How to Make a Drain Field

Updated July 20, 2017

A drain field is an important part of a septic system. When properly installed, a drain field takes liquid waste that has separated from solid waste in a septic tank or tanks and disperses it through a series of trenches in the ground called fill lines. Fill lines are filled with a filament, such as gravel which acts as a filtering mechanism as the liquid settles. Drain fields are usually installed in rural areas where city services are unavailable.

Decide what size septic tanks to install. For most homes, septic tanks range in size from 750 to 1500 gallons. The size of the septic tank or tanks (often two are used with one being used for an overflow tank) will determine the size of the drain field necessary to disperse the amount of liquid present in the septic tank.

Design the type and size of drain field you will need. It is best to do this before building the house. For example, in a gravity system, sometimes moving the planned location for the house a foot in any direction will allow a drain field to flow better. In the event a drain field cannot be set to flow in the proper direction to allow gravity to carry the waste, a pump may need to be installed at the septic tank. Distance and length of fill lines will be determined by the size of the septic tanks and type of soil. In areas where the soil has more clay, longer fill lines will need to be installed because liquid takes longer to settle through clay.

Take the drain field plan to the local health district for approval and/or permits. An inspector can often help you with the plans by being able to spot potential problems and/or suggest improvements. Check also to see if the drain field will need to be inspected between installation steps. For example, a health inspector might come to inspect the system after the fill lines are dug, the gravel is installed and before the fill lines are covered. This insures that the fill lines are positioned to drain and that the drain field is large enough for the system involved.

Use a rubber-tire back hoe/front end loader to dig the number of fill lines according to plans between 8 1/2 to 10 feet apart. Dig each one 1 foot deep with a drop of 1/8 inch per foot if a gravity system is being used.

Place a layer of fabric weed barrier in the bottom of each trench that will be wide enough to go around the gravel and the piping. Fill the bottom of each trench with 3/4 to 1 1/2-inch diameter gravel, along with 4-inch wide perforated pipe. Attach the perforated piping to a solid PVC pipe that runs downward from the septic tank. Perforated piping should be level. Call the health inspector if required before covering the fill lines.

Finish covering the fill lines with the fabric weed barrier. This will help prevent grass, small tree roots and weeds from growing into the piping and clogging up the fill lines. Place at least 1 foot of soil over the trenches to fill them, using the front end loader.


Back hoes can usually be rented at a local machine rental store. Check with the rental place to see if a reduction in cost is available if you rent the machine for a longer period of time. If an inspector has to come out during the installation process, this could delay finishing the job.


A back hoe falls under the category of heavy equipment. Although rubber-tire back hoes are of a smaller variety (about the size of a farm tractor), in order to be safe, you should know how to operate one.

Things You'll Need

  • Back hoe/front end loader
  • Gravel
  • Weed barrier
  • 4-inch wide PVC perforated piping
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About the Author

Ann Azbill has been writing online content since the spring of 2009. Her portfolio includes articles for eHow and Azbill holds an associate's degree in business.