Diagnosis

Sewer Leak in the Yard? Wet, Sunken or Green Patches

By San Jose Sewer Line Repair · November 11, 2026 · 8 min read

A patch of lawn that stays soft in the middle of a San Jose November is not proof of a sewer leak in the yard on its own. Many turn out to be irrigation. Others are the water service line. Some are the sewer, and those are the ones where guessing wrong costs the most. Here is a sensible order to work through it, and most of it you can do yourself in an afternoon.

Step one: map what is underneath

Before you interpret a wet spot, know what is under it. Three lines usually cross a front yard:

• The sewer lateral, running from the house cleanout toward the street, typically four to seven feet down, on the side of the house where the bathrooms are
• The water service, from the meter box at the curb to the house, usually two to three feet down, in a straighter and often different line
• Irrigation, six to twelve inches down, everywhere

If your patch is directly between the meter box and the house, suspect water first. If it lines up cleanout to street, suspect sewer. If it is under a sprinkler head, start there.

Step two: the meter test

This one takes ten minutes and eliminates the water service.

1. Turn off every fixture and appliance in the house, including the irrigation controller and any ice maker.
2. Open the meter box at the curb and look at the low-flow indicator — a small triangle or dial that spins on any flow.
3. Watch it for five minutes. Note the reading.
4. Come back in an hour with everything still off.

Movement with nothing running means a pressurized leak somewhere between the meter and the house. That is a water line problem, not a sewer problem, and it explains soft ground much faster than a sewer leak does, because it runs 24 hours a day.

Step three: shut the irrigation off for a week

Sounds obvious. It is skipped constantly. Turn the controller off entirely — in November your lawn will survive — and see whether the patch dries. A cracked lateral line or a leaking valve puts out gallons a minute for the few minutes it runs, and the water surfaces well away from the break.

Step four: what a sewer leak in the yard looks like

A leaking lateral does not usually make a puddle. It makes a slow, persistent condition, and the signs are as much about smell and plant behavior as about water:

The sunken part matters more than the wet part. When a clay joint opens, soil washes into the pipe and travels to the main. What is left behind is a void, and the void eventually shows as a sunken spot over the sewer line — a shallow trough in the lawn — or a hairline crack running across the driveway in the same direction as the pipe. A green stripe in the lawn that follows the same path is the other classic tell.

Step five: the dye test you can do at home

Buy fluorescent tracing dye or use a strong food coloring. Flush a generous amount down the toilet closest to the front of the house, then run a bathtub for two or three minutes to carry it out to the lateral. Check the wet patch over the next several hours. Colored water surfacing is a confirmed sewer leak. No color is not proof of the opposite — a deep leak may never surface visibly — but a positive result saves you a step.

A soil probe helps too: a long screwdriver or a piece of rebar pushed into the ground along the suspected line. Where the pipe has been washing soil out, the probe suddenly goes in easy.

Step six: get it located, then decide

When the yard says sewer, the next step is a camera inspection with a locate, $250 to $500 in San Jose. That is what most sewer line leak detection in San Jose comes down to in practice: underground sewer leak detection on a residential lateral is usually a camera and a sonde, not specialty equipment. The camera head carries a sonde, and a receiver on the surface marks the exact spot and depth in paint on your lawn. That converts a vague soft area into "cracked joint at 27 feet, five feet deep, two feet inside the sidewalk," which is the difference between a $2,500 spot repair and a $20,000 replacement quoted out of caution.

From there, the answers look like this:

• One broken joint, rest of the pipe sound: spot repair, $1,500 to $4,000 in open yard
• Multiple open joints, pipe still round: lining
• Washed-out bedding with a sag: dig and regrade that section
• Cracked pipe under a mature tree or a patio: trenchless, because the alternative removes the tree

One local note

Front-yard leaks in the older Rose Garden and Naglee Park blocks are often right at the property line, where a 1920s clay lateral meets a section replaced in the 1970s with ABS. Two materials, one poorly made coupling, decades of soil movement. If your patch is near the sidewalk rather than near the house, that transition is the first thing to look for on camera. The signs of a broken sewer line post covers what the same failure looks like from inside the house.

Mistakes that waste time and money

• Digging to look. A shovel hole over a suspected leak tells you almost nothing about a pipe five feet down, and it can hit the water service or an irrigation line on the way.
• Skipping the meter test. It is free, and a pressurized water leak is a different trade, a different repair and often a faster one.
• Reseeding the green stripe. Covering the symptom leaves the void under it growing.
• Accepting a full replacement quote without a locate. Until the camera marks the spot and depth, nobody knows whether this is one joint or the whole run.

What to tell the contractor when you call

Have a short list ready. It saves a visit and gets a more useful first quote:

1. Where the patch is, measured roughly from the house and the sidewalk.
2. How long it has been there and whether it changes after laundry or showers.
3. Results of the meter test and the week with irrigation off.
4. Whether a dye test showed color, and how long it took.
5. Where the cleanout is, and the approximate year the house was built.

Common questions

How do I tell a sewer leak from an irrigation leak?

Turn the irrigation controller off for a week and see whether the patch dries. Irrigation leaks show up after a watering cycle and the water is clear; a sewer leak is constant, worse after heavy household use, and may smell faintly or draw small drain flies. A dye test flushed from the nearest toilet can confirm it.

Why is there a sunken spot over my sewer line?

Usually because a joint has opened and soil has been washing into the pipe and away to the main. The ground above settles into the empty space. It is worth a camera run soon, because the void tends to grow and the pipe bedding goes with it.

Can a leaking sewer line cause a sinkhole?

It can cause a depression that keeps getting deeper, because the leak carries soil away over time. In a lawn that usually shows as a trough; under concrete it can show as cracking or a slab that rocks. Keep people and vehicles off a spot that is clearly sinking and have the line cameraed and located.

Is a wet patch over the sewer line a health risk?

It can be, because leaking wastewater carries bacteria. Keep children and pets away from the spot, do not dig into it, and wash your hands after checking it. Getting the line cameraed and the leak located soon limits the exposure.