter slope?

I can imagine turbulence at that spot, but as long as the flat slope has a sufficient drop and sufficient cover or insulation, I'm hopeful that the change in slopes is not a problem.

Any thoughts would be appreciated. On 2022-03-24 by Richard -

Reply by (mod) -

@Richard,

When there's only effluent in a pipe we worry less about solid waste slamming into the side of a pipe or elbow and sticking to form a clog (unless solids are escaping the septic tank)

But where flow slows down
OR leaves effluent in a sag (belly) in the pipe, freezing is likely.

A running toilet that sends a slow trickle of water into the drainfield can cause freezing too.

Try running a sewer line cam through the lines to see if there's a belly or sag.

And check for slow leaks, drips, running toilets.

by Richard

@Inspectapedia Com Moderator, thanks for your recommendations. The line has recently thawed and the camera is on the way, so any bellies soon will be rooted out, so to speak. I'll also put some thought and investigation into potential continuous water supplies. I'll report back here with any interesting findings.

How to make a 90 degree turn in toilet waste line

On a corner that involves toilet waste flow. Better to make the 90° corner with a 1° drop rate per usual or if trenching allows 4" deeper depth, make the corner with 2, 45° corners while dropping the 4"? Thanks On 2021-07-31 by hi guy -

Reply by (mod) -

@hi guy,

Wider sweep turns - 2 45s - will generally flow better than any sharper turn.

What's the maximum drop allowed on the effluent septic line?

We are installing a conventional tank near at our house. We are wanting to run the effluent to our aerobic tank 225 get away that's on place already.

What is the maximum drop % wise per foot for the effluent line?

The aerobic tank is about 8' lower than the conventional tank. - Thank you On 2021-07-20 by Ted -

Answer by inspectapedia.com.moderator (mod) - 1/4" per foot = 6.25% slope

@Ted,

If a sewage line slopes more than 1/4" per foot the water tends to leave the solids behind;

You may need to go to a grinder sewage pump and force main;

Or you can route line along the slope at 1/4" per foot then make turns to zig zag to the destination.

by Ted

@inspectapedia.com.moderator, if raw sewage yes. What about if it's been in a septic tank breaking down like it would be going to a leach field. There is no sewage in that, correct? That is what would be going to the aerobic tank.
Thanks

by inspectapedia.com.moderator (mod)

@Ted,

If you are only moving clarified a fluid then there are should be no solids that would be left behind in the Wastewater. But you certainly cannot consider that water to be sanitary. That's a different point.

by Ted

@inspectapedia.com.moderator, yes of course not sanitary. Would that clarified liquid be fine to flow at sub-grade @ a 8' drop in 225'? Thanks

Reply by (mod) -

Start with your building or Health Department to find out what design they will consider acceptable in your country and community. What does your contractor or septic engineer say?

by Ted

Thinking of running 2, 1k gal tanks in a series, then pump out to, 2 sprinklers rather than installing an aerobic tank. We are short on service people for maintenance and our ground would be better served i think with a sprinkle system. Thoughts? Thanks

How would a long distance of 500 feet to sloping down to the drain field be handled?

How would the septic tank and pipes be arranged when the approved drain field area is 500 feet from the house.

Also there is a rocky 40 foot hill along the run. The slope before and after the hill is relatively flat, sloping very gently towards the drain field. What are the issues involved and where should the septic tank be located.

Reply by (mod) - maintain the proper slope or zig-zag the lines down the slope; use an effluent pump if your line has to also go uphill in sections

The drain line needs to have consistent and proper slope of 1/8 to 1/4" per foot and should have a number of access points or cleanouts along the run.

The tank is usually located close to the building.

If you can't maintain that slope (500 ft. x 0.125 per foot) = 62.5 inches of drop required - a better design might be to use a septic effluent pump to push the septic tank output to the drainfield.

Watch out: if you're in a freezing climagte be sure to keep your piping below the frost line or to provide insulation and if necessary, heat.

 

Septic drain slopes back towards the house - clog prone: do I have to do it over?

The drain pipe at the point it goes out of the house is slightly slopes to the inside of the house
It stays full all the time.

Several times it has clogged at some point from house to septic tank.
Should I have the angle redone coming out of the house? On 2019-05-21 by Jimmy -

by (mod) - Problems and solutions for a septic or sewer line sloped the wrong way

JImmy

A sewer or drain line sloped in the wrong direction is going to have repeated clogs and in a freezing climate risks freeze-up, breakage, or worse a sewer backup into the building.

The whole sewer line ought to be inspected (perhaps with a sewer camera) and any slope errors should corrected.

If you cannot fix the slope because the septic tank isn't deep enough, I suggest converting to a "force main" sewer line that uses a grinder pump to push waste into the septic tank from the house.

...

Continue reading  at STEEP SLOPE SEPTIC DESIGNS or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see SEWER / SEPTIC LINES for STEEP SITES FAQs - questions & answers posted originally at the end of this page.

Or see these

Recommended Articles

• DIFFICULT SEPTIC SITE DESIGNS
• SEPTIC CONSULTANTS, DESIGNERS, ENGINEERS
• SEPTIC SYSTEM DESIGN ALTERNATIVES - home
• SEPTIC SYSTEM DESIGN BASICS - home
• SEWER / SEPTIC LINES at STEEP SITES
• SOIL PROPERTIES & BUILDING FAILURES
• STEEP SLOPE SEPTIC DESIGNS

Suggested citation for this web page

SEWER / SEPTIC LINES at STEEP SITES at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

Or see this

INDEX to RELATED ARTICLES: ARTICLE INDEX to DRAIN SEPTIC SEWER PIPES

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