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Drainfield: how a septic drainfield works and what limits it

A drainfield is the part of a septic system where the soil finishes treating the wastewater. In North Carolina it needs suitable soil, room for a repair area, and the setbacks the state rule sets. On a small lot, those limits often decide what can be built.

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Quick summary: This guide explains how a septic drainfield works, the parts it has, and the North Carolina rules that limit where it can go. It is for infill buyers, builders, and owners of lots without sewer. The key takeaway: the drainfield and its repair area need suitable soil and set distances from wells, water, and lot lines, and on a small lot that space often sets the size of the house.

What a drainfield is

A drainfield is the area of soil where a septic system releases its liquid and the soil completes the treatment. North Carolina’s rules call it the dispersal field: “the physical location where final treatment and dispersal of effluent occurs in the soil,” in the definitions at 15A N.C. Admin. Code 18E .0105. You also see the names leach field, drain field, and absorption field.

The drainfield is the part of the system that depends on the land. The tank can be bought and set on almost any lot. The soil under the drainfield cannot be bought.

How a septic drainfield works

A conventional system in North Carolina is a septic tank plus a gravity distribution drainfield, under 15A N.C. Admin. Code 18E .0902. Wastewater moves through three parts.

  1. The septic tank. A watertight tank receives the wastewater from the house. Solids settle and scum floats, bacteria digest organic matter, and the liquid, called effluent, flows out.
  2. Distribution. Pipes and a distribution device carry effluent from the tank to the field. Pipes in the field are called laterals. Some systems use a pump to dose the field under pressure.
  3. The trenches. Each trench is an excavation no wider than three feet, with gravel, crushed stone, or an approved product around one or more laterals. Effluent leaves the trench at its bottom and enters the soil.

For a conventional system, trenches are at least two feet wide and no more than three feet wide. Trench depth does not exceed 36 inches on the downslope side unless an agent approves it. Trenches sit level, at least three trench widths apart on center.

The soil does the final treatment as effluent moves down. The rule requires at least 12 inches of naturally occurring soil between the trench bottom and any limiting condition. It requires 18 inches of separation to a soil wetness condition when more than six inches of that separation is sandy Group I soil. Both rules are in 15A N.C. Admin. Code 18E .0901. The same rule sets the trench length from the design flow and the soil’s loading rate, so slower soils need longer trenches.

The repair area

North Carolina requires room for a second drainfield before it permits the first. A site must have available space for the initial system and a repair area, and a site without both is unsuitable, under 15A N.C. Admin. Code 18E .0508. The repair area is suitable soil reserved for a replacement if the first field fails.

The rule has an exception for older small lots. The repair area requirement does not apply to a lot described in a recorded deed or plat on or before January 1, 1983, when the lot is too small and the facility meets the flow and wastewater limits in the rule. The agent still sets the largest repair area the lot can hold. A lot that used the exception once loses it for a new initial system.

The short version: Count the drainfield twice. A lot needs suitable soil for the first field and the repair field, both clear of the setbacks, before a house plan matters.

Setbacks from the drainfield

The state rule sets minimum distances from every wastewater system to features around it, in Table IX of 15A N.C. Admin. Code 18E .0601. Some of the distances, as the rule states them:

Feature Minimum setback
Private drinking water well serving one single-family home 50 feet
Public, community, or shared water supply well 100 feet
Lake or pond, from normal water level 50 feet
Other streams and surface waters, from the ordinary high-water mark 50 feet
Any property line 10 feet
Building foundation 5 feet
Basement, cellar, or in-ground pool 15 feet
Any water line 10 feet
Another dispersal field, except the site’s own repair area 20 feet

Systems with a design flow over 3,000 gallons per day carry larger setbacks in a second table. The same rule bars drainfields and repair areas under impervious surfaces or areas with vehicle traffic, unless an approved system type allows it. A driveway cannot cross the field.

What limits a drainfield on a lot

Soil, slope, depth, water, and space limit a drainfield. Each one can remove part of a lot or all of it.

  • Soil texture and structure. Clays accept effluent slowly, so the trenches get longer. Sands accept it fast. The loading rate in the rule follows the soil group.
  • Soil depth. Soil depth to saprolite, rock, or parent material under 18 inches is unsuitable, under 15A N.C. Admin. Code 18E .0505.
  • Water table. A seasonal high water table cuts the separation the soil gives. Gray and mottled soil colors show where water sits part of the year.
  • Slope. Uniform stable slopes up to 65 percent are suitable, and steeper slopes are not, under 15A N.C. Admin. Code 18E .0502. The same rule rules out depressions, areas where surface water collects, and jurisdictional wetlands. Slope also adds to the soil depth the site needs.
  • Space. Setbacks from the lot lines, the house, a well, and a stream leave the usable area. The repair area must fit in it too.

An engineered or advanced pretreatment system can overcome some limits. It adds design, cost, and in many cases a maintenance contract. The soil and site evaluation decides which systems a lot can use. See Perc test: what it is and what it costs.

Take action: Before you buy an infill or rural lot without sewer, find out whether the drainfield, the repair area, and the setbacks leave room for the house you want. Start with Can the house you want fit on this lot?

Signs a drainfield is failing

The state rule defines a malfunctioning system by what you can see, in 15A N.C. Admin. Code 18E .1303. A system malfunctions when any of these occurs:

  • Sewage or effluent discharges to the ground surface, to surface water, or into groundwater.
  • Sewage or effluent backs up into the house, the drains, the tank, or the distribution system.
  • Effluent stands within three inches of finished grade over a trench. Two observations at least 24 hours apart and more than 24 hours after rain confirm it.
  • The tank needs pumping more than once a month to prevent those conditions.

The owner must contact the local health department when the system malfunctions. The same rule requires a tank pump-out when scum and sludge fill more than one-third of the liquid depth. It also forbids soil disturbance or compaction over the drainfield and the repair area.

Who answers drainfield questions

The local health department answers whether a lot can hold a drainfield, through its permits. A licensed soil scientist evaluates the soil, and a professional engineer designs engineered systems. A certified septic contractor installs the system. See Septic permits in North Carolina: the improvement permit, the construction authorization, and the operation permit for the permit order, and Sewer, septic, or a package plant and A water system nearby is not water for your project for when sewer is the better answer.

The public record shows the soil survey and, in many counties, the permits for existing systems. It cannot show the soil under a given trench or how the field has been used. A lot split that leaves the drainfield or repair area on the wrong side of a line can break the permit. See Lot split: can you split your lot.

Key recap

  • A drainfield is the soil area where a septic system disperses effluent and the soil finishes the treatment.
  • A conventional North Carolina system is a septic tank plus a gravity drainfield of trenches two to three feet wide.
  • A site needs suitable space for the initial drainfield and a repair area, with a narrow exception for small lots recorded on or before January 1, 1983.
  • Setbacks in the state rule include 50 feet from a private drinking water well and 10 feet from any property line.
  • Soil depth, a seasonal high water table, slope, and space limit where a drainfield can go.

Questions

How far does a drainfield have to be from a well in North Carolina?

At least 50 feet from a private drinking water well serving one single-family home, and 100 feet from a public, community, or shared well, under Table IX of 15A NCAC 18E .0601. The health department confirms the distances that apply to the lot.

Can you build over a drainfield?

No. The state rule keeps building foundations at least 5 feet from a wastewater system and bars drainfields and repair areas under impervious surfaces and vehicle traffic, unless an approved system allows it. Soil compaction over the field is also forbidden.

How do you know if a drainfield is failing?

Look for effluent on the ground surface, backups into the house, or wet ground over the trenches. The state rule calls a system malfunctioning when any of these occurs or the tank needs pumping more than once a month. Contact the local health department.

What is a drainfield repair area?

It is suitable soil the permit reserves for a replacement drainfield if the first one fails. North Carolina requires it on most sites before it permits the first system, and it must stay undisturbed.

References

Primary sources cited on this page, in APA style.

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