How to find your septic tank: paper first, shovel last

Every method below costs less than the one after it. Work down the list and stop when you have the lid. Most people never get past step three.

Step 1: pull the permit file before you touch a shovel

Southeastern Idaho Public Health holds the septic permit records for Bannock County systems. The original permit often includes a site diagram that shows where the tank and drain field sit relative to the house, plus the tank size and install date. On a system permitted in the last few decades that drawing can put you within a few feet of the lid before you step outside.

Two things to know about the file. Distances on the diagram were measured from the foundation as it stood on install day, so a later addition, deck, or attached garage can throw the reference point off. And on older acreage places the paper trail is thin or missing. An unpermitted hand-dug tank from decades back leaves no drawing at all, which is when the yard has to do the talking.

If you are buying rather than owning, the records pull is part of a septic inspection anyway, and the inspector locates and opens the tank as the first item on the list.

Step 2: let the house point at it

The sewer main leaves the house in one place, and the tank sits on that line. Find the exit and you have the direction.

1. In the basement or crawlspace, find where the main drain line goes through the wall or floor. On a slab house, the cleanout in the yard near the foundation marks the same spot.
2. Stand outside at that point and look downhill. Tanks around here usually sit 10 to 20 feet from the foundation on the downhill side, because the line runs on gravity and nobody trenched farther than they had to.
3. Look for a second cleanout, an inspection pipe, or a vent stub in the lawn along that line. Some installs left a short capped pipe over the tank inlet. It is not the lid, but it is inches from it.

Rule out the places a tank is almost never put: under a driveway, under a deck or patio that predates the system, under the well or inside the well’s setback, and uphill of the house.

Step 3: read the ground

The tank changes the ground above it in ways that show up if you know the season to look.

• Snow melts first over the lid. A working tank runs warmer than the soil around it. In January, the first bare or slumped patch in the snow on the downhill side of the house is very often sitting right on top of the tank. This is the one advantage a Pocatello winter gives you in this job.
• Grass grows differently over it. On the benches the tank sits in shallower, rockier fill than the native ground, so the strip above it goes brown first in July. On river-bottom lots it can run the other way, greener from the moisture. Either way, look for a rectangle, not a stripe. The long green stripe is the drain field, and you do not want to be digging there.
• The lid may already be showing. Older concrete lids sit a few inches down and frost heave brings them up over the years. Walk the area with your foot dragging and feel for a hard edge under the sod.

Step 4: probe, carefully

If the file, the house, and the ground have narrowed it to a patch of lawn, a probe finds the lid. A probe is a length of steel rod with a T handle, and a fiberglass driveway marker or a long screwdriver on a small area does the same job.

1. Call 811 first and wait for the locate. Gas, power, and phone lines run through the same yards as septic lines, and a probe through a gas service is a far worse afternoon than a lost lid.
2. Work in a grid along the sewer line’s direction from the foundation, every 12 inches or so, pushing the probe straight down. Sod over a tank here is usually a foot or two deep, so you are feeling for a hard flat stop at that depth, not the rocks that stop you at four inches on a bench lot.
3. Concrete gives a dull thud and stops the probe dead across a wide area. Fiberglass or polyethylene tanks sound hollow and flex a little. A single rock stops the probe in one spot and lets it through an inch to either side.
4. Once you hit the tank, probe outward to find its edges, then work the edges to find the lid. On most tanks the lid is a square or round section at one end or in the center, and on tanks from the 90s onward there may be two, one over the inlet and one over the outlet.

Push, do not hammer. A probe driven hard into an old concrete lid can crack it, and a cracked lid is a repair you did not have before you started looking.

What not to dig or drive on while you look

• The drain field. It is the long green stripe or the set of parallel lines downhill from the tank, and every trench in it is doing work. Do not probe it, do not drive the truck across it to get closer to the tank, and do not park the trailer there while the search is on.
• Anywhere the 811 locate marked. The paint and flags stay until you are done.
• Unknown pipes. A capped pipe in the lawn might be an inspection port or it might be the well line. Leave caps on until you know.

When to stop and call

If the probe grid turns up nothing along the whole line, the tank may be deeper than a foot or two, farther from the house than usual, or placed somewhere the house does not point at because the line takes a turn underground. At that point the search stops being a free afternoon and starts costing sod. A buried-lid locate and dig runs $50 to $150 on our pumping cost page, and it is usually folded into a pump-out visit rather than billed as a separate trip.

Put in a riser so this never happens again

A riser is a section of pipe, usually green plastic, that extends the tank’s access opening up to grade with a lid at the surface. Once it is in, the tank is found by anyone who walks the yard, in any season, without a shovel. Every later pump-out, inspection, and filter rinse gets faster and cheaper because the crew is not digging.

A riser installed at grade runs $250 to $450, priced by the depth to the lid, and it pays for itself in two visits. Most homeowners around here add one at the first pump-out after buying the place, since the lid is already open and the digging is already done. If you spent an afternoon with a probe finding the tank, the riser is the part of the job that makes the afternoon worth it.

Before you call, have three things ready: the address, roughly where the tank is or where the line leaves the house, and whether the lid is exposed, under sod, or under snow. That is everything an exact quote needs, and it is the difference between a quick visit and a shovel day.