Is my problem under the basement slab or in the side sewer?

Two pipes, two failure modes

Walk the route of your waste from the basement laundry to the street and you pass through two different systems. Inside the foundation, the building drain: in Seattle houses from roughly 1920 to 1975 that is almost always cast iron, hung from the joists or buried under the slab. A few feet outside the wall it becomes the side sewer — clay, concrete or Orangeburg depending on the decade — and runs to the main in the street or the alley. Each material fails in its own way, and each fix is priced and permitted differently, so "the sewer is bad" is not a diagnosis until you know which pipe.

How cast iron fails

Cast iron does not crack at joints the way clay does; it corrodes. The inside surface rusts into tubercles — a rough scale that narrows the bore and catches grease, hair and paper. Over decades the bottom of the pipe, where the flow runs, thins and eventually rusts through, leaving a channel: a pipe with no floor, draining into the soil beneath the slab. Symptoms are recurring clogs near the house, a musty or sewage smell in the basement, and, once channelled, damp or heaved floor near the pipe route.

How the side sewer fails

Covered in detail elsewhere in the Learning Center: clay loses its joints, concrete loses its crown, Orangeburg loses its shape. Symptoms are roots, rain-driven backups, wet strips in the yard, and offsets on camera.

Telling them apart

What each fix looks like

Under the slab

Scaled cast iron that is still intact is cleaned by hydrojetting with a descaling head, then lined from the cleanout so the rough bore becomes smooth and the thin floor is reinforced. Channelled or collapsed cast iron cannot be lined through; that section is exposed by cutting the slab, replaced with new pipe, and the floor patched. The camera and locator mark where the cut needs to be so it stays small.

In the yard

The side sewer is lined if it is round and mostly intact, or replaced — by bursting where the run allows, by trench where it is short and shallow — if it has collapsed or deformed. Yard work needs a side sewer permit from Seattle Public Utilities; the permit guide explains what that involves.

Why the distinction matters when you are comparing bids

A bid that says "replace sewer line" without saying which pipe, from where to where, is not comparable to one that does. One contractor may be pricing the yard only; another may include the run under the slab; a third may be lining one and replacing the other. Ask for the footage, ask where the material changes, and ask each bidder to state the start and end point of what they are pricing. The bid comparison article goes through the rest.