Typical drainfield layout - USDA - DJF Septic Drainfield FAQs

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Septic leachfield FAQs:

This article provides diagnostic and policy or code and design questions and answers about septic drainfields, soakbeds, leach fields, and similar onsite wastewater disposal systems.

This article series explains how we choose the size of a septic leachfield or soakaway bed or drainfield. We discuss several different conventional soil absorption systems: absorption fields: conventional trench, deep trench, shallow trench, cut-and-fill, and gravelless septic systems. Then we discuss septic absorption beds, and seepage pits.

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Questions & Answers about Septic Drainfields

These questions & answers about the size & layout of septic drainfields were posted originally at SEPTIC DRAINFIELD SIZE - home; you'll want to see that article too.

On 2017-07-13 by (mod) settlement & odors over & around a new septic drainfield; switching drainfield areas everyt 6 months?

Laura that sounds to me as if

- a sewer line or effluent line is broken, perhaps due to settlement of soil beneath it as can happen in new construction, especially if trenches aren't correctly dug or prepared

- sewage effluent is leaking out, causing the settlement you describe as a sink hole, as well as the odors

Such sink holes can also happen if earlier sitework buried debris, stumps, branches, junk on a property - something that may be discovered if settlement continues after piping is repaired and the holes are filled.

By the way in my OPINION, switching drainfield areas every 6 months won't do a thing to keep a system working nor to extend drainfield life insofar as that is intended to allow a thickened biomat to thin and thus to permit further absorption of effluent into the soil. A drainfield would need to rest for years before a biomat will be broken down.

If the drainfield switch is intended to let a wet area dry out that sounds as if the original design was a questionable one since wet soils cannot treat effluent effectively - in essence even if you're disposing of sewage effluent you're not treating it adequately so the system is contaminating the envrionment, groundwater, nearby wells, lakes, streams, or aquifers.

On 2017-07-13 by Laura Taylor

I have a new septic system, it was put in 10 month ago. I have to switch the lines every 6 month. Now here is the problem, my house started to smell like sewer, (now it does again)I now have a sink hole in my back yard.

They had 6 to 10 of dirt above the lines and instead of removing the dirt from around the edges ( the dirt was over my out buildings foundation and 9 in. above the bottom of my fence, and 8 in on my patio.) they remove all of it until it was some what flat.

The first sink hole is 3 ft deep and the second one is now 2 ft deep and the are growing to gather. Could someone tell me what can be going on?? I live in levelland tx and the soil in my back yard is like concrete. The leach fields are in a 30 ft by 30 ft square. The house is 1874 sq ft and 3 br, 2 bath.

On 2017-07-13 by (mod) ok to drill under neighbor's land to get to a remote septic or well area?


I can't give legal advice, nor could an attorney without knowing more about the property, locale, etc. but in general it's not likely that you could nor should consider drilling under someone's property at all and certainly not without permission and a legal easement.

You would need to approach the property owner(s) involved, obtain an easement, and also make darn sure that your drilling (there are subsurface pipe drilling surfaces that can do what you want) doesn't smack into something on their property like a well, pipe, septic, or electrical line. I'm not optimistic.

On 2017-07-13 by scott

I have a piece of lake front property in southern Michigan. It is a great buliding site it but does not have enough room on the lot to position a well and septic. We also own another property about 100 ft away that could be used for the drain and septic but that property does not directly connect to the lake property. My question is, can we legally and logictically tunnel under a another persons property to the vacant lot that we also own and position a septic system there.

On 2017-06-01 by (mod) septic tank sizing tables


We describe the general approach to determining the size of the septic drain field in the article above. There you will see that there isn't a single right answer to field size since it depends on the soil percolation rate and other site features.

You can read the general guidelines that relate septic tank size to number of building occupants or in some building code areas number of bathrooms at this link

On 2017-06-01 by Billy

For a two bath house what size of tanks and how much drain line do I need

On 2017-05-20 by (mod) buying an older home that sat empty for years: what to do about the septic system

It would be smart to have the septic tank pumped, and inspected for damage so that you can start from a known condition.

And if there's damage, such as a lost septic baffle or tank tee you will want to replace that. And thus you will know more about the condition of the drainfield.

Armed with that information and a newly pumped septic tank, use the website search box just above to find our article on


About the septic field size, I don't know what they are referring to buy squares. However I can assure you that 12 square feet would not be nearly large enough for a functional drainfield.

On 2017-05-19 by Bill

I recently purchased a house built in 1992. It sat empty for about 5 years before I bought it.. Three questions:

1, Do you think the leaching fields would have basically cleaned themselves since there was no waste being added that long?

2, I use the house about 1 week per month. I have hot had septic issues. Should I pump the tank?

3. The county permit said, in the filed for length of the leaching system, 12 squares. Based on a rough drawing of the system attached to the completed permit I am pretty sure it does not mean 12 square feet. What do you think it meant?

On 2017-05-10 by (mod) - septics on a steep slope require thoughtful design

there are certainly potential problems with building a drain field on a steep slope. A concern is that the waste water runs too fast down to the end of the pitch to lines bus failing to be distributed uniformly in the soil and causing a system or field failure.

Typically the design engineer will run the drain field lines across the slope rather than down the slope in order to avoid that problem. Use the search box above to find our article series on

steep slope septic systems

Question: neighbor building drainfield close to our property line: what are the setbacks?

14 Sept 2015 Tom said:

Our neighbour is about to put in a new leaching field and septic tank. Our leaching field is just a few feet away from his property line and he intends to put his field a few feet the other side of his property line. How close can two leaching fields be?



Generally your local building department or health department will specify a property line set-back requirement such as 10 feet from the edge of the drainfield.



Question: Quick4® Equalizer® 36 Chamb

(Oct 26, 2015) SteveBryant said:
Is there a guideline for how many feet of The
Quick4® Equalizer® 36 Chamber to use for a 1,000 gallon septic tank?

Question: leach line depth requirements for Ohio

(Dec 30, 2015) Todd Carson said:
I live in ne Ohio and was wondering what the depths are needed for the leach lines?


Depth is determined by the septic tank outlet and terrain, not necessarily below the frost line.

Question: why is there a grate over one of my septic field vents?

Ground water monitoring well, adapted from Shasta CA septic code (C) original source  cited in article(May 16, 2016) Dave Alfano said:

I have an 'H' shaped leach field with a distribution box in one of the 120' lines. There are 'risers' or inspection ports on the ends of each line that protrude several inches above ground level (4 total).

Three of the ports are slip-capped and one has as open grate. Why the grate? - seems like bugs and water can enter the field at this point and your website seems to indicate just capping for these ports. Thanks

Reply: Ground water monitoring well in septic field area?

I don't know, Dave; perhaps someone thinks it's a vent (if it's at the uphill end of line) or else, an ugly thought, someone put an escape outlet for the case of the field becoming saturated. It's not permitted, however, to discharge sewage to the surface.

As a more likely guess, for an apparent vent or riser pipe in a septic drainfield, you may be looking at a monitoring well. Some septic soakbed specifications such as in the U.S. in California include a requirement for a ground water monitoring well to assure that ground water is sufficiently below the bottom of the drainfield trench bottom.

Watch out: Don't confuse area surface drains set into a buried piping system to handle surface runoff with septic soakbed vents or inspection ports, and for goodness sake do not ever deliberately drain surface runoff into the soakbed - it'll become flooded and fail.



(May 24, 2016) Jan. said:

It looks like we need to put a leech field in our present home, originally built in 1971, in order to bring it up to state codes. The area in back of our home is heavily wooded and steep. Is there a certain type of application that is best for these conditions?


You'll want a septic engineer familiar with local septic codes and who can examine your site and its soil conditions, available area, slope, and locations of nearby well or waterways.




Question: is my leach field long enough ?

(June 27, 2016) JRains said:
I have a 4 bedroom house with an inch every 6 min. perc test. Is 300 feet of lateral lines with 2 foot gravel beds enough for my system?


It depends. See these articles

Question: expanding a peat moss septic

(June 30, 2016) Fred said:
I have a peat moss system that is to small for the house can I make it bigger



You can usually enlarge the existing system by extending it at it's end. Check with your original system designer and of course check property line clearances.

Search for PEAT FILTER SEPTICS for more information.

Question: number and length of "leech lines" for a 4 bedroom home

(Aug 9, 2016) jake said:
for a 4 bedroom home percolation is on inch in 3 min. How many leech lines and how long should they be.



Question: Codes for SCAT Sewage Collection and Treatment Regulations

(Aug 28, 2016) Ronald White said:
How do I find the correct codes for my SCAT program distribution box?



If by "SCAT" you refer to "Sewage Collection and Treatment (SCAT) Regulations" those are determined by the jurisdiction in which you live. For example in the U.S. in the state of Virginia, SCAT Regulations are at

while in the U.S. in the state of California you'd see

Check with your local building and health departments to see what SCAT regulations apply where you live.

Question: build a patio over a drainfield?

(Sept 2, 2016) Ignoramus said:
I want to build a patio but know that there is a leach field somewhere in the area
1. I am told NOTHING should cover the ground over a leach field - is that true?
2. If so, how do I find out exactly where the field is ( municipality does not have information on exactly where it is)? Can a septic cleaning company determine this?



"Nothing" is a bit broad. For example, typical shallow-root grasses are perfectly fine to cover a drainfield. Where no grass grows, a thin layer of gravel might also be OK. What we want to avoid is any cover that interferes with evaporation, any traffic that smashes the soil down, any growth that sends roots down into the system.

An experienced plumber or septic company can find the exact drainfield location using pipe locating equipment, though often that's not necessary as a visual inspection can often work for you. Search InspectApedia for "DRAINFIELD LOCATION" to read details.

Question: septic soakbed chamber length

(Oct 7, 2016) Sando said:
How many feet long the chambers have to be for 2 bath house with 4 people ?

Question: septic soakaway bed trench depth

(Oct 29, 2016) Leslie said:
How deep should the trenches be


Great question, Leslie.

Ideally we keep the trenches close to the surface, say trench-top 12-18" below ground and trench bottom (always) is 24" or more above the seasonal high-water table in the area. That'w because we want to support both anaerobic and aerobic bacteria in the drainfield to break down pathogens; if a trench is too deep it won't have much if any measurable oxygen and while it may dispose of sewage effluent it won't be treating it: we'll be discharging pathogens into the environment.

Nevertheless, millions of drainfield trenches are buried much deeper - as much as 8 feet deep here in northern Minnesota, perhaps to avoid freezing issues in very cold climates.


2016/11/24 Puzzled said:
Perc Rates for leach fields are commonly expressed in Minutes per Inch. The inspector said my perc rate of 2 MPI requires advanced treatment in California. When I check the Plumbing Code it says perc rates should be between 0.83 gallons per sq.ft. per day and 5.0 gallons per sq. ft. per day. How does this relate to MPI?


See a detailed reply and tables at PERC TEST STANDARDS


Continue reading SEPTIC DRAINFIELD SIZE - home, or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.



Or see SEPTIC DRAINFIELD LOCATION for help in finding an existing septic soakaway bed, leach field or drainfield

Also see 75-A.8 Subsurface treatment of effluent, absorption field requirements, leach field design criteria for example regulations describing the construction of septic drainfields.

Or see these

Key Septic Drainfield Articles

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


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