How to install septic tanks properly

Septic tanks routinely fail due to improper installation practices. Septic tanks can break if installed on rocks or on uneven surfaces. Broken septic tanks can leak waste water and pollute the environment. Septic tanks installed non-level or on improperly compacted soil can settle and render tank baffles less effective. Proper septic tank installation may also include adding insulation to protect against freezing in northern climates. Installing a septic tank properly requires the proper equipment, know-how and attention to detail.

Locate the inlet and the outlet of the septic tank. Both should be marked with paint or moulded letters in the concrete. Measure the holes if markings are not present. The height of the inlet hole is higher than that of the outlet hole.

Set the elevation using a laser level. Use the sewer stub at the house for the beginning elevation. Add the septic tank's inlet hole height, a 2-percent slope for the pipe and at least 2 inches of granular fill to bed the tank.

Excavate the septic tank hole with the excavator. Do not over-excavate the hole. Pile the excavated soil far enough away from the tank hole to prevent the hole walls from collapsing.

Level the hole with a rake. Remove all rocks from the excavation.

Pour at least 2 inches of granular fill into the excavation's bottom and level with a rake. This will provide a uniform support for the tank's bottom.

Check the elevations with the laser level to be sure there is sufficient slope from the home to the tank.

Confirm that the tank is level using a bubble level. Check the tank's levelness in several different areas as tank lids are often undulating.

Add insulation to the tank's top if the tank will have less than 2 feet of cover soil in northern climates. Use sheet insulation that is rated for underground use. Cut the sheets to tightly fit the tank's top and secure in place with small piles of soil.

Backfill the tank with the excavated soil. Ensure no rocks are present in the backfill as these can damage the tank. Compact the soil every 6 to 8 inches as it is backfilled.

Mound the soil over the tank's location to promote surface water run-off.


Septic tanks installed backward will not perform correctly.


Working around equipment is dangerous. Use safety equipment and keep work crews at safe distances.

Things You'll Need

  • Excavator
  • Shovel
  • Septic tank
  • Laser level
  • Level
  • Rake
  • Granular fill
  • Soil compactor
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About the Author

Stephen Hasty started writing in 2009. Covering technical articles and newsletters, his work has appeared in "The Kennebec Valley Plumbing Newsletter" and "Maine Leasing." Hasty holds a bachelor's degree from Saint Cloud State University, a real estate sales agent license and a master plumber license from the state of Maine.