How to find your septic tank and drain field

Start with the drawing someone already made

Septic systems are usually permitted and inspected by a local health or environmental department, and the U.S. EPA says you can contact that office for a copy of your property's septic record drawing, often called the as-built, along with the permits. The permit can also tell you when the system was installed and what the soil is like.

Some counties put these drawings online. Public Health – Seattle & King County in Washington, for example, has a search tool for septic records and will search its files if you email the parcel number and street address. In Oregon, the Department of Environmental Quality keeps records for some counties and finds them by parcel number, which the county assessor's office can give you.

Records are not always there. King County lists the usual reasons: the home was built before the early 1970s, the system went in without a permit, the designer never submitted final documents, or the address or parcel number has changed. It also warns that many records from before 1998 are less accurate or legible. The University of Georgia Extension adds two more places to look: the paperwork that came with the house, since the builder should have given the first owner a map, and old health department files, which on older homes may be drawn on something other than graph paper.

Read the yard for clues

According to EPA, some drain fields sit under a mound or small hill built when the system was installed. If you follow the plumbing outlet away from the house you may also find an access riser, which EPA describes as a black or green disc, or a probe point for the top of the tank, usually about 10 feet from the building.

The grass can help, though not always. UGA Extension notes that in dry weather the lawn may be drier over the shallow soil above the tank and greener over the drain field, where treated water is released. Slight depressions running in straight lines can also mark drain field trenches, especially if they line up with a drawing from the health department.

If you are not sure the house has a septic system at all, EPA suggests checking the property deed, building permit and plans, looking for a $0.00 sewer charge or no sewer bill, and asking neighbors. A rural address and a private well both make septic more likely.

Follow the sewer pipe out of the house

UGA Extension's method starts indoors. In a crawlspace or an unfinished basement, find where the wastewater pipes come together and leave through the wall. That pipe is the building sewer, and the tank is often 10 to 20 feet outside, directly in line with it. A clean-out plug outside the house can help you pick up the line. EPA's figure of about 10 feet and UGA's 10 to 20 feet are both typical distances, not rules.

From there UGA describes probing with a length of rebar or a similar metal rod. Before you push anything into the ground, call 811 to have utility lines marked; the service is free, but it only marks utility lines, not ones the homeowner owns, such as irrigation or outdoor lighting. Begin about 6 feet from the house in line with the building sewer, probe gently so you do not puncture the pipe, and move out a foot at a time until the rod hits something solid.

A tank is typically about 5 by 8 feet, according to UGA, and its top usually sits 1 to 4 feet below the surface. Probe again until the rod goes deeper, which marks an end of the tank, and find both ends. Rocks and old pipes can feel like a tank, so expect a few false starts. Wet soil after rain makes probing easier.

From the tank to the drain field

Once the tank is found, the drain field is easier. Wastewater leaves the tank through an outlet pipe, sometimes passing through a distribution box that splits it between several lines, and flows into the drain field, which EPA describes as a shallow, covered excavation in unsaturated soil. UGA suggests looking for greener strips of grass after dry weather to trace the lines.

If you still cannot find it, EPA says your local permitting authority can give you a list of septic professionals, who will walk the property and determine where the system and its parts are.

Safety and common mistakes

Never enter a septic tank. Oregon State University Extension warns that the tank produces toxic gases that can kill a person in a matter of minutes, and that no one should go in to help someone who has collapsed without a self-contained breathing apparatus.

Do not drive or park on the tank or drain field. UGA says this can damage or collapse the tank or lines and compact the soil. EPA adds that most drain fields are not designed for vehicles or heavy equipment and should not be covered with concrete or asphalt. Do not build on the area without checking with a local designer or permitting authority, and keep trees and shrubs away from it; EPA says native grasses and ground covers are the right planting.

Probing before calling 811. It is the step people skip, and the one UGA puts first, because a metal rod can find a gas, water or electric line before it finds the tank.

Write it down so it is not lost again

UGA's last instruction, once the tank is found, is to draw a rough sketch of its location relative to the house so it is not lost again. Oregon State University Extension suggests installing risers so the openings sit at or near the surface, and marking the tank's location so the openings are easy to find next time.

SepticLog keeps that sketch on your phone. Take a photo of the yard, draw a loop around the tank, put a marker on each lid and on the drain field, and add a note such as which wall the pipe leaves from. The picture sits next to your pumping history, ready to show whoever arrives with the truck. It is a sketch, not a survey, with no GPS or measuring tools.

Septic records, permits and setback rules differ by county and state. Call 811 before probing, never enter or lean into an open tank, and use a septic professional when you are unsure. This guide summarizes published guidance and is not a substitute for your local health or environmental department.

Frequently asked questions