Septic tank design depth:

To what depths are septic tanks or cesspools or seepage pits or drywells commonly buried?

Do we need to put the septic tank below the frost line to avoid freezing?

Here we describe the depth at which septic tanks are installed and we describe several factors that influence the acutal depth to which a septic tank (or cesspool or drywell or soak-pit) will be buried.

A companion article linked-to at the end of this one explains the septic tank depth sketch shown at the top of this page and describes in detail how to find the actual depth of an existing installed septic tank.

Septic Tank Installation Depth Q&A

[Click to enlarge any image]

These questions and answers about the design or desired depth for septic tanks were posted originally

at SEPTIC TANK DESIGN DEPTH. Please see the advice given there.

To find the actual depth of an existing septic tank also

see SEPTIC TANK DEPTH

If you're worried about a frozen septic tank

see SEPTIC TANK & DRAINFIELD FREEZE PROTECTION

On 2018-10-03 21:45:33.614701 by (mod) -

Anon:

Generally we keep the septic tank 10 feet or about 3 meters from the building wall. The distance can be greater than that but will be limited by the fact that going further makes us put the tank deeper in the ground - as we explain

at SEPTIC TANK DESIGN DEPTH.

Also you will want to see SEPTIC CLEARANCE DISTANCES

On 2018-10-03 y Anonymous

What is the minimum and maximum allowed distance from the house foundation to the septic tank? And what is the minimum and maximum depth for the top of the septic tank?

On 2017-11-12 by Dragos B

Where can I find length, angles and specs(volume, diameter) for 1 to 10.
Is any regulation about distance from nearest house, underground water?
Thanks

On 2017-10-14 by Anonymous - find the lowest price for septic tank service?

Aaron,

The best answer to your question of "who's the cheapest septic pumper in town" is at

HOW MUCH SHOULD YOU PAY

As a realtor I expect you to understand that paying the "cheapest" price is not always the least expensive in the end.

For example some low-price septic pumpers claim to empty the tank but bamboozle the homeowner (or realtor) into letting them put back into the tank most of what they pumped out - claiming the tank needs the water (not true), and risking pushiing floating solids into the drainfield, destroying it by by clogging.

On 2017-10-14 by Aaron Schave Realtor

When you have to have a septic tank pumped who's the cheapest In town?

On 2016-04-30 by (mod) - ok to have the top of septic tank exposed?

Yes but, the top covers or access ports must be secure, safe to walk-on, and childproof to avoid a potential fatality.

On 2016-04-30 by Jean Mcmeans

Is it ok to have the top of septic tank exposed?

On 2015-07-30 by (mod) - no need to inoculate septic tanks with bacteria, treatments, water

Austin:

There is no need to pre-fill the septic tank before use. Depending on your family size and tank size the tank will be full in a few days or more simply from normal use.

Similarly there is absolutely no need to inoculate the tank with bacteria, chemicals, or treatments. Don't do that either.

On 2015-07-30 by Austin

I am installing a septic tank in my new house. Do I need to fill it up with water before the household starts using the toilets? Or should it be empty and fill up with usage?

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

Question: septic tank depth must permit adequate drain line slope

(Feb 12, 2014) Anonymous said:

If my sewer discharge pipe is 6ft+ 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:

Anon. I agree completely. 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

...

Continue reading  at SEPTIC TANK DEPTH to find the depth to the cover of an existing septic tank or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see these

Recommended Articles

• SEPTIC CLEARANCE DISTANCES
• SEPTIC TANK COVERS
• SEPTIC TANK & DRAINFIELD FREEZE PROTECTION
• SEPTIC TANK DESIGN STRENGTH SPECS
• SEPTIC TANK, HOW TO FIND
  • DISTANCE TO SEPTIC TANK
  • FIND MAIN WASTE LINE EXIT
  • POSSIBLE SEPTIC TANK LOCATIONS
  • SEPTIC TANK COVERS
  • SEPTIC TANK DEPTH
  • SEPTIC TANK DESIGN DEPTH
  • SEPTIC TANK LOCATING EQUIPMENT
  • SEPTIC TANK LOCATION SKETCH
  • SEPTIC TANK RISERS
  • SEPTIC TANK GRASS or SNOWMELT
  • SEPTIC LOCATION VIDEO
  • VISUAL CLUES LOCATE the SEPTIC TANK
  • WHO KNOWS SEPTIC LOCATION?
• U.S. SEPTIC AUTHORITIES & DESIGN SPECIFICATIONS

Suggested citation for this web page

SEPTIC TANK DESIGN DEPTH FAQs 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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