Septic drainfield design & size determination: this article explains how we choose the size of a septic leachfield or soakaway bed or drainfield. We discuss rules of thumb used to set the size of a conventional septic drainfield. Septic drainfields, also called leach fields, absorption beds, soil absorption systems, soakaway beds, and leaching beds, perform the functions of septic effluent treatment and disposal in onsite wastewater treatment systems, conventionally called "septic systems".
Here 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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A conventional septic tank performs roughly 45% of the sewage treatment or less at a private home served by a septic system. The rest of the wastewater treatment and ultimately the liquid disposal occurs in the drainfield.
[Click to enlarge any image]
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.
The size of the absorption field needed (in square feet of area, presumably also unencumbered by trees, driveways, buildings, etc.) can range considerably depending on the soil percolation rate. Sketch at left - USDA.
A lot with a good percolation rate or "perc" of perhaps one inch of percolation in three minutes might require about 4500 square feet for a typical three bedroom home. If the same home were built where there was a poor a soil percolation rate of an hour per inch, 9000 square feet or more might be required for the absorption area.
Typically, septic leach fields (synonyms: drainfield, leach bed, soakaway bed, absorption bed) are built by placing perforated effluent distribution pipes in a field or bed of gravel. The field is a series of trenches that may be up to 100-feet long and 1 foot to 3 feet in width, separated by six feet or more, depending on local requirements, and sometimes constructed leaving space between the original lines to install replacement leach lines when needed. - paraphrasing USDA.
Drainfield size and location also have to take into account local zoning - setback requirements from property borders, setbacks from streams, wetlands, wells, water supply lines, and other encumbrances.
Below we summarize the basics of septic drainfield design, followed by more detailed septic field design specifications for common types of drainfields or soakaway beds.
In the most common design of drainfield, perforated pipes are buried in gravel-filled trenches to form the drainfield. [See our warnings at (3) Construction of Shallow Septic System Absorption Trenches]
[Click to enlarge any image]
Our second drainfield sketch (left, USDA) shows a slightly different view, in this case an isometric cross-section drawing of a septic drainfield trench.
The sketch above shows a cross section of a typical drainfield trench, and places below the trench the critical biomat as well as other septic field design areas and considerations. (Source US EPA who in turn obtained the drawing from Ayres Associates)
Where lot space does not permit drainfield trenches such as I've just described, a septic engineer may specify that seepage pits or galleys are to be installed. These fit in a smaller space since a single pit may be 6' to 8' in diameter. But the depth to which effluent is being delivered (4' or more) means that the sewage effluent is unlikely to be fully treated by a biomass. These systems may successfully "dispose" of effluent but they are probably not adequately "treating" it.
The Biomat: The formation, clogging, and measures to protect and extend the life of the biomat, or organism layer below and around soil absorption system effluent discharge piping is discussed at Septic System Absorption System Biomat Formation as a subchapter of this text.
Summary Table of Typical Disposal Field Specifications
|Drainfield / Soakbed / Seepage Bed Component||Specification||Comments|
|Absorption trench (leach line)|
Maximum slope of trench (grade)
|4 inches / 100 feet|
Minimum slope of trench (grade)
Other Soakbed / Leachfield Construction Specfications
These example specifications for a septic disposal field, also referred to as drainfield, leach field, soakbed, soak trenches etc., are based on Shasta County California's septic code cited below.
A high quality septic drainfield or leach bed design includes inspection ports or pipes that permit inspection of the condition of the field. Vertical pipes are placed, usually at the end of each drainfield trench or section or at critical or suspect areas near the drainfield to permit monitoring of liquid levels in the drainfield trenches. If a few ports are included outside the drainfield area they will aid distinguishing between a drainfield suffering local effluent saturation from an area groundwater problem that also impedes the drainfield operation.
Some installers trim the inspection ports to ground level, sealing each with a removable cap to permit inspection of the drainfield condition without interfering with mowing.
The following specification for septic drainfields or leaching beds, of various types, designs, and depths, is adapted and expanded from: New York State Appendix 75-A.8 Subsurface treatment, of New York's Wastewater Treatment Standards for Individual Household Systems. We also include excerpts and references from other U.S., Canadian, and other authorities at state and provincial levels, and from the US EPA Onsite Wastewater Treatment Systems Manual which is available free from the US EPA and is provided in a linked-to copy at the REFERENCES section of this article.
All effluent from septic tanks or aerobic tanks shall be discharged to a subsurface treatment system. Surface discharge of septic tank or aerobic unit effluent shall not be approved by the Department of Health or a local health department acting as its agent.
[DF NOTE: This section discusses the design requirements for septic absorption fields, also called leach fields, drain fields, drainfields, or conventional soil absorption systems.]
(i) The minimum distances that absorption fields shall be separated from other facilities are shown in Figure 1 and Table 2.
(ii) A minimum of four feet of useable soil shall exist above bedrock and groundwater with a minimum separation of two feet to the lowest part of any trench.
(iii) Absorption fields shall not be built under driveways, parts of buildings or under above-ground swimming pools or other areas subject to heavy loading. Surface waters shall be diverted from the vicinity of the system.
(i) The required length of absorption trench is determined from Table 4A based upon the percolation test results and confirmed by the soil evaluation. The maximum trench width for design purposes shall be 24 inches. Only 24 inches shall be allowed for absorption area calculations. Where trenches exceed 24 inches in width, calculations of absorptive area shall be based on a width of 24 inches.
(ii) Adjacent trenches shall be separated by at least four feet of undisturbed soil. Individual trenches shall be constructed parallel to the ground contours with trench bottoms as near level as possible. They need not be perfectly straight but abrupt changes in direction shall be avoided.
This table specifies the necessary length of a conventional septic drainfield trench as a function of the soil percolation rate and the anticipated daily wastewater flow from the building served.
Specifications for the required length of septic system drainfield trenches based on input flow rate and soil percolation rate are given in the table below.
Septic Wastewater Effluent Input Flow Rate (Gallons per Day) 
|Required number of feet of absorption trench, assuming a 2-ft wide conventional gravel trench|
|1 - 5||108||125||162||187||216||250||270||312||325||374|
|6 - 7||130||150||195||225||260||300||325||375||390||450|
|8 - 10||145||167||217||250||290||333||360||417||433||500|
|11 - 15||162||188||244||281||325||375||406||469||488||563|
|16 - 20||186||214||279||321||372||429||464||536||557||643|
|21 - 30||217||250||325||375||433||500||542||625||650||750|
|31 - 45||260||300||390||450||520||600||650||750||780||900|
|46 - 60||290||333||433||500||578||667||722||833||867||1000|
|Dosing not required
|Dosing system or alternative design is required|
 Original source: New York State NYS75-A.8 Table 4A.
 Conditions that require more than 1000 feet of septic drainfield trench must have an alternative dosing system design.
The following is adapted from our engineer's article summarizing "How Big Should the Septic Leach Field Be" found at HOW BIG SHOULD THE LEACH FIELD BE?.
Determining the required size of a leach field is a bit more complicated. The first thing to consider is the nature of the soil in which the leach field is to be constructed. Because water has to be absorbed in the soil, we need to know how fast it can be absorbed. This is called the percolation rate and is expressed as the time it takes for water in a test hole to decrease in level by one inch (minutes/inch).
We must also know the type of soil and whether seasonal changes in the natural level of groundwater will interfere with the satisfactory operation of the system. Seasonal groundwater must be more than four feet from the bottom of the leach field trenches. Judgments regarding the soil conditions and percolation rates are best left to a professional. If the soil percolates very quickly, (less than one minute per inch) or very slowly (greater than 60 minutes per inch) it will not be possible to install a standard leach field in the existing soil.
We must now determine the amount of water that has to be absorbed each day. As with the septic tank sizing, there are also "rules of thumb" that can be used to find out how much water must be absorbed each day for each bedroom in the house (expressed as gallons per day per bedroom).
For older houses (built before 1979) we must allow 150 gallons per day (gpd) per bedroom. For houses where the toilets are limited to no more than 3.5 gallons per flush and the faucets and showerheads are limited to 3 gallons per minute or less, we must allow 130 gpd per bedroom. For houses with water-saving toilets that use only one gallon per flush we allow 90 gpd per bedroom. The required flow rate is found by multiplying the appropriate flow by the number of bedrooms (in this case, we do not have to count a garbage disposal as a bedroom).
Knowing the rate at which water can be absorbed by the soil (the percolation rate) and the flow rate (in gallons per day), we can use the following table to calculate how many square feet of absorption field is needed. [Readers will notice that this table is similar to but less detailed than our typical state or board of health table above at Table 4A.]
What is the Required Size of the Septic Leach Field?
Minutes per Inch
Application Rate - Gallons
per Day per Square Foot
|1 - 5||1.2|
|6 - 07||1.0|
|8 - 10||0.9|
|11 - 15||0.8|
|16 - 20||0.7|
|21 - 30||0.6|
|31 - 45||0.5|
|46 - 60||0.45|
Soil with a percolation rate less than 1 minute per inch or more than 60 minutes per inch is unsuitable for a conventional system.
This table specifies the allowable wastewater application rate into the soil of a conventional septic system drainfield as a function of the soil percolation rate for percolation rates between 1 minute per inch to 60 minutes per inch. Soils with a percolation rate of less than 1 minute per inch should not be used for a conventional septic drainfield.
Readers will note that this table considers only the dimensions of the bottom of the drainfield trench in considering the effective soil absorption area. Typically a conventional drainfield trench is 2 ft. wide, so the effective absorption area is simply 2 ft. x field-length in ft.
TABLE 4B - SOIL APPLICATION RATES [Click to enlarge any image]
Soil Percolation Tests Perc Tests or Deep Hole Test for Soil Absorption Rate Testing - how to conduct soil testing for percolation rate determination
(i) Perforated distributor pipe shall be used in the trenches. Solid (non-perforated) pipe shall be used between the distribution box and the trenches. Perforated pipe shall be made of rigid or corrugated plastic and be labeled as fully meeting ASTM standards for use in septic systems. Corrugated plastic pipe delivered in coils is not to be used unless provision is made to prevent the recoiling or movement of the pipe after installation.
(ii) Aggregate shall mean washed gravel or crushed stone 3/4 - 1 1/2 inches in diameter. Larger diameter material or finer substances and run-of-bank gravel are unacceptable.
(iii) The aggregate shall be covered with a material that prevents soil from entering the aggregate after backfilling, yet must permit air and moisture to pass through. The preferred material for covering the aggregate is a permeable geotextile. Untreated building paper or a four inch layer of hay or straw is acceptable. Polyethylene and treated building paper are relatively impervious and shall not be used.
(i) Trench locations and depths should be marked by stakes before the trenches are excavated. The natural surface shall not be significantly disturbed. If the site is re graded or similarly disturbed, the soil shall be allowed to stabilize and new percolation tests conducted.
(ii) The trench depth shall be as shallow as possible, but not less than 18 inches. At least six inches of aggregate is placed below the distribution line and two inches above the line. The earth cover over the aggregate should not exceed 12 inches in order to enhance natural aeration and nitrogen uptake by plant life. Trenches shall be excavated to design depth with bottoms practically level. Heavy equipment shall be kept away from the field because the weight may permanently alter soil characteristics due to compaction, cause trench cave-ins, and/or mis-align and break pipe.
(iii) Trench bottoms are to be raked and immediately covered with at least six inches of aggregate.
(iv) Any smeared surfaces on the trench walls are to be raked. Distributor lines are carefully placed on the aggregate and covered with aggregate to a depth of at least two inches over the top of the pipe. Additional aggregate may be required to bring the top of the aggregate to within six to 12 inches of the surface.
(v) In gravity distribution systems, the pipe shall be carefully sloped at between 1/16 inch and 1/32 inch per foot. Grades shall be determined by an engineer's level, transit or carpenter's level.
(vi) After the upper aggregate is placed, the geotextile, untreated building paper, hay or straw is to be immediately installed and the trench backfilled with native soil. If the trenches cannot be immediately backfilled, they should be temporarily covered with an impervious material such as treated building paper to prevent sidewall collapse and siltation into the aggregate.
(vii) The earth backfill is to be mounded slightly above the original ground level to allow for settling and after settlement the entire area should be graded without the use of heavy equipment and seeded with grass.
[See our warnings at (3) Construction of Shallow Septic System Absorption Trenches]
Because they omit gravel, the soakaway bed or drainfield trench dimensions for no-rock or gravelless septic systems are different than for conventional drainfields. If you are working on a design for GRAVELLESS SEPTIC SYSTEMS continue reading or use the web-links at SEPTIC SYSTEM DESIGN ALTERNATIVES navigate the online septic systems book or our other online documents.
These are used on sites where an useable layer of soil is overlaid by three to five feet of impermeable soil.
(i) There shall be at least four feet of useable solid beneath the impermeable layer.
(ii) The required length of absorption trench is determined from Table 4A based upon percolation tests conducted in the underlying soil.
(i) Trenches are excavated at least two feet into the useable layer and backfilled with aggregate or coarse sandy material containing a low percentage of fines more permeable than the underlying material to a level 30 inches below the original ground surface.
(ii) An absorption trench system as described in Section 75-A.8(b) is constructed in the upper 30 inches of the excavation.
These systems are used where there is at least two feet but less than four feet of useable soil and/or separation to boundary conditions.
(i) A minimum two foot separation must be maintained between the bottom of each trench and all boundary conditions.
(ii) The bottom of each trench must not be above the original ground surface.
(iii) Material of the same permeability as the underlying original soil shall be used as fill material. The depth of the fill shall not be greater than 30 inches above the original ground elevation.
(iv) An absorption trench system as described in Section 75-A.8(b) is designed using the percolation of the underlying original soil.
(i) Heavy equipment shall be kept out of the absorption area.
(ii) Fill material is carefully placed within the absorption area.
(iii) The edge of the fill material shall be tapered at a slope of no greater than one vertical to three horizontal. On sloped sites a diversion ditch shall be placed on the uphill side to prevent runoff from entering the fill.
(iv) The absorption trench system is constructed in the fill material, extending into the existing natural soil.
I recently installed leaching chambers and before I could backfill the trenches it rained for a couple of hours, is it ok to use the wet backfill to fill the trenches? - B.D.
OPINION-DF: A competent onsite septic field construction inspection by an expert may find additional concerns that need to be addressed to assure a long drainfield life. That said, here are some things to consider:
CONTACT us to suggest alternative septic system designs and specifications.
(1) A cut and fill septic system is an absorption trench system installed on sites where impermeable soil overlays a permeable soil.
Cut and fill septic systems may be used where all the following conditions are found:
(i) A soil with a percolation rate slower than 60 minutes per inch, such as clay or clay loam, overlays a useable soil with a percolation rate faster than 60 minutes per inch;
(ii) At least three feet of useable soil is available beneath the tight soil;
(iii) All minimum vertical and horizontal separation distances can be maintained as described in Table 2.
(i) It shall provide for the removal of the overlaying unusable soil and replacement by soil having a percolation rate comparable with the underlying soil;
(ii) An absorption trench system is designed as described in Section 75-A.8(b).
(iii) The required length of absorption trench is based upon the percolation of the underlying soil or the fill material, whichever has the slower percolation (lower permeability).
(i) The area excavated and filled must provide at least a five foot buffer in each direction beyond the trenches.
(ii) The material placed above the trenches shall have a percolation rate faster than 60 minutes per inch.
(iii) Original surface material shall not be used as backfill above the trenches.
(iv) The surface area of the fill system must be mounded and graded to enhance the runoff of rainwater from the system and seeded to grass.
An absorption bed system operates on a principal similar to the absorption trench except that several laterals, rather than just one, are installed in a single excavation. This reduces the effective sidewall infiltration area per linear foot of lateral or leach line.
(i) A bed system may be built in soils with a percolation rate between one and 30 minutes per inch. A bed shall not be built where the soil evaluation indicates silty loam, clay loam, or clay.
(ii) Slope of the site shall not exceed eight percent.
(iii) Bed systems are more practical on sites that are long and narrow with a minimal slope.
(iv) All vertical and horizontal separation distance requirements shall be met.
(i) Pressure distribution is required for the installation of an absorption bed system. The local health department having jurisdiction may allow the use of siphon dosing on specific sites.
(ii) The maximum width of the bed shall be 20 feet. The maximum length of each lateral from a pressure manifold shall be 100 feet. Utilizing a center manifold system, a bed may then have a maximum length of 200 feet. Laterals for siphon dosing systems in beds are limited to 75 feet.
(iii) The depth of the bed shall be between 18 and 30 inches below original ground level.
(iv) Laterals shall be spaced five (5) feet apart. Two and one-half feet (2 1/2') must be provided between the laterals and the sidewalls. In the maximum width of 20 feet, only four laterals may be installed.
(v) Using pressure distribution with a center manifold, a bed system shall have maximum dimensions of 205 feet by 20 feet.
(vi) The required bed bottom area shall be calculated from the application rates shown in Table 5 - below
TABLE 5 ABSORPTION BED SEPTIC SYSTEMS -- REQUIRED BOTTOM AREA PERCOLATION RATE APPLICATION RATE MINUTES/INCH GALLONS/DAY/SQ. FT. ------------------ ------------------ 1 - 5 0.95 6 07/13/2010 - 07 0.80 8 - 10 0.70 11 - 15 0.60 16 - 20 0.55 21 - 30 0.45 30+ Not Acceptable
(i) Heavy construction equipment shall be kept outside the proposed bottom area of the bed. [See our warnings at (3) Construction of Shallow Septic System Absorption Trenches]
(ii) The required bed bottom area is excavated as level as practical. The bottom and sides of the excavation are hand raked to reduce soil smearing.
(iii) After excavation, a six inch layer of aggregate is placed across the bottom of the bed.
(iv) The laterals are laid level on the aggregate and covered with aggregate to a level two inches above the top of the pipe.
(v) The entire bed area is covered with a permeable geotextile. Untreated building paper or a four inch layer of loose hay or straw may be substituted if a permeable geotextile is unavailable.
If you scrolled down in this document to look for information on Seepage Pit construction, that subchapter discussing the design and use of seepage pits for onsite wastewater disposal is published as a separate web page: Seepage Pits.
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