Finishing Septic Systems

photo of unfinished yard.The finishing of the onsite system is not something to consider only after it has been installed. Instead, think about the finishing early in the planning stages, when discussing the location of driveways, sidewalks, decks, and gardens. For example, the soil treatment system cannot be under a driveway, the piping carrying effluent to the system may freeze in winter if it passes under a sidewalk, and a deck built over the tank can cause some serious maintenance problems.

Involve the homeowner in planning discussions, so they understand how some of the decisions for the location of the septic system will affect their plans for developing the rest of their lot. Be sure to consider future development, such as the location of swimming pools or outbuildings, and talk about firewood piles and other backyard activities. All of this thought will help minimize problems in the future. (Pictured at right: A system right after installation). 


Once the system has been constructed, the first part of the finishing is backfilling around the piping. The pipes must be proper materials as specified by the Department of Health. They also need to be located with a proper slope to minimize standing water. The minimum slope is one inch in eight feet. Pipes carrying solids (raw sewage) have a maximum slope of one inch in four feet. Pipes carrying just water have no maximum slope.

These pipes should lie on natural soil. If the soil is not natural, use rock or sand bedding to fill spaces around the pipes. You may also need to compact the soil under the pipes to minimize settling.

The piping needs to be properly glued together to minimize any leakage in or out. Leakage often leads to problems with tree roots. Roots can't enter the pipes except by cracks or leaks already present. They can't create holes in pipes. But once roots "find" cracks, they will enlarge the cracks as they grow. Roots growing inside pipes can cause slow drainage and in extreme cases, completely plug the pipe.

After the pipe is properly bedded, it may need to be insulated. Any time piping goes under an area from which snow will be removed, it should be insulated to minimize any chances of freezing. Proper insulation can be done two ways. One method is to use an insulated pipe product manufactured to include both pipe and insulation in one. The benefits of this method are good R-value and ease of installation. Another method is to bury three-foot wide, flat Styrofoam insulation board on top of the pipe, with the pipe centered beneath the board. You need approximately three feet of buried depth with one inch of insulation, because one inch Styrofoam is equivalent to about twelve inches of soil. So, depending on the depth at which the system will be located, the amount of insulation necessary will change: two feet of soil plus two inches of Styrofoam, one foot of soil plus three inches of Styrofoam. If there is four feet of soil above the pipe, no insulation is necessary.

Soil used to backfill around the system components will settle over the course of a month or two. When you're backfilling over trenches or around tanks or drop boxes, you need to mound the soil to allow for settling. You can figure on settling of about three to four inches per foot, but for heavier soils, the amount of settling will be greater. Using sand as a backfill is an option for heavier soils, to minimize some of the settling. But you can never be certain just how much settling will occur, so plan on waiting four to eight weeks to allow settling to take place, before you actually finish the system.

Drainage of Surface Water

Drainage is critical in the soil treatment system area. No surface water should be allowed to run over, pond over, or drain into the system. All rainwater, including drainage from roofs and driveways and any sump pump water, has to be routed away from the soil treatment system. Make sure there are no areas where water could stand above the tank(s), as well as over any of the piping.

Make sure the surface water is moved around or down gradient of the system, using landscaping, ditches, and/or shallowly buried draintile. When using tile to deal with surface water, make sure that the rock above it comes very close to the ground surface. Along with surface water, underground flow can also affect the system. Over time, natural drainage patterns create flow patterns in the soil. If at all possible, keep all parts of the system outside these natural subsurface drainage ways. Drain tile can be used to redirect subsurface flow away from the system.


The vegetative cover for the soil treatment system has three functions. It needs to create a ground cover to hold snow in the wintertime and to minimize erosion. It should take up and transpire some of the water entering the system. (Unfortunately, many plant materials that are excellent at using excess water also have invasive root growth, and are not desirable for landscaping the system.)

The third concern is that the vegetative cover look nice. Many perennials, particularly native prairie species, would be good choices, as would grasses, both native prairie species and common turfgrass plants. A combination of native grasses and flowers would provide excellent erosion control and water uptake, due to the deep root system of the grasses, along with the desired ornamental qualities of the prairie flowers.

Note that the vegetative cover is a permanent planting. Annual flower beds that would be replanted each year are not appropriate. Vegetable gardens should not be located over the soil treatment system, primarily because, like annual flower beds, they are tilled and replanted each spring, potentially disturbing the system below them. A vegetable garden would be more harmful to the on-site system than the system would be to the garden. Woody plants, such as shrubs, vines, and trees, should not be planted over the system.

photo of finished yardEstablishing a groundcover over the system should be a first priority. Temporary seeding of annual grasses such as oats, rye, or ryegrass can provide quick cover if the intended permanent vegetative cover cannot be planted right away. Sod can be laid at almost any time that frost is out of the ground. Seeding turfgrass species is most successful when done between mid- August and mid-September, although April to mid-May seedings are possible. Perennials can be planted either spring or fall; summer plantings are likely to need too much irrigation to become established.

The maintenance of the vegetative cover depends on plant materials chosen. It's important that the plants chosen be somewhat drought tolerant, because it's not acceptable to water plants growing over the system. Although low maintenance of the vegetative cover is best, periodic mowing of grass, and weeding of other plantings may be necessary. Check the system at least annually to be sure gophers or other animals haven't taken up residence. For more information about plantings over the soil treatment system, see Landscaping Septic Systems, FO-6986. (Pictured at right: A nice-looking mound). 


The risers for inspection and maintenance access should be at an elevation that prevents the intrusion of water into the tank and treatment system. There's no reason, though, for the risers to stick up so much that they are a maintenance headache or an eyesore. One way to camouflage the risers is to paint them green. Children should not play on or around the risers.

Operation Check

Finally, check to make sure that all of the system components are operating properly, including the tank and pumps. Now is the time to go over the entire system, its components, function, and capacity, with the homeowner. Review the maintenance schedule for the system and for the landscaping.