How a septic drain field is designed

Drain field design starts with two questions: how much wastewater will the house send to the field each day, and how fast can the soil on this lot absorb and treat it? A soil evaluation answers the second, the number of bedrooms usually stands in for the first, and the state or county code turns both into a required trench area, depth and layout. This page explains the method. It is not a substitute for a design.

Drain fields must be designed or approved by a licensed professional and permitted by your local health department before anyone digs. Codes for sizing, materials, depth and setbacks differ from state to state and often from county to county.

Step 1: Soil evaluation and the perc test

The soil does the treating, so the design depends on it. NC State Extension notes that a professional must examine the soil before installation and size the field to it.

What evaluators look for:

• Texture. Sandy soils drain fast, clay soils slowly. NC State says high-clay soils absorb too slowly, while very sandy soils or shallow bedrock may not treat wastewater before it reaches groundwater.
• Color. Uniform brown to red soil usually means good drainage. Gray soil or gray mottling means the soil is seasonally saturated, which NC State calls unacceptable for a drain field.
• Depth to a limiting layer, such as bedrock, dense soil or the seasonal high water table.
• Slope, drainageways, floodplains and shrink-swell soils, which Virginia Cooperative Extension lists as part of the evaluation.

The perc test. A percolation test measures how fast water drops in a soaked test hole, in minutes per inch (MPI). The University of Nebraska describes dividing the time by the drop in water level, and repeating until three readings in a row are within 10% of each other. Some states rely mainly on a perc test, while others, like Virginia, have a licensed soil evaluator assign a soil rate from the soil profile.

Nebraska’s guidance shows the limits:

Perc rate	What it means
Faster than 5 MPI	Effluent moves too fast to be treated
5 to 60 MPI	Range for a conventional drain field
Slower than 60 MPI	Effluent may not soak in and may surface; an engineer is required

Vertical separation is the depth of suitable, unsaturated soil under the trench. It varies a lot by state. Ohio State says most Ohio sites need at least 18 inches. Penn State says Pennsylvania requires at least 4 feet between the bottom of the gravel and the limiting zone. Where soil is too shallow, designers turn to mounds, drip or other alternatives.

Step 2: Daily flow and sizing

Codes usually size a home’s system by the number of bedrooms, not by how many people live there today. The code assigns a design flow in gallons per day (gpd) per bedroom, then divides by the soil’s loading rate.

Nebraska’s code is a clear example. The required trench area is:

Square feet of trench = daily design flow (gpd) / (5 / square root of the perc rate)

That produces a table like this (single-family home, square feet of trench):

Perc rate (MPI)	2 bedrooms	3 bedrooms	4 bedrooms
5 to 10	330	495	660
11 to 20	420	630	840
21 to 30	500	750	1,000
31 to 40	550	825	1,100
41 to 50	660	990	1,320
51 to 60	700	1,050	1,400

Source: University of Nebraska-Lincoln, from Nebraska Title 124. Other states use different flows and loading rates. Virginia, for example, requires about 218 square feet per bedroom at 20 MPI and 452 at 60 MPI, with a 400-square-foot minimum for a single-family home.

The same flow numbers size the tank. The septic tank size calculator gives a starting estimate.

Many jurisdictions also require a reserve area for a future replacement field. Virginia requires 50% in silty or clayey soils, and some counties require 100% or more.

Step 3: Trench vs. bed

Layout	What it is	Notes
Trenches	Narrow parallel trenches, each with one perforated pipe	Penn State gives widths of 12 to 72 inches. Trenches follow the land’s contour.
Seepage bed	One wide excavation with several pipes	Penn State gives 12 to 36 inches of aggregate depth. Nebraska requires more area for a bed than for trenches.
Mound	A built-up sand bed fed by a pump	Used where soil is shallow or the water table is high
Drip	Small pressurized tubing in the top 6 to 12 inches of soil	Needs a dose tank and electricity

Ohio State notes that soil treatment systems are long and narrow and must run along the contour of the lot. Running trenches across a slope keeps each one at an even depth. See types of septic systems for mounds and other alternatives.

Step 4: Drain field materials

Gravel, chambers and gravelless products

Material	How it works	Notes
Washed gravel or crushed stone	Surrounds a perforated pipe, holds effluent, spreads it across the trench bottom	The traditional choice. EPA notes gravel systems have a large footprint and need a nearby gravel supply.
Plastic chambers	Open-bottom arches that create a void over the soil; no stone	EPA says they are easier to deliver and build, can be made from recycled material, and suit high water tables, variable flows and areas without gravel.
Other gravelless products	Fabric-wrapped pipe, or bundles of synthetic aggregate	Approval and sizing vary by state

Some states allow a smaller field with chambers or other gravelless products, while others require the same area as gravel. Only your local code can say which applies.

Drain field piping

Size. Conventional gravity laterals typically use 4-inch perforated pipe. The University of Kentucky describes a 4-inch perforated distribution pipe near the bottom of each trench with about a foot of washed gravel or crushed rock. Pressure-dosed systems use pipe sized by the designer for the pump and layout.

Type. The two common choices are:

• Rigid perforated PVC. Usually stamped with its ASTM designation, so an inspector can confirm it meets code. ASTM D2729 is one of the specifications listed in ASTM F481 for perforated leach field pipe.
• Corrugated perforated polyethylene. Flexible and comes in long coils, but is typically unmarked. ASTM F481 says corrugated-interior pipe should not be used between the house and the septic tank, because the ridges can catch solids.

ASTM F481 also sets a minimum pipe stiffness, but it says the local approving authority decides which pipe is allowed. Use only the pipe your permit specifies.

Solid vs. perforated. The pipe from the house to the tank and from the tank to the distribution box is solid. Only the laterals inside the trenches are perforated.

Step 5: Depth and slope

Item	Typical guidance	Source
Trench depth	18 to 24 inches for conventional trenches	NC State
Gravel around pipe	12 to 18 inches deep, with about 12 inches of soil cover	NC State
Trench width and spacing	About 2 feet wide, 8 to 10 feet apart	University of Kentucky
Lateral slope	Level along the trench in pressure systems; trenches follow the contour	Purdue, Ohio State

Depths and widths differ between states, as the table shows. The soil cover over each trench also protects the pipes from freezing, according to Ohio State. The key design idea is that each trench bottom stays level, so effluent spreads along the whole length instead of running to the low end and overloading it. Purdue’s guide for pressure distribution says each lateral should be level over the entire length of its trench.

Step 6: Setbacks

Setbacks keep effluent away from wells, water and buildings. EPA notes that most states or local governments set a required distance between a septic system and a well. The numbers vary:

From	Nebraska (drain field)	Virginia (absorption trench)
Private drinking water well	100 ft	50 ft
Lakes, streams, surface water	50 ft	50 ft
Property line	5 ft	5 ft
Building foundation	10 to 30 ft, depending on foundation type	10 ft (20 ft for a basement)

Nebraska’s guide notes that local codes may be stricter than the state minimum. Always get setbacks from your county health department, and check where your neighbors’ wells are.

Who designs and permits a drain field

Expect this order of work:

1. A licensed soil evaluator, designer or engineer evaluates the site. In Virginia, the health department reviews designs from licensed onsite soil evaluators and professional engineers.
2. The designer prepares a plan sized to the soil and the house.
3. The county or state health department reviews and issues a permit.
4. A licensed installer builds it to the permitted plan. Many health departments inspect the work before it is covered, so ask about inspection steps when you apply.

To estimate what a new system might cost, try the septic system cost calculator. If you are replacing an old field, see signs of drain field failure and the drain field replacement cost guide. For a primer on how the field works once built, read the drain field guide.