How To Build a Septic System



Introduction: You can avoid the cost of hiring a septic designer and excavator, but don't let friends convince you 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 a septic system?  Its all here.

The excavator makes decisions every day that most homeowners make once in a lifetimeWhy not do it yourself? Doing anything yourself can be heroic or crazy depending on the outcome. In more than 10 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 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.

the building site before developmentThe 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 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 neighbor's house to the left in this photo is already built. The dirt dug from the two test holes can be seen at the back of the lot at the edge of the farm field 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 lots, every inch of space is needed. A survey is required to ensure that setbacks from property lines are met. Note the red steak marking the corner pin behind the transformer.

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

The map is vital. 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.  Go to your local health records to see what your health department requires here and to see if you can make your own map.hand drawn scale sketch of the site with test pits shown

The test pits and site exploration are by backhoe. Two 5 to 7-foot or deeper pits are required by local health in this and 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.  This is a better test than the old perc test because it allows a wider look at the soil.

Design the drainfield as big as local health tells you. This size is based on soil type and house size (bedrooms). The chart below is an example of one that health inspectors use to make sure designers provide enough drainfield to meet local and state regulations.  Use this chart only to check your site requirements supplied by your local health department.  The soil expert will pay particular attention to the soil found at a depth of three to four feet below the finish grade at the location of the future drainfield. This is the soil that will receive the bulk of the "sewage effluent " and provide treatment.  For background on treatment processes, see Septic System FAQs. Although the test pits can be dug by hand, very few homeowners are willing to take this task on when a few minutes with a back-hoe solves the problem.

Backhoe operators generally charge $100 or so 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 open and close them at one time if health department personnel will allow this and be on the site to approve the soil assessment. 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 a septic system so even though you know what you are doing, don't forget to listen.

When you stop exploring soil and begin construction, there is often no going back.  The site at this point may be ruined forever by getting it wrong.

Drainfield location is generally downslope from the tank.  In our example, there is little extra space on the site so the drainfield location is more or less fixed.  Whatever soil is in this location, will have to be accepted as a given. The designer may have to negotiate with the contractor and the owner if there is not enough space for everything. You will generally not be able to clip a little off one of the trenches to fit in the corner of the driveway.

On a larger property, if the test holes showed a poor soil type (usually silty or restrictive soil, solid rock or water), another pit or two or three could be dug to get a better location for the drainfield. Fill in the poor test pits before local health sees them. In most counties this is customary and not considered deceptive.  Local health will only evaluate what you show them and 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. In most counties this expert is a consultant to the property owner and is known as a designer, an engineer or a soil scientist. In some counties, the health inspector or "sanitarian" will perform this duty. In some increasingly rare cases the local health inspector will both design the septic system, and perform the final inspection of the job too.

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. Most test pits involve four or more soil types within each pit.

How to use the chart: The drainfield size is not dependent on the number of bathrooms or fixtures in the home. Most jurisdictions use the number of bedrooms as a way to size drainfields.  The number of people in the house, usually no more than two to a bedroom, and their water use is how the flow rate is established.  This flow rate of about three to four hundred 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.

By the way, a square foot of gravel 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".

To confuse things further, you may in some areas substitute a couple of rows of plastic vaults known as Infiltrators (a trade name) instead of the gravel. These vaults hook together like a freight train and are very easy to build. Most health jurisdictions have recognized the value and efficiency of the vault technology. Also, some health inspectors give "credit" for some of the sidewall of the trench for vault or gravel systems. You may get 400 Sq Ft of "credit" for our trench that actually contains only 300 Sq Ft of ground. This may be vital space on a tight site.

Costs: To determine the retail cost of your drainfield, figure that vaults will cost $95 each in place ($25 - 30 at the store), and drainrock type drainfields in place will cost roughly $4 per square foot (drainrock is $15-20 / ton delivered). The tank is about $500 delivered. Some jobs require a D-Box with fittings and seals for about $130 total, and perhaps filter-fabric to keep dirt out of the drainrock at $30 to $50 depending on trench length (fabric not needed with vaults). Hiring an excavator costs another $7-900 for pipe laying, sewer line excavation final backfilling and other tasks, or more depending on job layout and slopes. Click here for further cost estimates.

In most counties, in most states, the new plastic vault technology has replaced the traditional gravel drainfield.  Although, the Septic Design CD available on this site provides for both gravel and vault drainfields. Eventually the vaults will become the standard drainfield type. Notice that the vaults provide treatment with less area depending on the soil type. Usually local health will allow a quarter to a third less drainfield with the vaults. This is because of the superior storage volume provided by the vaults compared to drainrock.

Are you now ready to determine the area of trench needed for your home. Remember, dig a hole or two and check the soil type.

Soil Class Soil Type: take a soil sample 3 to 4 feet below grade in the drainfield area by digging a pit 2 Bedroom House 3 Bedroom House 4 Bedroom House
# 1 Coarse Sand 200 sq ft gravel or 10 vaults 300 sq ft gravel or 14 vaults 400 sq ft gravel or 18 vaults
# 2 Medium Sand 240 sq ft gravel or 12 vaults 360 sq ft gravel or 16 vaults 480 sq ft gravel or 21 vaults
# 3 Fine Sand - Loamy Coarse Sand - Loamy Med Sand 300 sq ft gravel or 10 vaults 450 sq ft gravel or 15 vaults 600 sq ft gravel or 20 vaults
# 4 Very Fine Sand - Loamy Fine Sand - All Loams 400 sq ft gravel or 14 vaults 600 sq ft gravel or 20 vaults 800 sq ft gravel or 26 vaults
# 5 All Silt Loams of Good Structure 540 sq ft gravel or 18 vaults 800 sq ft gravel or 26 vaults 1070 sq ft gravel or 35 vaults
# 6 Other Silt Loams - All Clay Loams - All Clays 1200 sq ft gravel or 39 vaults 1800 sq ft gravel or 60 vaults 2400 sq ft gravel or 78 vaults

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

Drainfield layout requires equal sized trenches. From the septic tank, at least two equal sized trenches or lines must be designed.  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.

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.

Study the drawings shown here.   Drawings tell others how to build a septic system.  These two sheets represent the complete drawing set for a three-bedroom house on our model property. 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.

septic design drawing of the proposed septic system detail sheet of the septic design

You now must wait for the permit to be approved before going on to the construction phase.  Ask local health what the customary time frame is for permit application review.

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 starting 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.

How To Build a Septic System - Step1The layout stage of the job transfers the design to the ground. The Layout of all parts of the system must be projected onto the ground.  Before you begin, be sure that the drawing is accurate and that it matches the site exactly.  Compare the site drawing on your approved design to a plat map of the site to make sure that the drawings match and show the correct scale.  On the drawings, add up all strings of dimensions to make sure that a critical measurement has not been missed.  Find and mark the corners of the property.  Do not use existing fences or the advise of neighbors unless you are sure of the property lines from a legal survey.

Here, the crew is checking the location of the third trench.  A long (100 feet plus) tape, a twenty-five-foot tape, stakes and spray paint are the tools of choice.  You must be deliberate here because ground that has been excavated cannot generally be filled in and reused without problems.  The system will only work if the bottom of the trench sits in undisturbed ground and dead level.

Be careful not to begin excavation until local health has approved your plans, no mater how eager you are to get started.  If the inspector requires 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."

checking the levels of all components - Step 2 in How to Build a Septic SystemElevations are critical for all components. The tripod on the sidewalk on the left is a laser level device.  With the special level rod, an assistant may walk around and find the elevation of any point of the system within sight of the tripod by listening for a series of beeps.

This tool is particularly useful to ensure that the trenches are not over excavated.  Constant attention to elevation is the key to a successful job.   Older technology such as a surveyor's transit may be used, but don't attempt construction with a standard construction bubble type level. These tools are available anywhere to rent.  The health inspector must be called for a final inspection of the job before any backfilling occurs.  However, the tanks, pipes and vaults should be backfilled around their sides during construction.

The crew is using the top of the septic tank in the foreground as a work table .  Notice the difference in elevation between the top of the tank and the surface of the nearby sidewalk.  This tank will be less than a foot underground.

Pipe Slope and Type for Proper Drainage. A word here about pipes and drainage. PVC (Polly Vinyl Chloride) pipe comes in a variety of sizes and types. Pipes in a gravity system are 4 inch diameter. ASTM 3034 or ASTM Schedule 40 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 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. Some counties allow the thin wall pipe between the tank and the drainfield. 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). Less slope and the flow is too slow to clear the pipe. More slope and the water drains off too quickly and the solids will become stranded and cause a blockage. Do not ignore this slope requirement for any reason, or the spot will become a perpetual problem in the system.

If the yard down to the tank slopes too much, the sewer may have to be dropped straight down in a series of steps. 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" instead of a 90 degree elbow to allow proper cleaning with a snake or roto-rooter. Also put in a clean-out (sweep "T" with a riser and cap) to direct a snake (a metal probe for cleaning out blockages from the surface) 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 although some health departments want them there anyway. If the ground slopes towards the house from the drainfield area, or if the site is flat, you may need a pump in the tank to deliver the effluent to the proper elevation.

After the septic tank, and after the D-Box all the way to the drainfield, the slope may be as little as 1/64" per foot if the excavation is expert. The effluent line has no solids in it so the slope is only required to keep the pipe clear of liquid. The effluent line may drop down at any angle for the same reason. 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 clearly shows the layout. The septic tank can be seen on the bottom right of the view.  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 house site.

A concrete tank in our area runs about $500 delivered up to 40 miles, and further for a few dollars more.  Fiberglass 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. 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 above, 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.  This is to provide proper soil moisture for compaction of the dirt around the pipes and parts.

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 this job in one pass.

installer connects the D-Box to the vaults - Connections are Step 3 in How To Build a Septic System 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 view, 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 from the plans.  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. 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 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.

Finishing the Job:  Final backfilling after the final health department inspection will require special attention to soil moisture and soil compaction as well.  Some excavators will "Walk" the trenches with the rear wheel of the backhoe, but this may collapse the vaults if done too aggressively.  The soil must 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 importantNever Underestimate the Importance of Details: This image 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. This clean-out if placed near the driveway can be used as an RV dump as well. 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.

The plans for our example system called for an invert (bottom of the pipe) elevation of no more than thirteen inches below grade.  Contractors who leave this detail to the plumber may wish they had not.  Raising the house plumbing, or lowering the septic tank and cutting dirt off the yard to lower finish grade are expensive ways to "fix" this problem.

Some 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 below on rare nights), I have never seen a sewer line freeze that was built at the proper slope however shallow.  Tanks too are safe even when placed slightly above ground such as under a deck.  In our area, water lines here are being buried at 3 to 4 feet to prevent freezing.  Sewer lines and septic tanks follow a different set of rules than water lines. 

Pressure systems however must be protected from freezing.  I favor full drainback of all transport lines, manifolds and even laterals. An exception to this rule is when the laterals are designed to be placed on the ground inside the vaults. In this case, only drain the laterals into the drainfield - this will reduce growth of anaerobes inside the parts of the system and keep the orifices clear. On second thought, don't put laterals on the ground at all. Suspend them from the vaults for complete drainage (good for aerobes), or strap them to concrete blocks with plastic ties and then put them above the ground. Future postings will go over the details of pressure system design including protective strategies, (the first and most important of which is hiring a good designer). Much further north, things are different. Tanks are insulated with foam insulation, deeper systems are used, and heating elements are sometimes added. Vast areas of the planet are unsuitable or poor for septic. Solid rock, for instance, or perma-frost. However where people like to live, in most cases are suitable places for septic already. And if not, technology steps in, in marginal and fringe areas with more complex septic systems. 

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 tag for the system from the inspector to prove it got a final health inspection. 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: 09/11/2004   How to Build a Septic System
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