Why does my yard smell like sewage after it rains?

Start with where the smell is strongest

Smell is a bad locator on its own, so walk the property after the next rain and pay attention to three places: the cleanout (a capped pipe usually near the foundation or property line), the strip of ground between the house and the street, and the curb or nearest manhole. Where the smell peaks tells you which of the three common causes you are dealing with.

Cause 1: a leaking side sewer

The side sewer is the pipe from your house to the city main. In most Seattle houses built before the 1970s it is vitrified clay or concrete in short sections, and in a smaller number it is Orangeburg fibre pipe. Sections separate at the joints as the ground moves and as roots pry them apart. Between rains the leak drains into the soil and you notice nothing. After a soaking, the ground is already saturated, effluent has nowhere to go, and the smell comes up — usually along a narrow band that is greener and spongier than the lawn on either side.

The confirmation is a camera inspection. The camera shows the open joint or crack, and the locator on the camera head marks its position and depth on the surface. That matters because the fix is very different depending on whether the defect is one joint under the lawn or the whole run under a driveway and a mature tree — see spot repair vs. full replacement.

Cause 2: an open or failed cleanout

Cleanouts get mowed over, driven over, and left uncapped by the last person who cleared a clog. An open cleanout vents sewer gas constantly, and a low one fills with rainwater that then drains into your sewer, which is why the smell can be paired with a slow house after storms. Check the cap; if it is missing, cracked, or sitting in a puddle, that alone can explain the smell.

Cause 3: the combined sewer system

Much of central and older Seattle — parts of Capitol Hill, Queen Anne, Ballard, and the neighborhoods around Lake Union and the Ship Canal — was built on a combined system where roof drains, street runoff and household sewage share one pipe. In a big storm those pipes run full, and the system is designed to overflow at set points rather than back up into houses. Around those points and the manholes feeding them, a storm can bring a distinct sewage smell that fades as the flow drops. That is a city matter, not a defect in your line, and the combined vs. separated sewers article explains how to find out which system your block is on.

A simple way to sort the three

Why it is worth acting before the next dry spell

A leaking line is quiet in summer, which is why so many are found during a sale rather than by the owner. A joint that is leaking effluent out is also letting groundwater and roots in, and the roots are what eventually stop the pipe. Finding it while it is a smell rather than a backup means the repair can usually be trenchless and scheduled, not dug and urgent.