How deep is my sewer line, and how do you find exactly where it runs?

Typical depths in Seattle

A side sewer leaves the house below the lowest fixture and falls toward the main at a steady grade, usually a quarter inch per foot or more. Near the foundation it is often four to six feet down; by the time it reaches the property line it can be eight; where the city main is deep — common on the flats near the water and on hillsides where the street is well below the house — it can be twelve to fifteen feet at the connection. Houses in Queen Anne, Magnolia and the slopes of West Seattle regularly have deep street ends; postwar ramblers on flat lots in Lake City or Shoreline are usually shallow.

How the route is found

1. The camera goes in from a cleanout, a pulled toilet, or a roof vent. The head carries a small radio transmitter (the sonde).
2. The locator follows it above ground. A handheld receiver picks up the sonde's signal and reads out horizontal position and depth. At each point of interest — a root mass, an offset, a belly, the property line, the connection — the operator stops the camera, finds the peak signal, and paints a mark on the surface with depth written beside it.
3. The marks become the map. A run of paint dots from the house to the street, each with a depth, is the actual route of your line, including the bends and the alley connection nobody remembered.

This is a routine part of a proper camera inspection, and the walkthrough describes the whole visit.

What the trace changes

• Spot repair size. Knowing a defect is at 31 feet and 6.5 feet deep, under lawn, makes it a small planned dig rather than an exploration.
• Method. A ten-foot-deep line under a driveway is a pipe bursting or lining job; a four-foot line under grass may be simpler to trench.
• Who owns the problem. Depth and position tell you whether the defect is on your lot, in the right-of-way, or on a neighbor's property — which decides the permit, and sometimes who pays.
• Landscaping and structures. A trace that runs under the new patio changes the conversation before anyone rents an excavator.

Depth and the permit

Seattle's side sewer permit asks for the route and depth, and the inspector will want to see that any excavation is properly shored once it passes about four feet. A trace done before the application means the drawing on the permit matches the pipe in the ground, which avoids the revision that follows when a crew finds the line three feet from where the old sketch said.

Common surprises

Lines that jog around an old tree, then continue in a different direction. Lines that go out the back to an alley main rather than the street. Two houses sharing one lateral downstream of a wye in one yard. A line that turns under the neighbor's garage. A "cleanout" that turns out to be a downspout drain. These are the reasons a locate is worth doing before a bid is written, not after the trench is open.

Marking for the long term

Paint fades. Ask for a simple sketch with distances and depths, and keep it with the house documents along with the video. The next contractor, the next buyer, and the permit reviewer at Seattle Public Utilities will all want it, and it saves repeating the trace.