How to design a leach field septic system

Updated January 11, 2018

A leach field works by dispersing liquids from a septic tank into the soil over a large area. Waste is then directed away from the septic tank through drains and spreads out into the ground. Bacteria remove organic material from the liquids as it moves through the earth, thus cleaning it before it reaches an aquifer or comes to the surface to evaporate. Building a leach field is labour intensive, but the design is fairly straightforward.

Connect a solid pipe at least 4 inches in diameter to your septic tank. Attach the other end to a distribution box. Waste will flow out of the septic tank and into the distribution box.

Dig three 1 foot deep trenches parallel to each other in a straight line away from the distribution box. The pipes will be attached to the distribution tank at one end. Dig trenches that are at least 20 feet long, but not too close to driveways, property boundaries or other obstacles.

Even out the bottoms of the trenches and then line them with 2 inches of gravel. Smooth out the gravel. Lay straight lengths of perforated pipe in the trenches.

Attach the ends of the pipes near the distribution box to the box with shorter lengths of perforated pipe. Use U-shaped connectors to connect the joints.

Cover the pipes to about 2 inches below ground level with more gravel. Smooth the gravel out and cover it with straw. Put topsoil on the straw and landscape the area with grass or other plants. Avoid planting trees with roots that could invade the leach field.


Not all septic systems use a distribution box. Some are connected directly to the leach field. Make sure that the leach field drains away from buildings. Have a percolation test done to make sure that your soil type can support a leach field. The bottom of the distribution box should be level with the pipes in the leach field.

Things You'll Need

  • 5 foot solid pipe
  • Distribution box
  • Backhoe, shovel or spade
  • 60 feet of 4-inch diameter perforated pipe (more or less depending on the length of the trenches)
  • Pipe connectors
  • Gravel
  • Straw
  • Topsoil
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About the Author

Meg Jernigan has been writing for more than 30 years. She specializes in travel, cooking and interior decorating. Her offline credits include copy editing full-length books and creating marketing copy for nonprofit organizations. Jernigan attended George Washington University, majoring in speech and drama.