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How To Build Septic Tank Systems

in 6 easy steps

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 Introduction: You can avoid the cost of hiring a septic designer and excavator, but don't let friends convince The excavator makes decisions every day that most homeowners never make once in a lifetimeyou that these jobs are without value. Be aware that like buying property this aspect of homesteading, if not handled with cold reason and great care, could ruin Thanksgiving dinner and many restful nights sleep. How to build septic tank systems?  It's all here.

Why not do it yourself ? Doing anything yourself can be heroic or crazy depending on the outcome. In more than 25 years spent designing site layouts and septic systems in the scenic countryside of beautiful Washington State, I have seen more than one home-site ruined forever by sloppy excavation, poor water well placement and the dozens of other pitfalls awaiting an eager baron or baroness heading up the country with a heart full of dreams.

Homeowners can save money and prove to their friends that they have mastered this aspect of house construction. However, be warned that a few down days with a rented backhoe can quickly eat up any anticipated saving by doing-it-yourself. Remember also that digging up a power line and darkening your block cannot only embarrass you but it could cost you more in repairs than your entire project budget.

The skills of an experienced septic designer or excavator increase in value with smaller sites and in poor soil conditions. If you know in your heart that you lack these skills, don't risk your peace-of-mind.

1. The site evaluation is step one: Get a scale map of your property if you don't have one.  The scale drawing below is from an application for a septic tank system that was drawn by a professional septic designer and submitted to the local health department. The site, classified as a difficult one, is for a small lot in a rural subdivision with a community water system (tank in distance).  At least a water well does not have to be worked into the layout. 

the building site before development showing two test pits at the rearhand drawn scale sketch of the site with test pits shownThe closest neighbor's house can be seen in the photo to the left. The dirt dug from the two test holes can be seen at the back of the lot at the edge of the farm field above and on either side of the green power transformer. The site is ready for the site evaluation with local health. Design and construction will be cramped. With small and tight lots, every inch of space is needed. A survey is usually advised to ensure that setbacks from property lines are met. Note the red survey steak marking the lot corner pin behind the transformer.

Local health will usually provide a package of forms and information explaining the application process. The helpfulness of health inspectors is generally better in the rural counties, but they will not tell you how to build septic tank systems. That information follows;

Important note: The site should not be cleared, scraped, leveled or otherwise disturbed until the details of the site layout have been figured out. If a water well is required on the site, it should be witched or located only after a place has been found for the septic system.

Besides the description of soil texture and depth, the site evaluation usually has a scale hand drawn map like the one above. The map must show the location of the test pits.  Most health departments require this evaluation to be done by a licensed person.  Also, the surrounding conditions are shown in scale including property lines, buildings, wells, pipes, paved areas, surrounding septic systems if known, roads, easements, trees and banks.  You must also show slope direction, drainage ways, surface water, and surrounding land uses.  You can go to your local health records to see what your health department requires here and to see if you can make your own map.

video clip showing how a site evaluation (formerly known as a perc test,) is performed The test pits and site exploration are by backhoe:  A couple of 6 foot or deeper pits are usually required by local health in most counties. The pits or "test holes" have replaced the traditional percolation or "perc" test that used to be common in most areas.   Today, the soil expert must have one or two or more pits dug with a back-hoe 5 or more feet deep, and wide enough and sloped so that he or she may walk down into them and sample the soil in the side walls. See the video clip above right.

Soil stability and safety are always considered for the inspectors and for casual members of the public. Kids are drawn to equipment and holes in the dirt. Leaving the holes open for later inspection is inferior to having local health attend the site inspection and close things up in a single trip for the backhoe. If the holes must be kept open, say for a later visit by local health, barricade or safely cover the holes to keep the public out.

The site evaluation by backhoe is a better test than the old perc test because it allows a wider look at the soil.

2. Soil classification is next: The soil classification system used in most places in the USA (and in some other countries), is called the US Department of Agriculture Soil Conservation Classification System. Be aware that the ability to determine such qualities as "soil texture" and "soil structure" comes with experience. If you blow the soil classification, you may wind up with less or more drainfield than you need. More drainfield means needless expense. Less drainfield means early failure, usually with guests over.

The most restrictive soil type found in any of the pits in the area of the drainfield should guide your choice in the case of mixed or confusing soils anywhere within the drainfield area. Most test pits involve four or more soil types as you look down through the soil profile within each pit so study the soil directly below where the drainfield will be (between 36 inches and 48 inches below grade usually).

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3. You now determine the area of trench for your  home: Dig a couple of six foot holes at each end of your drainfield  area and check the soil type.

How to use the chart: In spite of what you think, the drainfield size is not dependent on the number of bathrooms or fixtures in the home. Almost all health departments use the number of bedrooms in a home as a way to size drainfields. The number of people in the house (usually two to a bedroom), and their usual daily water use (usually sixty gallons per person per day), is how the flow rate is established. This flow rate of about three hundred sixty gallons of water use per day inside a three bedroom house determines the amount of sewage that must be sent to the drainfield. Once you have decided what type of soil is found under your future drainfield, look up the drainfield area required for your house size in the chart below and you have the required drainfield size. Note: The chart below was compiled from the rules of various states and many years of practical experience. Your county may have very different sizes and by law you must use the chart from your local health department.

Drainfield Size Chart

Soil Class


Soil Type: take a soil sample 5 ft below grade in drainfield area by digging a person sized test pit at each end

2 Bedroom House 240 gallons of sewage/ day

3 Bedroom House 360 gallons of sewage/ day

4 Bedroom House 480 gallons of sewage/ day

Commercial Projects  for every 100 gallons of sewage/ day

# 1

Coarse Sand - most sand soils will require pressure distribution if gravel is present and/or the lot is small

200 sq ft gravel or

10 six ft vaults or

16 four ft vaults

300 sq ft gravel or

14 six ft vaults or

22 four ft vaults

400 sq ft gravel or

18 six ft vaults or

28 four ft vaults

84 sq ft gravel or

3.6 six ft vaults or

5.6 four ft vaults

# 2

Medium Sand

240 sq ft gravel or

12 six ft vaults or

20 four ft vaults

360 sq ft gravel or

16 six ft vaults or

25 four ft vaults

480 sq ft gravel or

21 six ft vaults or

33 four ft vaults

100 sq ft gravel or

4.3 six ft vaults or

6.7 four ft vaults

# 3

Fine Sand - Loamy Coarse Sand - Loamy Med Sand

300 sq ft gravel or

10 six ft vaults or

16 four ft vaults

450 sq ft gravel or

15 six ft vaults or

24 four ft vaults

600 sq ft gravel or

20 six ft vaults or

32 four ft vaults

125 sq ft gravel or

4.1 six ft vaults or

6.5 four ft vaults

# 4

Very Fine Sand - Loamy Fine Sand - All Loams

400 sq ft gravel or

14 six ft vaults or

22 four ft vaults

600 sq ft gravel or

20 six ft vaults or

32 four ft vaults

800 sq ft gravel or

26 six ft vaults or

42 four ft vaults

167 sq ft gravel or

5.4 six ft vaults or

8.4 four ft vaults

# 5

All Silt Loams of Good Structure

540 sq ft gravel or

18 six ft vaults or

28 four ft vaults

800 sq ft gravel or

26 six ft vaults or

42 four ft vaults

1070 sq ft gravel or

35 six ft vaults or

55 four ft vaults

223 sq ft gravel or

7.2 six ft vaults or

11.2 four ft vaults

# 6

Other Silt Loams - All Clay Loams - All Clays Note: some clays may not qualify for any system

1200 sq ft gravel or

39 six ft vaults or

62 four ft vaults

1800 sq ft gravel or

60 six ft vaults or

94 four ft vaults

2400 sq ft gravel or

78 six ft vaults or

122 four ft vaults

500 sq ft gravel or

16.0 six ft vaults or

25.0 four ft vaults

bed type drainfields allowed if the soil type is fine sand or coarser - pressure distribution required with all bed designs and if soil is extremely gravelly or very gravelly plus a sand lining may be required under the drainfield

one square foot of drainfield is measured like a carpet looking down from the top - "sidewall" is not considered although some health

departments continue to give "credit" for up to 6 inches of sidewall calculated by measuring the perimeter of the bed or trenches - using

sidewall confuses calculations and has been discredited due to the fact that water in soil seldom moves sideways except when saturated

Return to Drainfield Design     Return to Site Evaluation "Perc Test"

Drainfield layout requires at least 2 equal sized trenches. From the septic tank a single drain line is no longer advised.  The separation of flow into two, three or more lines is accomplished with a distribution box or "D-box" to split the flow.  In the D-box pipes are equipped with simple flow control valves in the form of eccentric plugs that evenly split the flow between lines.  The effluent (sewage that has gone through the septic tank)  flows downhill from the tank outlet, through the D-box and down to the individual trenches where it spills out onto the floor of each trench where treatment starts in the soil at that location. Don't forget, the individual trenches are NOT sloped, but are dead level from one end to the other.

Your local health department rules. Your county health department has rules and guidelines to follow. Sometimes rules are the same as state guidelines, but sometimes more restrictive rules special to your county must be followed.  These rules include depths and setbacks and construction details.   Such things as how far you can place the drainfield from a water well (usually 100 feet), a building (usually 10 feet), a water line (usually 10 feet), a stream, pond or lake (75 to 100 feet), the septic tank (generally 5 feet), or even a tree (5 to 50 feet depending on species) cuts and banks (varies state to state, and county to county) are spelled out. They will specify how deep the trenches can be (usually no deeper than three feet max from final grade down to the floor of the trench), and even whether you may use the plastic vault technology shown here.  The key to getting the correct results from your health department people, is to present your ideas clearly and completely in your drawings. There is often some back and forth with the health department. You may be asked several times to return with fresh drawings to meet all of the site requirements and rules that the department has on its books.

Before you begin drawing your project, you must have all the rules from local health. This may be in the form of a two-page handout or a thick ordinance of dozens of pages.

By the way, a square foot of gravel in a drainfield is measured like a carpet, covering a 12 inch by 12 inch piece of ground, except the gravel is one foot thick. Therefore, a drainfield trench "covering" 300 square feet, is a pit, three feet wide and one hundred feet long, with a foot deep of gravel in the bottom. (In reality, you will use two trenches at 50 feet long each.) The gravel is covered with a tough but thin fabric to keep the dirt out called filter fabric. The whole thing is buried with a cover (backfill) of one to two feet of native soil. The thickness of backfill depends on the desired trench depth. The "gravel" is not really gravel at all, but a uniform clean washed rock with granules one and a half inches in diameter known to a gravel pit operator as "inch-and-a-half drainrock".

trench type drainfield before backfilling for a larger rural homePVC Vault Technology: You may now in most areas substitute rows of plastic chambers instead of the old school leachfield gravel. Infiltrator and Hancor are two companies who supply this product shown to the right. The vaults hook together like a freight train and are very easy to transport and build. The green pipes are inspection/squirt ports for the homeowner looking into the drainfield especially if trouble occurs down the road. The ports are also used for periodic squirt testing and are cut down to the finish depth after the drainfield is backfilled. Construction details for this house and dozens of other systems are shown in our drawings.

Excavators and installers usually prefer the vaults over drainrock once they have tried them. Most health jurisdictions are recognizing the value and efficiency of the vault technology and the use of vaults is becoming widespread across the USA. Gravel is used for geometric reasons and can be cheaper particularly if the job is close to the supply. Gravel is also used if livestock or vehicular traffic could invade the drainfield. Drainrock is a little more robust than most vaults.

Also, some health inspectors give "credit" for the increased storage capacity of the vaults over gravel systems. Each linear foot of standard vault in the ground yields around 10 gallons of storage. A linear foot of 3 ft wide gravel trench yields less than half of that even with the highest quality drainrock. You may get 400 Sq Ft of "credit" for our trench that actually contains only 300 Sq Ft of ground. This may save vital space on a tight site.

Vaults now come in several widths, lengths and heights. However, the standard width is a little less than three feet wide (but generally 3 feet is the width used for design and space planning) and two standard lengths 6 and 4 feet.

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Time To Prepare the Drawings:

The Septic Design CD available on this web-site allows you to create plans that may be modified and edited to meet the needs of your local health department whatever they are. Often a few simple notes or other changes can satisfy the specific rules of most health jurisdictions, but notes and requirements must usually be typed onto the plans. Drawings are the language of construction of anything. Besides, this is not your grandfathers septic system. Our drawings talk the new lingo.

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Study the drawings shown below. Permit drawings also tell others how to build septic tank systems. These two sheets represent the complete drawing set for a gravity design for the three-bedroom house on our model property shown below. The level of detail required may depend on the inspector's preference.  However, all buildings, walkways, property lines, retaining walls and the location of the original test holes must be shown.

4. Time to apply for the permit: Now Make Your Application to Local Health: Submit your plans at the counter of the local health department. You will have to fill out the application form and pay the permit fee. The example septic system drawings for r our model property are shown below. You now must wait for the drawings to be reviewed and approved before going on to the construction phase.  Ask local health what the customary time frame is for permit application review. It should not take more than two or three weeks at the most to receive the approved construction permit in the mail unless there is a local backlog, a staff shortage or a deliberately slow process in your area.

click on the picture of the septic design drawings for a close-up look

Local Health Will Approve Your Drawings usually with a signature and a date. Construction should usually begin within one year of that date to avoid losing the permit and having to start from scratch with a new fee. You should get a copy of the permit in the mail once the drawings are approved. You must follow all the notes and details on the drawings exactly. Look for and follow the printed instructions and any special notes that local health adds to the permit face.  Local health will inspect your work before anything is covered so plan your job carefully. Click on the drawings for a close-up. You will naturally apply your logo and address on the plans.

The left hand drawing shows the Site Plan with all of the required information in scale. The drawings are for printing on standard 8.5 x 11 copy paper. This format is the most handy size for filing and favored by health departments for storage convenience. Some health departments have gone all electronic and our drawings convert into PDF files with ease. Click on the drawings above to see them in PDF format. 

Using the Gravity Drawing Archive:

The drawing on the right above is our drawing GTGV3WDD from our CD for gravity septic designs. You can see it in our archive about half way down on the page under the topic "Vault designs using 3 trenches only." Once you have a plan view layout for your system, find the matching detail page for your trench cross section, trench layout and standard notes.


5. Finally it's time to start building the septic tank system:How To Build a Septic System - Step1

The layout stage of the job transfers the design to the ground. Excavation is like sewing. The Layout of all parts of the design (the pattern) must be projected onto the site (the cloth) and cutting in more than once will increase wastage. If there was a caution for young excavators it would be never to over excavate. Measuring twice and cutting once applies here. Before you begin, be sure that the drawing is accurate and that it matches the site exactly. Compare the site plan on your approved design to a plat map of the site if you have to, to make sure that the drawings match and show the correct scale. On the approved drawings from the health department, add up all strings of dimensions to make sure that a critical measurement has not been missed. Always find and mark the corners of the property. Do not use existing fences or the advice of neighbors unless you are sure of the property lines from a legal survey.

Here in the image to the left, the construction crew is checking the location of the third trench.  A long 200 foot tape, a Lufkin twenty-five-foot tape, wooden stakes and a red rattle can of spray paint are the tools of choice for this stage. Gravity systems will only work well if the bottom of the trench sits in undisturbed ground and is level. Sewer slope does NOT apply in the drainfield. Level means dead level. Read this caution again if you are not with me.

Be careful not to begin excavation until local health has approved your plans, no mater how eager you are to get started. Even though you have applied for and paid for a permit - If the inspector has not approved the design, they may require layout changes. He or she will not listen to excuses such as "we had to get the backhoe back, so we started without the permit." RULE: Once you have applied, listen to the inspector and follow all instructions, or start reading this page again from the top.

checking the levels of all components - Step 2 in How to Build a Septic SystemMeasure Twice Cut Once: The tripod on the sidewalk on the left side of the picture to the right is a laser level. This tool has been around for twenty years and developed with public money. Do not begin this job without one. With the special level rod, an assistant may walk around the site and find the elevation of any point of the excavation within an inch (2 cm) anywhere within sight of the tripod by listening to a beep from the box on the rod when you are "on elevation."

This tool (the laser level) is particularly useful to ensure that the trenches are not over excavated.  Constant attention to elevation is the key to a successful job. Older leveling technology such as a surveyor's transit may be used as long as you know how to use it, but don't ever think you know how to build septic tank systems with a standard construction bubble type level. I do not care if your level is 20 feet long. Laser levels and all the needed tools are available everywhere to rent

Once you are done excavating, the health inspector must be called for a final inspection of the job. Then backfilling occurs. The tanks, pipes and vaults can and should be backfilled around their sides during construction though. More counties are requiring leak testing of the tank these days. Vacuum testing, pressure testing or water testing of all tanks may be required in your area. Backfilling around the sides of concrete tanks may be postponed until the final inspection to check for leaks if desired by the county inspector. Most plastic tanks will distort under such stress and must be backfilled at the same time as they are filled with water for the first time.

In the picture to the right, the crew is using the top of the septic tank in the foreground as a work table and a datum. Notice the difference in elevation between the top of the tank and the surface of the nearby sidewalk (hint - about a foot or 30 cm.) This tank will be less than a foot underground when the lawn is planted.

the excavator stands beside the D-Box showing three lines following the proper effluent line slope down into the drainfieldPipe Slope and Type for Proper Drainage: A word here about pipes and drainage. PVC (polyvinyl chloride) pipe comes in a variety of sizes and types. Pipes in a gravity system are either 4 inch diameter. ASTM 3034 or 4 inch diameter ASTM Schedule 40. These pipe types are both good for sewer lines between the house and the tank. These pipes can not be crushed by stepping on them. ASTM 2729 (perforated drainfield pipe is made of this thin stuff) and ASTM CL160 (known as Class-160) are too thin to stand up to being driven over with a car, etc when placed in shallow trenches (a normal condition in modern septic systems.) Some counties allow the thin wall pipe between the tank and the drainfield - we do not.

Pipe Slope: There are only 2 pipe slopes used in septic systems, the sewer slope (contains "solids") and effluent line slope (no solids.) The "building sewer" must be sloped (sometimes called fall) at between 1/8" per foot and 1/4" per foot. In other words the pipe must drop at least one inch for every 8 feet of sewer line, and not more than one inch for every 4 feet all the way to the septic tank. Less slope and the flow is too slow to clear the pipe. More slope and the water theoretically drains off too quickly and the solids will become stranded and cause a blockage. This idea of maximum slope no greater than 1/4" per foot has been challenged recently, but an engineering justification is required to allow this in practice. Almost all health departments require this minimum slope of 1/4" per foot and will require "stair stepping" down steep slopes. However, do not ignore the minimum slope requirement in a sewer line for any reason, or that spot on your property will become a perpetual problem in the system. Effluent lines can slope less than the sewer, and more. They can "fall" with as little as 1/64th of an inch per foot (experienced excavators only) and effluent lines drop at any maximum slope you choose.

Drainfield Slope: It never hurts to remind everyone learning how to build septic tank systems, that the drainfield trenches and beds whether vaults or gravel are built dead flat with no slope at all end-to-end or side-to-side. Some ancient drainfield plans call for a slope in the drainfield, but this practice went out with the manufacture of clay sewer pipe. Resist the advice of friends and neighbors to put a slope in the drainfield. Trenches can naturally be at different elevations based on the ground contours, but each trench or bed (leaching bed) should be level within an inch up or down over the entire bed or each individual length of trench. Your system will last longer.

If the yard down to the tank from the house slopes too much, the sewer must be dropped straight down in a series of drops followed by proper sewer slope to the next drop. The drops are good places for cleanouts. Remember also that any right angle bends in the sewer pipe are not allowed when changing direction in the sewer, down or side-to-side. Always use two 45 degree "elbows" or "90 degree sweeps" instead of a 90 degree elbow to allow proper cleaning with a snake or roto-rooter. Also put in a clean-out to direct a snake (a metal probe for cleaning out blockages from the surface) starting at the outside of the house toward the tank every 50 feet (100 feet absolute maximum) in the sewer line. Clean-outs and 45 degree elbows are not needed in the effluent line. If the ground slopes towards the house from the drainfield area, or if the site is flat, you may need to pump the effluent from the elevation of the septic tank up to the drainfield (or leach bed.)

After the septic tank, and after the D-Box all the way to the drainfield, the slope may be effluent line slope, as little as 1/64" per foot. This requires expert excavation practice. The effluent line may drop down at any angle. All effluent lines must drain fully and not have a "sag" in the line that could cause pools to form and in cooler climates cause a line freeze. Click here or here for more about freezing septic systems.

view of the almost completed septic system from aboveThis view of the site shows the layout:

The septic tank can be seen on the right of the view between the first trench and the patio.  On this job, and normally, the septic tank is delivered from the tank manufacturer and lowered from the truck into a hole prepared by the excavator.  The tank hole has a flat floor at the exact depth.  Call the tank supplier to get the exact tank height and depths of the inlet and outlet.  In rocky ground, two or three inches of pea gravel may be needed to protect the tank bottom. The septic tank is often the first thing to be added to a new house site.

A concrete tank in our area runs about $550 delivered up to 40 miles, and further for a few dollars more.  Fiberglass tanks and ribbed polyethylene septic tanks are not usually advised as they are not only more expensive but they may not be sturdy enough to do the job.   Flexible tanks tend to distort over time particularly when they are pumped out.   Don't trust a tank that can't safely be filled with water in the parking lot without distorting or leaking. Steel tanks are still used in remote, colder locations such as Alaska, but sewage is corrosive to steel.  Metal components have a limited life in septic systems and should be avoided if possible .

In the view to the left, the backhoe is working on the third trench. The vaults are in place in the first two trenches and the vault units seen near the backhoe are waiting to go into the third trench when it is finished.  The site is being watered with the hose as work progresses.  The site may need to be sprinkled for a day or more prior to construction for the same reason.  Although this keeps dust down, the primary reason for adding water to dry soil is to provide proper moisture for compaction of the dirt around the pipes and parts. Water is usually used to soak the sides of the tank to compact the soil and fill in voids to avoid sink holes in the lawn. Very wet or very cold weather is not the time to build septic tank systems. Good excavators avoid working in extreme conditions except in emergencies. Working in poor weather usually involves return trips to smooth and refinish surface grades and is more expensive.

Before the vaults are placed in the trench, the side walls of the trench are roughened with a garden rake. This important step prevents the formation of bacterial scum on the trench walls called "smearing" which can cause early failure of the drainfield.  In some areas, the excavators weld teeth to the side of the backhoe bucket to do installer connects the D-Box to the vaults - Connections are Step 3 in How To Build a Septic System this job in one pass.

Septic systems are plumbing systems:

In this view, the concrete D-box has been placed where it belongs and the effluent lines are being pushed into the seals.  Although all the other pipe joints are glued together, where the lines enter the tank and D-box, the pipes are pushed into the special seals without glue. Older construction methods require lines to be sealed into D-boxes and tanks with concrete grout.  To the left bottom of the picture to the right, the yellow plastic rotatable flow control seals are waiting to be placed into the pipe ends to evenly distribute the flow between the trenches. Next to the excavator is the level rod for checking the level of the D-box and the pipes. The system works because of the difference in elevation between the sewer line leaving the house, the septic tank, the D-box, the effluent lines to the trenches and the floor elevations of all of the trenches themselves. These final elevations must be taken from the approved plans (see above.)  The finish grade and backfill will have to cover everything at the required depth when you are done.  Often the top of the septic tank is used as a datum or benchmark.  The floor of a well house, an existing slab, any reasonably immovable object will qualify as a benchmark but many of these things are built after the septic. Property corners on a site can change elevation.

looking into the D-BoxDistribution must be even:

 The D-box is ready to go. The flow control has been set by pouring a bucket of water into the box and rotating the seals to make the opening in each seal break the surface at the same elevation.  This evenly distributes the flow between the three outlet lines leading to the three trenches.  The inlet to the d-box is on the right and does not have a flow control seal.

The sandy dirt around the pipes has been walked and compacted to ensure that the lines will not be disturbed during backfilling.  The backfill will form a slight hump over the drainfield so that eventual settling will not cause low spots over the drainfield over time.  Note the expandable urethane foam sealing the effluent line at the top of the view to the left where the northeast effluent line disappears into the vault.  This attention to detail by some excavators distinguishes the best from the rest.   Skill and attention to details will help you more than anything else to ensure that the septic system will not fail within its useful life of fifteen to twenty-five years.  Most Owners' say at the first meeting with the designer "I don't want to have to mess with it." They will call the contractors back to the site at the first sign of trouble.  Doing it yourself won't hand you this option.

6. Finishing up, final inspection & backfill:  Final health sign-off can be obtained only after calling the local health department for a final inspection. They will tell you to fix anything you missed based on the approved plans. Hopefully you followed the plans closely. After all, you spent the time and money getting the plans and getting them approved.

close up of health official badge Some installers do not always follow all the notes and details. Some self installers either forget or avoid the final health inspection, or getting a health permit at all. This once was considered heroic behavior. However, these days people are not getting away with these careless practices in most jurisdictions. Appraisers, lenders and insurance companies check for permits before writing loans and policies. With us pulling out of a deep  recession littered with distressed and poorly managed property, there is a renewed interest in these companies only underwriting solid property value. Missing permits identify risky property - do not  gamble with the value of your dream.

Backfilling will require special attention to soil moisture and soil compaction as well. You should backfill and compact the sides of tanks, vaults and pipes as you build, mostly by filling in layers and compacting by walking along the fill areas until it "feels" solid. Some excavators will "Walk" the trenches with the rear wheel of the backhoe, but this may collapse the vaults if done too aggressively.  The backfill soil must have no stones, be slightly damp and not wet.  If soil compaction is ignored, the lawn will settle, sink-holes can appear, and the tank itself may shift during a heavy storm.

Backfilling may have to happen even though the site is soaking wet or frozen.  A visit later in the year may be required to fix the dips prior to the laying of sod.  Weather conditions should be considered in all excavation projects.  Putting off the job may be easier in the long run than fighting the elements.  Good excavators avoid working in the worst weather unless the client understands the risks and is willing to pay extra for precautions.  Remember, slightly damp (not wet, not bone dry) ground is the easiest to work with.

details of the septic system are all important - this image shows the cleanout before backfillingNever Underestimate the Importance of Details: This image to the right shows a clean-out. The sewer line to our project shown here just outside the house passes through the foundation wall about ten inches below the finish grade.  The sweep and riser and cap (shown here white), allow the sewer line to be "snaked" out all the way to the septic tank without working from the crawl space, a good maintenance feature. Note the 45 degree elbow below the cap, directing the cleanout downstream toward the tank.  back to pipe slope

The important point here is that the sewer line should not pass through the foundation below the footing.  The example in the small inset photo shows the wrong way to do this.   That sewer line in the inset photo is much too deep to meet the depth requirements for the drainfield that local health will require.

Side note: A cleanout identical to this one is often provided beside the driveway or near the RV parking area to act as an RV dump.

How to Design the Grade Elevations of Septic Systems:

The plans for our example on the right called for an invert (bottom of the pipe) elevation at the outside wall of no more than thirteen inches below finish grade. Contractors who leave this detail to the plumber may wish they had not. Redoing the house plumbing is one fix and shaving dirt off the surface of the yard is the other. In most health regulations, the floor of the drainfield trench can not be deeper than 3 feet from finish grade of the yard.

On a mostly level site, set your depth at the drainfield trenches first. Start from the floor elevation of the lowest trench (remember - in most jurisdictions NO deeper than 36 inches from finish grade.) Remember too, the floor of trench has NO SLOPE and is dead flat end to end. The 4 inch diameter effluent line pipe enters the drainfield trench with the "invert elevation" (bottom of the pipe) 6 inches to 8 inches above the floor of the trench. Next the D-Box is located upstream of the drainfield. All effluent lines must drain fully into the drainfield from the D-Box. Always avoid "bellies" or dips in the effluent lines which will trap effluent. To inspect long lines, pour 5 gallons of water in the top of a dry line to yield an equal volume into a bucket at the bottom.

Now determine the invert elevation of the effluent line that enters the highest trench (8 inches above the floor of the highest trench.) Place this effluent line with minimal slope up to the D-Box which will now fix the elevation of the D-Box. Add 3 inches for drop inside this box. From the inlet of the D-Box we follow the effluent line up to the septic tank outlet following the same effluent line slope rule. This will determine the outlet elevation, and therefore the vertical placement of the septic tank. Between the outlet and the inlet of the tank, add 3 more inches for the internal working of the septic tank. This brings you up to the septic tank inlet invert elevation. From this septic tank inlet elevation we figure the route up to the house foundation following proper sewer slope. This gives us the pipe invert elevation of the sewer leaving the house as shown in the above photo to inform the plumbers. 

Some old school excavators will try to justify a deeper excavation for freeze protection. In several years of occasionally hard freezes in central Washington State (we see 25 degrees F below zero on rare nights), I have never seen a sewer line freeze that was built at the proper slope however shallow. Tanks too are safe from freezing even when placed slightly above ground such as under a deck. Water lines however are buried at 3 to 4 feet or much more in northern areas to prevent freezing. Sewer lines and septic tanks follow a different set of rules than water lines because septic system lines are seldom deeper than a foot or 2 below grade.

Pressure systems however are protected from freezing in a slightly different way. I favor full drainback of all transport lines, manifolds and even laterals. In vault type drainfields the one inch diameter lateral lines are strapped to the underside of the vaults with zip ties. Enough downward pointing orifices must be provided for complete lateral drainage. If you are not following all of these technical suggestions, consider hiring a good professional designer for drawing up your plans, particularly if you are required to provide pressure distribution. 

Much further north, things are different. Tanks are insulated with foam insulation, deeper systems are allowed and heating elements are sometimes added. Vast areas of the planet are unsuitable or poor for septic. Solid rock, for instance, or permafrost will not provide safe treatment. However the places people chose to live, in most cases are suitable places for septic already. And if not, technology steps in, on marginal and fringe areas with more complex (and expensive) septic systems.

the result of sticker shock over the cost of a septic system - monopoly tycoon guy falling over backwardCosts: With constantly changing cost of oil and fuel, costs are going up generally. Excavators run equipment all day and therefore have increasing fuel bills. Septic systems have many parts made of plastic (petroleum based) and these parts are often manufactured far away and are trucked to the site. Designers and regulators spend time in the field and a lot on the road often to remote sites in trucks. Septic is likely the only choice for sewage treatment from wilderness to farm to resort and even within some city limits. Mostly experts rather than homeowners are required to design systems in most areas. As long as full time public health is available in your county, you will likely not be able to avoid getting a permit. Public health fees for permits to build septic systems are like taxes and they help to fund public health services. The cost of public health is seldom figured into construction. Fees or taxes if you prefer are due with your permit. Local health also manages a huge database of systems and sets design and construction standards for a large and broad industry, and a green one at that. Licenses for qualified excavators, designers, inspectors, pumpers and other waste managers are administered at state and local levels through local boards of health and many state level relationships. 

Drainfield Cost: To determine the retail cost of your drainfield, figure that 4 foot plastic vaults will cost $65 - $75 in place if an excavator does it. Older style drainrock type drainfields in place will cost roughly $5 - $6 per square foot (drainrock is $18-25 / ton delivered with a cubic yard of drainrock representing about 1.3 tons of material). Drainrock runs about $450 for a 360 sq ft (typical 3 bedroom) drainfield plus $250 for delivery (depending on distance from the pit regardless of who is driving the dump truck.) You must include the cost of digging out the drainfield area, placing and shoveling rock and placing and assembling the perforated pipes (this is mostly equipment and shovel time.) Some rock is lost by all good excavators who leave the dirty bottom of the pile on the ground.

The Septic Tank: The standard 1000 gallon tank is about $800 - $1100 delivered. 1000 gal should be plenty, but some health departments require larger tanks, presumably to accommodate the negligent homeowner who thinks pumping is not required - ever. For larger tanks add a dollar per gallon so a 1250 gallon (next size up) is $950 + $250 = $1200. Plastic tanks will cost you a lot more whether you go for one of the many flimsy ones or one of the good few. Because of their weight, concrete tanks are always regional. However, many new plastic tanks are competing for a sprouting national market. This website is not confident that plastic tanks are ever superior to standard concrete designs except in a few rare instances - fly-in or poorly accessed locations for instance.

Other Items: Some jobs require a D-Box with fittings and seals for about $70 to $130 total depending on the number of outlets. Gravel and foam peanut drainfields require filter-fabric to keep dirt out of the drainrock before backfilling at $30 to $50 depending on trench length (fabric and center pipes are not needed with vaults for standard gravity systems like this one.) Excavation costs another $4000 - $6000 for digging the tank hole, pipe laying, sewer line excavation final backfilling and other tasks, or a lot more depending on job layout, fighting slope, or say a cramped site with overhead or underground obstacles. Effluent lines and sewer pipe are usually 4 inch diameter ASTM 3034 PVC or for the sewers, the heavier Schedule 40 ABS. Pipe costs between $3 to $4 per foot and comes in 20 foot lengths. Forget about using cheaper ASTM 2729 for your sewers as a money saving move - too flimsy. Your tank may need two plastic or concrete access risers and a filter. $300-$400. There are pipe fittings, epoxy to seal the risers, and other incidentals for a few hundred more. Click here for further cost estimates.

Additional Tips on the Site Evaluation (Perc Test):

For digging test holes, backhoe operators generally charge between $500 and nothing depending on driving distance, and whether the test holes will lead to a construction job on the property. Shop around. Remember also that someone can fall into the pits and the property owner is usually liable so plan to have them filled in as soon as you can. Cover the pits with plywood and place barriers around them if you are forced to wait until local health has checked them. You may be able to dig and fill them in at the same time if health department personnel show up for a site evaluation with all parties present (the designer, excavator, local health inspector and owner). The system (or civil engineer) designer sets up this meeting. The owner does not have to be at the meeting and can be represented by the designer. However, many designers (and I am one) prefer to have the owner there if the designer and owner (or developer) have only met on the phone. Checks for the permits and the designer's fee can be secured by the designer and building plans and preferences and site planning ideas can be shared.

How Long Should a Site Evaluation Take?: The site evaluation on a normal simple lot should take less than an hour. Complicated soils and difficult sites with rock outcroppings or shallow water tables or slopes can take two or more hours. Each six foot deep hole takes only five minutes or less to dig, ten minutes to log and study and five to 15 minutes to fill in or more depending on surface finish, sod replacement, irrigation repairs and any unexpected findings. Health personnel should show up once the two to four holes are dug and should only be on the site for ten to twenty minutes if all goes well. Difficult soils and sensitive areas can involve many more holes and can take much more than twice as long for all parties. Beyond the site evaluation, the designer should spend around an hour or more on the site to locate surrounding wells, locate all buildings, structures, trees, shorelines, banks, map contours, roads, easements, fences, power lines and all underground utilities (although the excavator is generally responsible to call for locating underground utilities).

Unlicensed equipment operators can usually dig the pits for you but only experienced people can locate the drainfield area and arrange the parts of your site to your best advantage. Any county licensed  excavator knows how to build septic tank systems so even though you know what you are doing, don't forget to listen when you get the chance.

What if My Site Fails the Evaluation?!:  The site evaluation can show that you have a "difficult site." Here is some advice about proceeding with your project in spite of finding "problems" during your site evaluation.

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Who Can Evaluate the Site? The local health department is qualified, but in many states health inspectors will no longer perform your assessment. In most counties these days, the expert is a consultant to the property owner and is known as a state or county licensed designer, a state licensed engineer or a registered soil scientist. Who can perform evaluations depends almost exclusively on the state rules. Homeowners can often build the system but few jurisdictions allow owners to evaluate the soil because of the potential risk to public health.

In the past, and in some counties even today, local health inspectors (sanitarians) will do this for you and hand you the drainfield location, trench length and tank size. Your local health department will tell you over the phone who can evaluate the site or even send you a list of qualified people. Most local health departments have the rules and the application forms available online.

Where to Put the Drainfield: Drainfield location is generally downslope from the tank.  Our example above is a tight site. There is little extra space on the site so the drainfield location is more or less fixed.  Whatever soil is found in this location, will have to be accepted as a given. You may find that there is not enough area on a tight site for the drainfield and the required replacement area. It is then up to the owner to scale back the project by dropping a bedroom, one space in the garage or the spot for a future pool.

Do not expect local health to allow you to trim even a few inches off one of the the trenches to make the drainfield fit in a tight spot unless such relaxations are customary. The work of health inspectors is public, very public. The behavior of health inspectors is under constant and sometimes unsympathetic review by neighbors, developers and their local politicians. Inspectors who give out too many favors will eventually have to face charges of playing favorites. My 25+ years of practice in various jurisdictions has never caught a health inspector trying to pull off any serious shenanigans. 

On a larger property, if the test holes show poor soil (usually silty or restrictive soil, solid rock or water in the test hole), another pit or two or three is dug to get a better location for the drainfield. Fill in the poor test pits before local health sees them if you want. In most counties this is customary and not considered deceptive. Local health will only evaluate what you show them. Your job is to put the best appearance on your property for the evaluation in the location where you want the drainfield. Finding a favorable location for the drainfield and then sizing it to match the ability of the soil to absorb water is the job of the soil expert.

Do not worry about the soil expert tearing up the site during the exploration phase. As with medical diagnostics, the knowledge gained will outweigh the risk.  

The Last Word is Have Fun But Be Cautious. Every site has its own special qualities and potential traps. Without the knowledge of the various details of designing and building your septic system, you may wind up spending a dollar to save a dime.  If you are doing the work yourself, you may miss an important detail like getting a final inspection tag for the system from the inspector to prove compliance with all standards. Tricks of the trade only come to designers and excavators who have learned to successfully complete job after job. Sometimes a project is better left to those who can make it look simple.

However if you DO do-it-yourself, and it all works out, every plumbing moment in your day will be that much more satisfying.

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Last Revised: 11/11/2019   How to Build Septic Tank Systems
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