Civil-Environmental Engineering

Frequently Asked Questions*

2) What are the typical requirements for gravity leachfields?


Each county has it own requirements for specifics such as maximum allowable trench length, depth, and so forth.  Take a look at the summaries listed on the right side of the Installers Page.



In general, leachfield trenches should:

  • ​Be as shallow as possible/allowed (this insures that the microbes that breakdown the organics get plenty of oxygen).

  • Allow at least the minimum separation to groundwater (i.e. the depth between the trench bottom and top of the groundwater).​

  • Run along the ground's natural contours as much as possible.

  • Have at least the minimum allowed undisturbed soil between trenches.

  • Have sides free from smearing (during the wet season, the sides may need to be raked after digging to insure good absorption).

  • Have the bottom raked or furrowed about 1" deep along the direction of the contours.

  • Be filled with 3/4" to 2-1/2" clean and washed drain rock,  covering the full length of the trench (the depth over and under the pipe depends on the county, but is usually at least 2" over and 12" under).

  • Be covered with filter fabric (to keep the dirt out of the gravel), then the required depth of carefully selected backfill (often 12").

  • Maintain the required setbacks to all surrounding features, either on-site or off site (for example, neighboring wells).

  • Be placed so as to allow a 100% replacement area to be used in case of primary field failure.


​Standard leachfield trenches typically should NOT:

  • Be covered with asphalt or concrete.

  • Be deeper than the allowed maximum depth.

  • Be subjected to vehicular, livestock, or frequent foot traffic.

  • Be on steep slopes (3:1 or greater).​

  • Be used in 100-yr flood zones or with flows in excess of 1500 gpd (analysis required).

Supply line should:


  • Be a four inch schedule 40 ABS  line at 1/8" to 1/4" drop per foot.​

  • Have a double, or back-to-back, cleanout between the tank and the house.

  • Have a cleanout at any bend or elbow required and at 100 ft intervals minimum.

  • Be as short as practical, but no less than 20' from house to tank.


Septic Tanks should:

  • ​​Meet the county size requirements.

  • Be placed so as to meet all setback requirements (such as distance from building foundations).

  • Be set on a level bed of at least 3" of sand or pea gravel.

  • Be weighted or anchored in high ground water areas sufficient to prevent floating when empty.

  • Have watertight risers (usually 24") that are sealed to the tank, with covers secured by corrosion resistant fasteners.

  • Have the ground surface graded to divert water away from the risers, or the risers should end 2" above grade.

  • Show no measurable leakage over 24 hours when full of water 2" into the riser.

  • Have inlet and outlet sanitary tees, a first compartment that is 2/3 of the total, and 10% volume above the liquid level for scum storage.


​Tight-lines between the tank and field should:​


  • Be schedule 40 ABS or SDR 35 and have a drop of at least 1/16" per ft.



Perforated laterals should:

  • ​Be SDR 35 or triple wall ASTM F810, 4" diameter pipe.

  • Be nearly level (see county requirements for maximum allowed deviations).

  • Have 1/2" holes that are at the 5:00 and 7:00 positions on not more 5" centers.

  • Have an end cap or end cleanout.

Counties frequently require:

  • ​Distribution boxes be bedded on sand or pea gravel (& sometimes on a concrete pad), with outlets level, and inlet at least one inch above outlets.  Distribution box is usally to be a minimum of 5 ft from the leach trench.

  • Observation ports at the ends of each trench (4" dia., with slotted bottom and capped top), down to the bottom of the drainrock.

  • A gravelless system has the same size requirements as a standard trench system.

  • A bed type system (instead of trenches) must have an increased flooded bottom area (see county requirements).


It is good practice to:

  • Provide an effluent filter at the tank outlet to prevent clogging the leachfield.

  • Use flow equalization devices and check the distribution boxes with water to assure equal flow.

  • Replant the surface of the field with shallow rooted grasses.

*The following page is a summary for quick reference only and may not represent the latest standards.  For complete and up-to-date information, see each counties official website.