Photograph of  a conventional septic tank during installation. Required Septic Tank Depth

  • SEPTIC TANK DEPTH - CONTENTS: What is the usual septic tank depth below the ground surface? How deep should the septic tank be buried? Septic tank location and depth - knowing how deep a septic tank is supposed to be buried can help find a septic tank at an unfamiliar property
  • POST a QUESTION or READ FAQs about how deep the septic tank should be placed and how deep in the ground it may be found at various building sites

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Septic tank depth: to what depths are septic tanks or cesspools or seepage pits or drywells commonly buried?

Here we describe the depth at which septic tanks are installed and we explain the use of septic tank risers to make it easier to pump out, clean, or service deeply-buried septic tanks.

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SEPTIC TANK DEPTH - Notes on How Deep we Put the Septic Tank

This septic tank depth article is part of our septic system article series that summarizes guidelines on the required septic tank size based on anticipated level of daily gallons of sewage wastewater flow. The typical residential septic tank size required for a given average daily sewage wastewater flow in gallons is provided in a table of septic tank sizes. Also
see DRAIN LINE DEPTH 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.

Septic tanks can be installed pretty much at any depth in the soil. Even in freezing climates, the septic tank serving an occupied home or even an unoccupied one is unlikely to freeze, partly because of latent heat the bottom of the septic tank receives from the earth and partly because of the heat generated by the bacterial action going on in the septic tank. (DO NOT add antifreeze to a septic system.)

Factors Determining Septic Tank Depth

The principal factors that determine the actual depth at which a septic tank is likely to be buried (and thus how deep you may have to dig to find the septic tank) at a particular site include:

  • Sewer line depth: The depth at which the lowest sewer line leaves the building which the septic tank serves. Since usually we rely on gravity to move sewage from the building to the septic tank, the tank will be lower than the exiting waste line of the building it serves
  • Site conditions of shape, rocks, obstructions: the location at which the contractor found site conditions suitable to bury the septic tank. If a site has bedrock or large boulders close to the surface, the tank may be located elsewhere; the further the tank is located from the building, if the system uses gravity to move sewage, the deeper the tank will be.
  • Keep septic tanks high: we don't put the septic tank any deeper than necessary, since we are usually moving effluent from the septic tank to the drainfield also by gravity. Plumbers usually install sewer lines to slope down from inlet to outlet, at 1/8" per foot to 1/4" per foot of linear run of the waste pipe.

    We don't want the septic drainfield to be buried at unnecessary depth since the absence of oxygen deep in the soil will prevent some wanted bacterial action that we need to break down and process septic effluent.
  • If a sewage ejector pump or grinder pump system is used to move sewage from the building to the septic tank, of course we can place the septic tank anywhere, including uphill from the building.
  • If a sewage effluent pump is used to move septic effluent from the septic tank to the drainfield, then of course we can place the tank "downhill" from the drainfield as well.
  • Growing grass: if the septic tank is just 2 or 3 inches below ground surface you may as well have left the top of the tank exposed, since you won't get grass to grow in such thin soil. Adding backfill to 6" to 12" may be enough to get grass to grow over the septic system - this is only a cosmetic issue, not a functional one.

Deep septic tanks should have a service riser installed. Septic risers are large-diameter "wells" that are placed over a septic tank inlet baffle access port (and possibly outlet) to permit easy access for septic tank pumping, inspection, and baffle repair. If the septic tank happens to be buried more than just a few inches below ground surface, good practice includes installing a septic riser, a large diameter pipe that gives good access to the septic tank for inspection and cleaning.

Details are at SEPTIC TANK RISERS.


Continue reading at SEPTIC TANK LOCATING EQUIPMENT or select a topic from the More Reading links shown below.


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SEPTIC TANK DEPTH at - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

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Technical Reviewers & References

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3 --> e-related issues; drilled and dug wells, gravel and pipe, chamber-type, and gravity septic systems; pump stations; common problems with well installation; and remedies for poor septic situations. Woodson also discusses ways to increase profits by avoiding cost overruns.
  • Country Plumbing: Living with a Septic System, Hartigan, Gerry: $ 9.95; ALAN C HOOD & TP; Quoting an Amazon reviewer's comment, with which we agree--DF:This book is informative as far as it goes and might be most useful for someone with an older system. But it was written in the early 1980s. A lot has changed since then. In particular, the book doesn't cover any of the newer systems that are used more and more nowadays in some parts of the country -- sand mounds, aeration systems, lagoons, etc.
  • Design Manuals for Septic Systems