Required Septic Tank Depth FAQs

Q&A on sSeptic tank design depth - how deep should we put the septic tank?

Septic Tank Top Not Covered by Fill FAQs

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

Is it ok to have the top of septic tank exposed? On 2016-04-30 by Jean Mcmeans

by (mod) - yes but ...

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

Septic Tank Depth / Fill to Prevent Freezing FAQs

How much fill do we put over a septic tank on a hill to avoid freezing in winter

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! (May 16, 2014) Anonymous

Reply: fill to prevent septic tank freezing is usually not needed

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.

See details at SEPTIC TANK & DRAINFIELD FREEZE 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: OK here's how

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

Septic Tank Top Strength vs Backfill Weight FAQs

Our septic tank lid collapsed under the weight of three feet of soil

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? (Nov 28, 2014) Scott C.

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

Septic Tank Depth Below Sewer Line Exit Q&A

Does the septic tank have to be lower than the sewer line exiting the house?

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. (Feb 12, 2014) Anonymous

Reply: Yes - here's the detail

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