Septic Tank Depth:

This article describes how to find the depth of a buried septic tank. Sometimes we can get the septic tank depth immediately by simple visual observation. But if the septic tank lid or top cover has been buried and we've not dug it up recently it may take some measurements and estimating to have a good idea just how deep the septic tank top is below ground.

A companion article linked at the end of this one gives a separate description of the design depth for septic tanks - how deep should a septic tank be?

Our page top photo shows us measuring the distance from house to septic tank nearest-corner (blue arrow) and also a septic tank cleanout-riser (red arrow). Looking through the septic tank riser top told us that we'd not have to remove much soil to expose the top of this septic tank.

How To Find the Actual Depth of a Septic Tank Below the Ground Surface

[Click to enlarge any image]

Article Series Contents

• SEPTIC TANK DEPTH - this article
• SEPTIC TANK DESIGN DEPTH - separate article

Reader Question: septic tank depth must permit adequate drain line slope

(Feb 12, 2014) Anonymous said: ©

If my sewer discharge pipe is 6 ft+ below the ground surface does the septic tank inlet not have to be slightly lower in order for the waste to flow into the tank. Can a person use an effluent pump to discharge the waste to the field or do you use the effluent pump before the septic tank.

Reply:

A waste line drains by gravity and slopes between 1/8" and 1/4" per linear foot. If your drain line had to run 20 feet from house outlet to tank inlet, the tank inlet has to be at least (20 x .125) " lower than the elevation of the waste line where it exits the building (unless your septic system is using a sewage pump or ejector pump).

At 1/8" per foot minimum slope that's (20 ft x 0.125 " per foot = just 2.5 inches below the house main waste line depth - the fat green arrow #9 in our sketch above. At 1/4" per foot that would place your septic tank inlet depth at 5 inches lower than the green arrow #9.

Our schematic above shows the measurements involved in finding the depth of a septic tank and should assist in understanding and picturing the septic tank depth location steps that we give just below.

How to Estimate, then Find the Actual Depth to the Septic Tank Lid in 10 Steps or Less

1. Find the septic tank: First, if we don't know where the septic tank is buried, we have to find the septic tank location. If there are septic tank risers, or clean-out ports poking up above ground then we know the tank location and we can go on to subsequent steps in this list.
   
   But if there are no visual clues (pipes above ground) then we can still make some intelligent guesses at where the tank is by inspecting the building piping and the site conditions to see where a septic tank could reasonably be expected to have been buried.
   
   Details of these procedures are
   
   at SEPTIC TANK, HOW TO FIND where you'll see several approaches.
2. Inspect Septic Tank Cleanouts or Risers: if there are one or more septic tank cleanout ports or risers or access ports (arrow #4 in our septic tank depth sketch above) that are above ground or close to ground level and perhaps marked by a stone, it's pretty easy: open the access port top or cover and measure with a tape the distance down to the top of the sewage effluent. See the orange arrow #6 in our septic tank depth sketch above.
   
   If the septic tank is in use and is full to its normal level, the top of waste in the tank will be a few inches under the under-side of the septic tank lid.
   
   If the tank is steel, plastic or fiberglass, then that's about the depth to the surface of the septic tank lid, with some variation if the upper surface of the septic tank is round rather than flat. If the septic tank is made of concrete then the dept from ground top to septic tank top is about 4-12" less than the depth of the septic sewage level from ground top.
   
   Also see SEPTIC TANK RISERS
3. Use the building main drain exit point + distance x sewer line slope: If there are no septic tank risers or access ports that we can find, there are still some clues about possible septic tank depth, provided that the system works by gravity.
   
   A typical septic system into which sewage or wastewater flows by gravity must be lower than building's main drain exit point, and sewer piping between the building main drain and the entry port of the septic tank must slope between 1/8" and 1/4" (or more) per foot for wastewater to flow into the septic tank.
   
   Our septic tank depth sketch above shows all of the measurements and measurement points that might be involved in finding the septic tank depth using this approach.
   
   More about typical sewer line drain piping depths is
   
   at DRAIN FREEZE PROTECTION
   
   With these concepts if we know the distance of the septic tank from the building we can calculate the approximate septic tank minimum depth below ground. We'll need to take into consideration the terrain shape. When ground slopes away from the building foundation wall (which we hope it does) then the septic tank depth will of course reflect that slope as well (as we show in our sketch).
4. Main drain exit point: Find the point at which the building main drain exits the foundation wall - Green Arrow #9 in our sketch.
5. Distance to septic tank: Measure the distance to the septic tank's nearest corner - Gray Arrow #10 in the septic tank sketch
6. Main drain exit depth: Measure or estimate the depth below ground at which the building main drain exits the structure. This is the outdoor sewer line starting depth - Green Arrow #9 in our sketch.
7. Compute minimum depth of the septic tank wastewater entry: Multiply the exit point to septic tank distance by 1/8" per foot to get the minimum depth of the septic tank entry piping connection. This will be the depth of the bottom of the septic tank wastewater entry pipe. This depth will be the same as the top of the septic tank wastewater level - Orange Arrow #6 in our sketch above
8. Measurements give the minimum or the actual depth to the top of the septic tank - Red Arrow #5 in our sketch - will be about a 12" to 24" above this entry point in step 4, allowing for the additional thickness or dimension of:
• the diameter of the entering septic or sewage wastewater pipe, typically 4" to 6" for residential properties
  or
• the air space between the top of the wastewater in the septic tank and the under-side of the septic tank lid, typically this distance is 6-8" in a septic tank.
  Watch out: if the sewage or septic wastewater is right up to the under-side of the septic tank then something is seriously wrong.
  
  See SEPTIC TANK LEVELS of SEWAGE
  
  and (plus)
• the thickness of the septic tank lid itself, ranging from about 1/8" to about 4" depending on septic tank material and tank lid structural requirements discussed
  
  at SEPTIC TANK DESIGN STRENGTH SPECS
  and (plus)
• the height of any septic tank cleanout or access risers and their lids - Blue Arrow #4 in our septic tank depth sketch above
  
  See SEPTIC TANK RISERS -
  and (plus)
• the depth of soil backfill over the septic tank lid or septic tank riser lid, ranging from 0" (which means you should see it) to just a few inches (which means grass may be dead in this area) to 6-12" or even more.
9. Consider (add or subtract) the effects of ground slope - Gray Horizontal line #7 in our sketch above - on possible septic tank depth below a horizontal line projected from the house sewer line exit point to the septic tank.
   
   If the terrain slopes away from the building by more than 1/8" to 1/4" per foot, then the septic tank will of course be further below a horizontal line projected from the building and it may be closer to the ground surface since the sewer line in this case will already be sloping more than the standard slope range of 1/8-1/4" per foot.
10. Dig: ultimately the precise depth to the septic tank top from the ground surface - Red Arrow #5 in our septic tank sketch - will be confirmed by excavating down to the top.
    
    Naturally site conditions and installer discoveries during original septic tank installation can cause considerable variation in septic tank depth, from septic tanks that are actually above ground to those that are deeper than the minimum depth for proper sewer line slope.

But it's reasonable to consider that an excavator who is digging the hole to install a septic tank would have little reason to dig deeper than necessary to bury the tank deeper than the depths we described

at SEPTIC TANK INSTALLATION DEPTH discussed earlier.

Our photo (left) is a close-up of the excavation to expose a septic tank cover as shown at the top of this page. This septic tank is located in Northern Minnesota. The page top photo includes a red arrow pointing to a permanently-installed (though a bit small) septic tank riser used for periodic septic tank pumping.

Also see DRAIN FREEZE PROTECTION where we describe the depth of sewer and septic system piping.

This chapter also explains how to calculate septic tank volume based on septic tank inside dimensions measured in feet, and we discuss the sizing, installation, and functions of septic tank tees to prevent septic system clogging.

Links to related septic system testing and design information are given at the ARTICLE INDEX.

 

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Reader Q&A - also see RECOMMENDED ARTICLES & FAQs

On 2021-05-11 by Mary Ann - circular settlement seen in our yard after hookup to public sewer

Our current house was built in 1954 in PA, with original cesspool or septic in backyard. In 1962, house was connected to public sewer

. Each spring, there ~4 foot diameter circular settling (gently tapers to about 6 inches deep at center) in the ground above where we believe the old cesspool/septic is located.

We are concerned the tank was not properly decommissioned. So 2 questions:

1) What type of engineer would we need to hire to investigate this? and

2), Is there any legal recourse if the tank was NOT properly decommissioned by either the plumbing firm who made the public hookup and/or local/state agency who was responsible to oversee/ensure proper decommissioning?

On 2021-05-12 - by (mod) -

@Mary Ann,

Typically you'd hire not an engineer - but a septic contractor - who would excavate sufficiently to discover if there is a safety hazard (an unsafe tank cover and the possiblity of someone falling-in - which can be very serious, even fatal) and to let you know if additional excavation and/or fill are needed.

Meanwhile, rope off and keep people away from the area of subsidence and mark it as unsafe.

Legal recourse? In my OPINION that's the wrong focus where immediate safety hazards deserve first the steps I cite above.

On 2021-01-29 by timothy mathews - can my septic tank be buried 12 feet deep?

can I use a a 12 ft deep tank for septic tank

On 2021-01-29 - by (mod) -

That's a bit deeper than we usually see for a residential septic tank - but the depth isn't the whole question; it's overall tank shape - see the tank size tables above on this page where septic tank shape and dimensions are included.

On 2020-11-11 by Kedir

How deep is a septic tank be below ground water table

On 2020-11-15 - by (mod) - How deep is a septic tank be below ground water table

Fair question Kedir but that's not a specification I've seen.

The top of a septic tank is usually close to ground surface, sometimes 2-4 deep, with variations as we explain in the article above.

Certainly the bottom of the tank might be in seasonal groundwater, but at the very least the tank must be installed and sealed to be darn sure that groundwater never leaks into the septic tank nor its piping.

Watch out: if the septic soakaway or drainfield or leachfield is also ever being flooded or is less than 24" above the seasonal high ground-water level, then the system cannot successfully treat pathogens and is likely to contaminate the envrionment including nearby water wells or waterways.

On 2020-10-21 by Anthony - how deep will my septic tank be if ithe sewer line starts into the ground 5 ft. away?

If my tank is set 6 Feet from the house and the sewage line starts running into the ground at the 5 Ft. Mark how many inches is it from ground level to the intrance

On 2020-10-21 - by (mod) -

Sorry Anthony but nobody can say for sure without doing some digging.

A waste line, properly installed, slopes 1/8" to 1/4" per foot of horizontal run. So over just one foot, the bottom of your horizontal waste pipe would be just 1/8 to 1/4" lower than where it started.

At that depth and over only one foot of horizontal run you'd be looking right at the actual septic tank.

So we must be missing something here: perhaps an elbow in the piping?

On 2020-05-13 by Anonymous - why is your septic system buried so deeply?

why is your septic system buried so deeply (4 feet down) in your yard?

On 2020-05-13 - by (mod) -

Anon

The depth of a septic tank or drainfield might be chosen by the installer because of terrain shape or features or even frost depth. For example in Northern Minnesota a conventional drainfield may be deeper than in high warm sandy soil in Florida.

But

Watch out: a deep drainfield may "work" in that it successfully **disposes** of septic effluent, but a field that's too deep is discharging sewage effluent (containing bacterial and other pathogens) into the environment, into local water streams, lakes wells, without sufficient treatment by soil bacteria.

That's because deeper in the soil there is less oxygen so we have only anearobic bacteia (half of the bacterial crowd that's needed to break down septic pathogens) and won't have aerobic bacteria (the other half, who want more oxygen).

On 2020-03-10 by Bill - My Yard is saturated and the field lines can move water - chop a hole in the septic tank top to let the sewage out?

This is not normal rainy season... the massive amount of rain is rare and causing many people problems.

My Yard is saturated and the field lines can move water. it took 4 sunny days to get things working again. my tank is old. I dont think it vents.

A plumber told me came and snaked my sewer pipe to tank and said he thought the water had backed up back into the pipes creating a clog.. One plumber told me quickly with no detail. If it was mine i would cut a 4" hole in the top and left it pour out

I suppose to prevent it backing up in the pipe. think I should vent it? Or cut 4" hole and cover it with something?.. There is no run off like many tank i see.

On 2020-03-10 - by (mod) - dumping raw sewage onto the ground is illegal as well as of course creating a public health hazard

Thanks for the follow up Bill.

What the plumber told you about allowing your sewage to drain out onto the yard surface might work to allow you to flush toilets and run plumbing fixtures in the house

however in just about every country state or province, dumping raw sewage onto the ground is illegal as well as of course creating a public health hazard for you and your neighbors.

I realize there's already a public health hazard, but a longer-term plan to avoid episodic discharge of raw sewage is probably what's needed.

Unfortunately sometimes we can't give the answer that people would like because their posed "solution",

Like punching a hole in a sewer pipe or in the top of a septic tank or anywhere else to allow sewage to spill out onto the ground to get drains flowing. is illegal, unsanitary, improper and puts People's Health at risk. In no case is dumping raw sewage onto the surface ever acceptable.

When a septic system is even seasonally flooded it's not working during that time. That's why I suggested that ultimately you would need a mound or raised bed septic for that site. I'm sorry to have to be the bearer of bad news but I think your local Health Department would probably say pretty much the same thing. If not I'd be very interested, and so would others in what else was proposed.

In no case is dumping raw sewage onto the surface ever acceptable. You're basicalliy putting sewage into your well, those of your neighbors, as well as into nearby bodies of water, streams, lakes, or even just into areas where your own family may walk and become infected.

Septic effluent is moved from a septic thank to a septic mound by an effluent pump.

In the ARTICLE INDEX Tale a look at MOUND SEPTIC SYSTEMS for detalls.

Also see

• SEPTIC FAILURE SIGNS
• SEPTIC FAILURE CRITERIA

On 2020-03-10 by Bill

longer-term plan, Like moving to a better property.
That is what I hoping for

On 2020-03-10 by (mod)

Understandable, Bill.

I mean that ultimately whoever owns the present property is going to need a different septic design.

See SEPTIC DRAINFIELD FAILURE DIAGNOSIS

On 2019-06-21 by Dave - can I cover the septic tank riser with concrete?

Question, can I cover the riser with the concrete lead from the septic tank at ground level?

On 2019-06-21 by (mod) -

Dave I don't know what concrete lead is

YOu can cover a riser with a safe, removable cover that's childproof.

It can be buried but of course you'd want to note its exact location with a drawing.

On 2018-11-15 by Lori parsons - effectiveness of a "finger system" septic that's 4 ft. deep

How effective is a finger system buried 4 feet deep

On 2018-12-12 by (mod) - finger septic system buried 4 feet deep

Lori

In my OPINION any effluent dispersal system in soil that's more than about 24" down will be oxygen-starved and so may successfully dispose of effluent but may not treat it - thus releasing contaminants to the environment.

The effluent disposal effectiveness depends on much more than depth, including soil type, percolation rate, installation specification, trench design, and of course septic tank pumping frequency and condition. One missing baffle can quickly destroy a drainfield.

On 2016-07-14 by Jack - can I put a pool over or close to a septic tank?

Is there a certain depth you can dig above a septic system? (Not the leach field). I want to put a pool in, but may need to dig a 18 inch trench about going across the system. Is this possible? thanks.

jack

On 2016-07-15 by (mod) -

At https://inspectapedia.com/septic/Leach_Field_Failure_Causes.php we point out that putting a swimming pool over or even near a septic drainfield spells d-r-a-i-n-f-i-e-l-d F-a-i-l-u-r-e.

Also see SEPTIC to POOL DISTANCE at https://inspectapedia.com/septic/Septic_to_Pool.php

A related septic field catastrophe is driving equipment over the soakbed or field area.

Cutting across the septic field with a trench MIGHT be possible if you dig by hand so as to avoid compressing soil or breaking pipes below, AND if the top of the highest soakbed pipe is deeper than the bottom of your trench, AND if your local building authorities will permit it. You don't say what the trench is for. There are required distances between potable water piping and septic systems and piping.

On 2015-11-14 by Kenneth Poor - much dirt backfill is needed for a septic tank in a freeze area?

How much dirt backfill is needed for a septic tank in a freeze area?

On 2015-11-14 by Anonymous

Kenneth

More Reading or in the ARTICLE INDEX

take a look at the recommended article

SEPTIC TANK DESIGN DEPTH

On 2015-06-11 by Anonymous - septic tank depth matters if the house is much higher?

I am building a new house, I have a septic design. If I put the leach field in first how do I know what the depth of the tank should be and the angle of the sewer pipe coming out of the house. I'm thinking if I put in the leach field first and then put the tank in so I get my 1/8 to 1/4" slope that would be correct

But what if my house sets a lot higher than the tank. Does the slope coming out of the house matter.

On 2015-11-14 by Anonymous

You'll find that if the house is well above the location of the septic tank that won't create a problem provided the proper septic tank inlet baffle is maintained in good condition.

More Reading or in the ARTICLE INDEX

take a look at the recommended article

SEPTIC TANK DESIGN DEPTH

Question: fill over septic tank on a hill

(May 16, 2014) Anonymous said: We have a septic tank which is on a sloped hill, and are wondering how much fill we need to put between it and a retaining wall to keep it from freezing in the winter. One side will be completely buried. Thanks for any advice!

Reply:

Anon, in some locations even an above ground septic tank survives moderate freezing locations without insulation, as being in-use it's warmed by inflowing wastewater.

But a tank that's not in use or is in very cold areas indeed may need protection.The quantity of insulation needed to avoid freezing depends on how cold is the exposure - something you don't state.

Take a look at the insulating values of earth found at

SOIL R-VALUES

- for help in deciding how much fill you need.

A very general comment is to take a look at the frost line depth for your area.

Watch out: even very large amounts of insulation won't prevent freezing of an inactive plumbing system or component in very cold areas. Rather the insulation slows the time to freeze but it can also slow solar or geothermal gain that counteracts freezing.

Also see this discussion about whether or not to turn off water in a winterized building:

WATER TURN OFF?

Reader comment: we still want to insulate our septic tank

Grace said: Thanks for your response to my question yesterday!

We're in North Idaho, and are planning to build a retaining wall on the downhill side of the tank. Would you recommend insulating that side of the tank (which will then have fill (up to 1-3ft) with rigid foam?

I've also understood that the moisture content of the soil greatly influences the insulative value and am planning to put tarp near the surface of the soil to drain surface water away from the tank.
Thanks for any other advice!

Reply:

Use a solid closed cell foam insulating board that does not absorb water. 4-6" of solid closed cell insulating board might help.

Question: septic tank lid collapsed under soil weight

(Nov 28, 2014) Scott C. said:
I have a septic system with a power assisted pump tank. The lid collapsed under the weight of the soil it was buried under. There was about 3'2" of clay type soil that the installer covered it with. The lid thickness is 4". Is that up to standard for that much soil?

Reply:

Scott, If you want to investigate the adequacy of the lid over a septic tank you need a bit of data. Septic tanks are indeed rated for different weights and loads.

The thickness of the septic tank lid alone is not enough to know whether or not it was adequate - as we don't know if it contained steel reinforcement or if it did, on what spacing and using what material, nor do we know the quality of the concrete, nor its history (e.g. prior vehicle traffic over the tank) nor do we know the septic tank and size.

See SEPTIC TANK DESIGN STRENGTH SPECS

...

Continue reading  at SEPTIC TANK DESIGN DEPTH to read about the recommended depths for septic tanks, or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see these

Recommended Articles

• SEPTIC DRAINFIELD SIZE for specifications for disposal field trenches, layout, size, setbacks, trench depth, etc.
• SEPTIC TANK DESIGN DEPTH
• SEPTIC TANK, HOW TO FIND
  
• PUMPER TRUCKS & VACUUM PUMPS SOURCES / SPECS for a discussion of the ability to pump out septic tanks at depth or distance from the pumper truck
• U.S. SEPTIC AUTHORITIES & DESIGN SPECIFICATIONS - sources of septic system design, installation, inspection, testing, codes & regulations

Suggested citation for this web page

SEPTIC TANK DEPTH 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 SEPTIC SYSTEMS

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Citations & References

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