Septic drainfield design & size FAQs:

This article provides diagnostic and policy or code and design questions and answers about the required size of 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.

Questions & Answers about Septic Drainfield Size, Piping, & Features

These questions and answers about determining the size of a leachfield or soakaway bed size were posted originally at SEPTIC DRAINFIELD SIZE - be sure to read that article.

On 2020-03-04 by Mod - maximum distance between septic tank and the D-box

David

There's not a code-specified maximum distance between septic tank and the D-box, but there will be a technical or practical distance limitation since the main effluent line from tank to D_box and from D-box to absorption trenches has to slope (typically 1/8" to 1/4" per foot).

A very long run that pushes drainfield lines too deep will have you building a system that lacks soil oxygen and thus doesn't treat the effluent adequately (not enough bacterial action) so then contaminates the environment, groundwater, wells.

On 2020-03-04 by David

What is maximum length of line between septic tank and diverter box ?

On 2020-02-26 - by (mod) - how do I know how big the drainfield needs to be?

Terry

The two most-basic considerations in setting the size of a septic system drainfield are the daily wastewater flow in volume and the percolation rate of the soil. There are other variables such as the type of effluent disposal system used: sprays, gravel trenches, or no-rock gravel-less systems.

The basics of drainfield sizing are in the article above on this page. Please take a look and don't hesitate to follow up with specific questions if any of that is unclear as then I'll know what we need to make easier to understand.

On 2020-02-26 by terry dudley

how do I know how big the drainfield needs to be

On 2020-01-03 - by (mod) -

Tommy typically you keep 1/8" to 1/4" per foot of horizontal distance.

On 2020-01-02 by Tommy

What's the minimum drop from septic tank to drain field?

On 2019-12-18 - by (mod) -

Scott:

The article above gives some "bottom area" requirements for septic drainfields based on daily wastewater flow, but you will want to see as well

GRAVELLESS SEPTIC SYSTEMS

for details of chamber type septic systems.

Honestly an off-the-cuff answer is not really reliable since in addition to daily wastewater flow, the area needed absolutely depends on the soil percolation rate of your absorption field.

On 2019-12-18 by Scott Bulloch

Using 24” septic leaching Chamber approximately how many feet of lateral line do I need for a house with 2 1/2 baths

On 2019-10-23 - by (mod) - For a 450sqt leach field how many 22" infiltrator connections would I need?

Hugh

Take a look at CHAMBER SEPTIC SYSTEMS where at the end of that article you'll see links to additional descriptions of different types & brands of no-rock or gravelless septics. The number of connections for any of these depends on the number of lines to be laid out.

Infiltrator Systems, Inc.provides chamber products TEl: 800-718-2754 Old Saybrook CT USA. Website: http://www.infiltratorsystems.com/

Infiltrator Systems provides state specific design manuals for no-rock chamber systems for specific states in the U.S.

There you will find that the "right answer" to how many Infiltrator septic system connections are needed will get a bit more detailed: it will also depend on the Infiltrator septic system product that you select.

Example: An Infiltrator Quick4 high capacity 14" chamber system is 34"W x 53"L x 14"H (I didn't find a 22-inch product so perhaps you can point me to the manufacturer and product specifications for the brand and model you're actually considering).

These chamber septics give an actual physical length and an "equivalent" length (as if it were a conventional drainfield trench). The "Effective length" of the model I cited above is 48" - but that does NOT speak to number of connections and thus we need to take care not to mix these two length descriptions.

If you really need 450 sq.ft. of infiltration area for your septic you consider only the length and width of the trench or product.

E.g. 34"W x 53"L x 14"H translates to 34"W x 53"L or 1,802 sq.in. pe chamber section.

Your 450 sq.ft. specification = 450 sqft x 144 (sq. in. in a sq. ft) = 64,800 sq.in.

64,800 total sq. in. / 1802 sq. in. per section = 40 chambers. You need connectors + end caps depending on the number of rows in which your chambers will be connected. That's 38 connectors + 2 ends at the very least, probably more depending on the actual arrangement.

Watch out: all of this is theoretical arm-waving and is NOT site-specific. The good news is if you call the actual company whose product you're buying they'll be thrilled to do the actual calculations for you for their product.

And all of that assumes your local septic engineer has already done a soil perc test and site survey and has told you that 450 is the right sqft. of drainfield chamber bottom area you need.

On 2019-10-23 8 by Hugh

For a 450sqt leach field how many 22" infiltrator connections would I need?

On 2019-08-23 - by (mod) - drainfield size or leachfield trench length required.

Dan

Thanks for asking the critical question on drainfield size or leachfield trench length required.

Take a look in the article above on this page at the table titled

What is the Required Size of the Septic Leach Field?

to see the answer to your question.

Keep in mind that it is not possible to answer your question as posed: you need to do a soil percolation rate test or "perc test" on the drainfield soils to see the absorption rate specific to your property.

A drainfield with a slow perc rate needs more feet of drainfield line or as you put it "sewage run" than one with a fast perc rate.

On 2019-08-23 by dan

how many feet of sewage run do i need for a two bedroom cottage

On 2019-07-06 by (mod) - can we cut off the tops of tall septic line access pipes?

Rock

If the pvc pipes and caps are access points for cleanout or for inspection, gluing them in place doesn't sound sensible to me.

You can shorten those projecting pipes down to ground level but in doing so you want to be sure that the caps and pipes are water tight so that running surface water doesn't flood and destroy the septic soakaway beds.

On 2019-07-05 by Rock

We have a new septic system (less than two years old) and can see PVC pipes with caps glued tight extending 18 to 24 inches above our lawn.

Can these be cut shorter and recapped? Is there a minimum height required?

On 2019-05-22 by mario

i want to find out for 70 people just toilets drainage is sand what size or square footage of leach lines i require

On 2019-05-01 by (mod) - hooking two buildings to a single tank and drainfield

There are both legal and functional questions about hooking two buildings to a single tank and drainfield including respecting property lines, and come bring a future sale, and perhaps overloading the existing system.

Before you try that approach, whose appeal I understand, you need to know the capacity of the existing septic installation and the present Wastewater load or number of users or number of bedrooms compared with the total number if you combine the two cabins. That's a question then to review with your local County Health Department as thet are the final legal Authority

On 2019-04-30 by Larkin Wilson

Hello sir, can you hook two small two-bedroom cabins to one septic tank & one leech field in the Matanuska Valley? thank you Larkin Wilson

On 2019-02-01 by (mod) - how big is a drainfield?

Dot

Septic drainfield length and with sizes are given in tables in the article above - the answer to your general question is


.... it depends


1. on the soil properties and soil percolation rate

2. the type of septic system

3. the daily wastewater flow volume

4. other factors listed in the article

On 2019-02-01 by Dorothy

What is the width and length of a drainfield ?

On 2019-01-25 by Chris - What do you know about the supposed remedy for failed drainfields of pumping restaurant-grade (35%) hydrogen peroxide (~55 gal's) into the distribution lines

Thanks Dan, I'll disregard that option ... am touring the site w/ an AOSE this Saturday ... hopefully his recommendations will do the trick!

On 2019-01-24 by (mod) -

Chris

Please see this article

OTHER DRAINFIELD RESTORE APPROACHES

On 2019-01-24 by Chris

Dan , many thanks for your (honest) prior response -- good to know there's folks out there that share good advice.

What do you know about the supposed remedy for failed drainfields of pumping restaurant-grade (35%) hydrogen peroxide (~55 gal's) into the distribution lines to break up and dissolve the organic matter that lines and blocks the trenches?

Thanks, Chris

On 2019-01-06 by (mod) -

Eric

Thanks for the question and septic history.

I agree that there is virtually NO independent, un-biased data or research that would claim that jetting or other drainfield rejuvenation approaches produce a lasting result (though there are one or two that somewhat work but at the cost of contaminating the environment with caustics).

I would like to offer a more reliable answer to your "will our new septic design be ok and exceed the original" but I can't.

What's needed is

- an onsite inspection of the site for drainage, space, location, and implementation details such as trench location, dimension, materials, gravel, drainage

combined with

- soil perc testing that tells us the soil properties.

It is the combination of soil perc rate and the anticipated daily wastewater flow that give a septic designer a guide to just how much leach area is needed to adequately both dispose of and treat the sewage effluent.

Finally: I imagine you're quite correct that the inability to pump the septic tank on a regular schedule (again based on usage levels) is what ultimately did in the fields; while I've seen an occasional (rare) septic system that lasted nearly 100 years, more usually 20 years of life isn't bad for a septic field.

On 2019-01-04 by eric ingvardsen

Hello,
We are installing a new leach field in between a semi-failed field designed as follows: At the centered outlet T, a 10' space to the right and 10' space to the left is followed by 90 degree elbows that form two 50' leach lines.

The original design is TRENCH AND GRAVEL - about 2' wide, and 4' of gravel under the leach pipe.

Soil is southwest desert sand. We are hoping to install a BED design that will be approximately 10 to 12 feet wide by approximately 50 feet long, filled to about the same depth of gravel as the original trenches.

3 leach lines will connected to the original left line just near the T connection from the septic tank outlet line. From there, the 3 lines (with no distribution box) will be interconnected at the other end as well. The final design will be a rectangular shaped "ring" in order to use as much of the bed as possible. No - we don't have a pressurized system with a pump nor are we wanting to do this.

Our system performed without issue for 19 years of very heavy peak period usage. We are a 4 bedroom house - 3 rooms used for a B&B. Our issue was surely caused by limited access to the septic tank itself when regular 3 year pump out couldn't completely remove the solids.

Eventually the leach lines got clogged. We water snake jetted the leach lines and then pressure forced fresh water with a bladder from the tank outlet pipe in hopes of reviving the drain field but this didn't prove completely successful.

This was after full access was made to the tank - something that should have been done years ago. I strongly feel the original leach line trenches would have continued to perform had they not been clogged. This was ultimately made worse by an errant toilet running for over 4 hours and packing the solids.

Note: The existing lines will still remain connected.

My question: We are doing this project ourselves - today we have a backhoe rented and will begin the dig. Will this new design EFFECTIVELY EXCEED THE ORIGINAL DESIGN?

I realize we are losing side wall absorption (hence interconnecting the other end of the 3 leach lines - with leach pipe of course), but the intention is to hopefully have an overbuilt design that will perform better and longer.

Consequently, we had to dig a relief pit to survive the last 3 weeks of business season. We reopen in March. Hope the vision is clear for the question? Thanks

On 2018-12-30 by (mod) -

Chris

If you can direct surface water and runoff away from the drainfield that may help, but frankly from your description it sounds to me as if the drainfield is in failure and is inadequate in original design as it's not sufficiently above the high water table.

Even when water levels fall enough that you're not seeing sewage backup the system remains failed and inadequate. A septic system that only backs up "sometimes" is still an inadequate one.

Add to that depressing picture the fact of global warming, climate change, increased intensity of storms and flooding, and one would expect a durable, sanitary solution for your drainfield to require its replacement with at least an elevated mound or raised bed septic of sufficient height.

Sorry.

On 2018-12-30 by Chris

I have a drainfield that has become saturated w/ the record-setting rains we've had throughout 2018, which has evidently created hydrostatic pressure on top of the drainfield that has pushed water up the pipe from the distribution box to the tank and, on a few occasions, into the basement drain of the house.

A large portion of the water that drained onto the drainfield came from surface flow from an adjacent uphill lot, so I created a drainage trench and system to redirect this surface flow to the rear of the lot; however, although this system alleviated part of the problem, it appears that a substantial amount of subsurface water is also "stacking up" from the adjacent downhill lot on the opposite side of my house, due to insufficient downward gradient.

I'm looking for a solution of some kind that will avoid the need to replace the entire drainfield w/ an expensive alternative system -- does anyone have any suggestions?

On 2018-10-07 7 by (mod) -

I think you need to look at the location of the well with respect not to the septic tank but to the drain field, or to both. A paragraph and a better answer is of course going to come when you have your water supply tested for potability.

On 2018-10-06 by Clyde Kuemmerle

Our septic tank (next to house) drains down hill into a field (orchard). The old well is 250' from the tank.Can this well again provide save drinking water for the house?

On 2018-10-04 by (mod) -

Karen

Local code requirements will vary;
so check with your local building or health department.

In general a new leach line trench will be 5' from the old ones so as to have sufficient working space.
The trench depth, width (often 2 ft), gravel, and other specifications are the same as the original.

The length and total area needed for leach lines depends on the soil percolation rate for your site and the daily wastewater flow volume.

On 2018-10-04 by Karen

I have an old leachline system that has started standing in sewage. I have had a trench dug next to the old line. How deep and wide does it need to be?

On 2018-09-13 by (mod) -

Kenneth

To determine the required drainfield size for a property you find

1. the daily wastewater load volume in gallons - commonly based on number of bedrooms, or occupants for residential properties

2. the soil percolation rate - by performing a soil perc test - see SEPTIC SOIL & PERC TESTS at inspectapedia.com/septic/Soil-Percolation-Tests.php

3. Then you'd use the septic leachbed (drainfield, soakaway, soakbed, leachfield are synonyms) tables in the article above on this page to find the required drainfield size.

The septic field sizing tables, as you'll see in the data, translate the combination of daily wastewater volume and soil percolation (absorption) rate into the absorption area required.

In some tables you'll see that as "maximum application rate"

For example in our table titled

Absorption Bed Septic Bottom Area Size Required

Example: one of the data lines tells us that if the soil percolation rate is 8-10 minutes per inch then the maximum wastewater application rate for the field will be 0.70 gallons per day per square foot of absorption bed bottom area.

Let's calculate out a further example for the same data line.

If our septic absorption field bottom area is 100 sq.ft. then

Total Daily Wastewater Volume (that can be handled) = 0.7 gpd/sq/ft. x 100 sq.ft. = 70 gallons. Which is, obviously, not much when you consider that Americans use as much as 200 gallons of water per day in their homes, per person for bathing, toilets, cooking.

On 2018-09-13 by Kenneth Mattison

How is leachbed calculated and sized ?

On 2018-09-08 by (mod) -

Trent

Thank you for asking a really important question.

To have room to give a detailed answer and to list common factors that could explain why your EZflow septic system has failed in just 5 years of service, I repeat your question and answer it

at NO ROCK SEPTIC SYSTEM LIFE

Please take a look and let me know what you think of that information and of what questions remain.

On 2018-09-13 - by (mod) -

how to determine drainfield size

Kenneth

To determine the required drainfield size for a property you find

1. the daily wastewater load volume in gallons - commonly based on number of bedrooms, or occupants for residential properties

2. the soil percolation rate - by performing a soil perc test - see SEPTIC SOIL & PERC TESTS at inspectapedia.com/septic/Soil-Percolation-Tests.php

3. Then you'd use the septic leachbed (drainfield, soakaway, soakbed, leachfield are synonyms) tables in the article above on this page to find the required drainfield size.

The septic field sizing tables, as you'll see in the data, translate the combination of daily wastewater volume and soil percolation (absorption) rate into the absorption area required.

In some tables you'll see that as "maximum application rate"

For example in our table titled

Absorption Bed Septic Bottom Area Size Required

Example: one of the data lines tells us that if the soil percolation rate is 8-10 minutes per inch then the maximum wastewater application rate for the field will be 0.70 gallons per day per square foot of absorption bed bottom area.

Let's calculate out a further example for the same data line.

If our septic absorption field bottom area is 100 sq.ft. then

Total Daily Wastewater Volume (that can be handled) = 0.7 gpd/sq/ft. x 100 sq.ft. = 70 gallons. Which is, obviously, not much when you consider that Americans use as much as 200 gallons of water per day in their homes, per person for bathing, toilets, cooking.

 

On 2018-08-28 by (mod) - 5 foot area for septic field trench

There are different approaches to separating versus interspersing replacement septic absorption beds or drain fields. For example you may leave space between leach line trenches to install a replacement trenches later.

Most standards want two -three feet or more between the edges of those trenches. With a two foot wide trench that makes each effective trench area 5 ft. in width.

On 2018-08-28 by Anonymous

How close can an old field be to a new field?

On 2018-08-25 by (mod) - septic tank size is not the same as drainfield size procedure

Terry the septic TANK size depends on the total daily wastewater volume not just the number of occupants (though that's a key factor as you'll see in the table on this page).
Some septic tank sizes are at SEPTIC TANK SIZE https://inspectapedia.com/septic/Septic_Tank_Size_Tables.php

BUT that too is very incomplete: the critical factor for you is less tank size and more what is going to happen to the wastewater effluent that leaves the septic tank ( a tank is normally always full so 100 gallons in = 100 gallons of effluent out into the drainfield)

Rather it's then the drainfield size that will be critical for you - you need space to fit a field that can handle the daily wastewater flow OR if you don't have enough space for a conventional soakaway bed your local building or health department may accept a proposal for a small onsite wastewater treatment plant that discharges acceptably clean water to the environment.

To answer that question you need an onsite septic engineer who examines the space at your site, the setback requirements for tank and field, and who does soil tests to get the soil percolation rate - a slow perc soil needs more areas of soakaway

Commendable, Terry, I don't blame you for avoiding people who are unpleasant.
It's not just the unpleasantness it's that you can expect poor workmanship and resistance to any requests you might have.

You'd start by identifying the brand of aerobic system you have installed,

continue by obtaining all of the installation, operation, and maintenance information that the manufacturer has for that system,

and after reading that, you might discuss training with the manufacturer if they're open to that.

Watch out: some septic system chores involve extremely-dangerous steps that should not be taken without training and that can never be performed alone - for example entering or falling into a septic tank, even just leaning over an open septic tank can mean a quick ugly death.

On 2018-08-09 by (mod) -

I can't say, Monique but certainly there's a difference between overflow of toilet fill water and sewage backup.

Neither will be sanitary since the fill water that overflows comes out through te toilet bowl, but there could be 2 underlying problems: a clogged toilet drain (start by having the drain checked and cleared) AND a fill that kept on filling way too long - you had to shut off the water supply.

Clear the drain then have the septic tank inspected for signs of blockage such as high sewage levels.

On 2018-08-09 by Monique

my toilet on the first floor overflowed with clear water, enough to flood the first floor. It finally stopped once i shut the main coming into the house and plunged the toilet. I had the septic people come to clean filter and empty the tank. what are the chances this was septic related? or just a clog in the line? Thank you

...

Continue reading  SEPTIC DRAINFIELD SIZE - topic home, or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see these

Recommended Articles

• SEPTIC DRAINFIELD INSPECTION & TEST - home
  • PLUMBING DRAIN NOISE DIAGNOSIS.
  • SEPTIC DISPOSAL FIELD CLOGGING INSPECTION
  • SEPTIC DRAINFIELD FAILURE CAUSES
  • SEPTIC DRAINFIELD FAILURE DIAGNOSIS
  • SEPTIC DRAINFIELD LOCATION
  • SEPTIC DRAINFIELD RESTORERS?
  • SEPTIC DRAINFIELD SIZE
  • SEPTIC DRAINFIELD SHAPE
  • SEPTIC ODORS INDICATING DRAINFIELD FAILURE
• SEPTIC DRAWINGS
• SEPTIC SOIL & PERC TESTS

Suggested citation for this web page

SEPTIC DRAINFIELD SIZE FAQs-2 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 DRAINFIELDS & DBOXES

Or use the SEARCH BOX found below to Ask a Question or Search InspectApedia

Ask a Question or Search InspectApedia

Questions & answers or comments about septic drainfield or soakaway bed size or capacity requirements & design.

Try the search box just below, or if you prefer, post a question or comment in the Comments box below and we will respond promptly.

Note: appearance of your Comment below may be delayed: if your comment contains an image, photograph, web link, or text that looks to the software as if it might be a web link, your posting will appear after it has been approved by a moderator. Apologies for the delay.

Only one image can be added per comment but you can post as many comments, and therefore images, as you like.
You will not receive a notification when a response to your question has been posted.
Please bookmark this page to make it easy for you to check back for our response.
Our Comment Box is provided by Countable Web Productions countable.ca

Citations & References

In addition to any citations in the article above, a full list is available on request.

---

ADVERTISEMENT

---