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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.
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I could not find which type of pipes are better to use for a drainfield/leachfield. I have seen 3 different types at lowes.
1. PVC pipe with 3 rows of holes on the bottom.
2. Black flexible corrugated pipe with 2 rows of holes on the bottom
3. Black flexible corrugated pipe that has small slits instead of holes and it is all the way around the circumference of the pipe.
I am not sure which to get and the price is about the same for all 3 of them. - D Smith 6/25/11
All of these will work if you are using gravel-filled trenches, and will work longer and better if you install an effluent filter at the septic tank outlet so as to avoid pipe clogging. In gravelless systems the galleys may use more narrow slits (and in some designs also a covering of geotextile fabric) to avoid soils washing INTO the galley or piping.
In general I avoid installing piping with holes facing "UP" in conventional drainfield trenches.
If I built a system 22x36 with 3 long 30' and 8' between them and the 8' on both sides of the center being leach line also how much effective field would I have? Ed 10/16/11
I'm sorry, Ed, but I don't think there is a straight forward answer to your question of how much more effective a drainfield will be with your specifications - first of all, what are we comparing it to ? And what are the soil characteristics? What is the design load or wastewater volume to be disposed-of? You don't say. The design and its efficacy depend on various characteristics that you haven't stated.
I do agree that leaving space between the leach lines is a good design that allows for future field replacement between the existing trenches.
i am trying to determine the size of an existing conventional drainfield. how do i go about that? - Alan H. 1/17/12
Alan, on determining the size of an existing septic drainfield, if you are unable to find as-built plans or a site map for the septic fields, you will want to review the advice given at SEPTIC DRAINFIELD LOCATION (article links listed at the "More Reading" links at the bottom of this article ) on how to find the location of the drainfield. The rough size and shape of that area will be as close as you'll get without some digging and probing.
my septic and leach field have been in for 30 yrs. working great. the state of PA wnt to expand their flood easement over the leach field. Will this cause a problem if i need to replace it 5yrs down the road? Example getting permits etc. - Todd 3/3/1012
Todd I'm unsure what the flood easement means in your case but two conditions give us concern:
1. any vehicle traffic passing over your leach field is likely to damage if not destroy it, leading to a need for costly repairs and perhaps relocation of the field
2. If the flood easement means that your leachfield is subject to actual flooding, you know that the fields will not function in any sanitary way when saturated.
I just moved to a brand new house and that has a septic. I have septic first time in my life. I have some guest due to family occasion. Alarm goes off every other day. I am told that septic tank pumps 400 gallons a day. If we use more than 400, it sets off alarm. We have 1250 gallon tank and other tank is 1000 gallons that pumps water to drainfield.
I have a big family so I will have guest several times a year. What should I do? Can I pump it manually e.g. total 800 gallons a day until guests are gone? or Should I buy a small pump e.g. 2 hp and pump water out on a small ditch? Can I do that? What is the solution? What is maximum gallons I can have pumped? My drain field is 6 ft deep and about 500 total length. - Jay 5/7/12
Jay, it sounds from your note as if the septic system was designed to handle a daily maximum wastewater volume of 400 gallons per day. I'd start by asking a local wastewater system engineer or installer who is familiar with yoru system design to assure that everything is currently working properly since if not, some repair action is needed.
Next, in my OPINION if your regular usage is exceeding the design capacity of the system, you'll need to address that problem by a combination of steps to minimize water usage to lighten the load on the system combined with an expansion of system capacity. To do anything else might seem to "work" in the short run, but actually the result is discharging untreated or insufficiently treated wastewater into the environment - contaminating groundwater, potentially contaminating local wells etc., and risking a near term system failure.
What is the required size for a septic system for 70 units - RV Trailer Park 6/1/12
For commercial property with no bedrooms how is the size of the drainfield determined? - Anon 7/18/12
Good question, Anon.
For an RV Trailer Park, using government design manuals  as a guide, we estimate that daily wastewater volume will be 75 to 125 gallons pe day per person, with a typical volume of 100 gpd. The drainfield will be designed to handle (100 gallons x number of visitors) per day.
For a commercial property with no bedrooms, the wastewater design volumes vary enormously depending on the type of facility. More details are at COMMERCIAL SEPTIC DESIGN
Sizing for commercial drainfields and septic tanks) is more difficult than for residential installations. Residential designs start with a simple assumption of the number of occupants and asn average daily wastewater volume (common is 150 gallons/bedroom or 75 gallons per day per person, though some sources use larger numbers).
We often hear complaints from people who say "we are just two people living in a four-bedroom home, why does our septic system have to be designed to handle eight people (4 bedrooms x 2)?" A sensible answer is that the wastewater system should be designed to handle the number of occupants that the building is designed for.
But commercial installations vary widely in the wastewater volume used per person per day depending on the type of facility, the number of visitors to it, how long they stay there, and what activities they pursue.
For example a gas station at a turnpike may have thousands of visitors per day, many of whom use the toilet facilities - that's why we stop at a rest stop - even though the typical length of visit is relatively short.
The US EPA Wastewater manual as well as some U.S. state DEC/DEP wastewater specifications guidelines have published a series of tables of ranges of wastewater production for different types of facilities per visitor or user along with other sources of possible usage volume (such as number of parking spaces).
In a separate article COMMERCIAL SEPTIC DESIGN [in process] we provide excerpts from that larger body of information.
(June 23, 2014) email@example.com said:
can i add to my existing leach lines because i think a tree entered one of my lines i have not opened my ground up yet
Certainly, provided your site has adequate space and the addition meets clearance or property setback requirements, adding additional drainfield capacity is generally a good move.
You might want first to try to diangose the trouble more accurately. If a tree is close to an existing drainfield line and the new line is not quite some distance away, the new line is likely to become invaded and blocked as well - unless you remove the tree and stump.
(Apr 6, 2015) Anonymous said:
Have the strict rules on number of bedrooms in an existing system been softened due
to water meter records?
Not that I know about, certainly not on a country level. You don't say where you live. Tell us or check with your local building or health department.
(Apr 27, 2015) Anne said:
Can I put the tank close to the house and run the drain field across from the house on the other side of a narrow (one car) driveway?
Check your local codes on the required minimum distance between septic tank and building foundation. And watch out that some idiot doesn't damage your house foundation during excavation.
To run a septic line under a drive you'll need Schedule 80 PVC pipe as well as proper installation (depth, bedded in sand, no sharp rocks in backfill) . Keep in mind it's not just the weight of your car, it's the weight of other heavier vehicles that may pull into the drive over the life of the building.
(May 12, 2015) ken said:
I have two field lines. I can swich field lines. One is 330 ft the other 250 ft. one has gravel the other dont. both are 10 inc gravels pipe.
i think it may be to deep its 36-40 inch deep i don't have good ground seems like the ground doesn't soak the water quick enough grass doesn't grow there should i take off 8-10 inch of dirt off the topthen paint grass seed. i have to switch every three weeks. please advice. thanks
Ken it sounds as if you need two simultaneous drainfields.
You could go to an alternating bed septic design, but clogged soils may recover in years but not weeks.
(June 2, 2015) JAMES M. PESZKO said:
I am a first time user of your Q&A. I am very impressed already! I have some of these very same questions.
I am not going not going to pay some ? (fill in with your words) a huge amount of money for something I know I can fix myself.. Yeah, I might be an EE but have studied a lot about pipes, soils, etc. (Civil engineering) Can't be that difficult.
Thanks for the note JMP. Some of my best friends are E.E.'s and some are stunningly sharp.
On the other hand, watch out for the "IBM Problem" - my coinage for people who are smart, know they're smart, but don't know that they don't know something. One of my buddies built a stunning deck - with very precise cuts and fasteners aligned to within a millimeter. But she didn't know that drywall screws are not structural fasters.
So it's also smart to ask questions and find out who knows. (Which is not me, I'm only fourteen years old.)
And I agree also about asking questions of your "expert" - some are certified, licensed, but lack experience in residential construction.
The risk may be a "safe" design solution but one that's unnecessary. In Hyde Park a civil engineer specified a major excavation, house lift, and new foundation for a foundation problem that would normally be solved by a helical pier.
His design worked and was safe but it cost about 10X what was suggested later by a foundation engineer who knew about helical piers and residential foundations.
I also agree that often a smart, careful homeowner who will take the time to actually read the proper design or other reference materials for a home repair topic may do a more careful, more thorough job than a stranger who may be less motivated and who may be in more of a hurry.
Working together makes us smarter.
(June 4, 2015) Mike said:
We moved into our new house in Sept of 2014, by the first of March 2015 the toilets began gargling when flushing. I contacted my contractor and he suggested that the drain field may be frozen (I had covered it with 6" of straw) but whatever I pumped it a couple times to get through the winter.
Once spring was here, there was wet spots on the drain field and the toilets eventually gargled again.
The soil here is pretty much clay, they had put in 300' of drain field. We are planning on digging this up and replacing it. What is the best solution to this, as far a the soil material to haul in? I'm in North Dakota. thanks
I'd look into a mound or raised bed septic system if your local health department will approve it.
(June 5, 2015) Todd said:
Our field lines are seeping up through the ground and will need to be replaced. The home is 20 years old and the health department knowingly without our knowledge approved perk test in 1995 after all indicators was red.
Now we have issues and inspector mentioned it to my wife when he came out that he knew it would be bad. Should the health department be liable for currently replacement cost of $15,000 for their negligence?
I understand the frustration one would feel on thinking that a building official didn't do a proper job.
OPINION: But you'll have a tough time proving that twenty years ago someone approved a septic design plan that was improper.
The arguments can include change of soil conditions, failure of the property owners to properly maintain the system, normal aging and soil clogging, or who knows what. I'm no lawyer (I abandoned law school after learning the difference between law and justice back in 1966)
but I suspect your lawyer would tell you that unless you can obtain a clear written statement that places responsibility on someone you won't get further than enriching the attorneys involved.
You've heard perhaps the phrase
You can't fight city hall.
In fact, you can sue city hall, depending on where you live. For example in New York article 78 of the CIvil Practice Law and Rules gives you that right, but as other writers have pointed out, the law requires that you exhaust your administrative remedies first.
In turn that means you can expect to be exhausted and financially destitute before your local municipality is going to even consider paying damages for a opinion expressed by an official off the record two decades ago.
That's the case unless you can actually prove a violation of the law.
And I imagine there will be a group of active and mostly retired code officials, health department officials, a septic design engineer, original builder, septic contractor or excavator, all standing in a circle pointing to one another.
(June 6, 2015) bob said:
From the septic tank we have a slope that drops about 4 feet in 20 feet from the septic tank, is this to much of slope to install a leach field.
Yes you can install a septic system when steep slopes are involved, Bob, though extra costs will be involved. The design specifics will depend on whether the steep slope is just between a septic tank and a drainfield that are each located on relatively level areas or the steep slope is the area into which the drainfield itself has to be located.
Generally the solution is to install leach lines that run parallel to the slope or fall line of the hill.
(Aug 5, 2015) Anonymous said:
I'm looking at buying a two bedroom home. The home built in 1960 had an old metal septic tank.
As it was leaking, the owner had it replaced with a 1000 gallon poly tank. While the instalation was taking place, I was able to see and measure the length of the two existing lateral lines from the T junction. One line was 13' and the other was 12'.
Are these lines big enough to handle the input when I have 6 to 10 people staying over? thanks Jim
No, that sounds unlikely. In fact unless your soil is unclogged gravel it's unlikely that a total of 15 feet of drainfield line is adequate for a family of two at modern wastewater usage levels.
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.
Continue reading SEPTIC DRAINFIELD SIZE - home, or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.
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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.
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