Short answer
When every fixture in the house drains slowly at once, the restriction is downstream of all of them: the building drain or the side sewer to the street. One slow sink is a local clog you can often clear yourself; whole-house slowness is a main-line problem, most often roots at a joint, a sag, or scale in old cast iron. That needs a cleanout-based clearing and a camera to find the cause.
Local clog or main line: the one-minute test
Drainage in a house is a tree. Fixtures feed branches, branches feed the building drain under the floor, and the building drain feeds the side sewer out to the city main. A problem on a twig affects one fixture. A problem on the trunk affects everything. So the test is simple: is it one drain, or all of them?
- One fixture slow — hair, soap, grease or a toy in that fixture's trap or branch. Often clearable with a hand snake.
- Everything on one floor slow, other floors fine — a branch serving that bathroom group.
- Everything slow, lowest fixtures worst — the building drain or side sewer. This article.
What narrows a main line in Seattle houses
Roots at the joints
Clay and concrete side sewers were laid in short lengths, and every joint is a seam a root can find. Big-leaf maple, cedar and laurel hedges are the usual culprits. Roots enter as hair-fine threads, thicken into mats, and catch every wipe and paper towel that passes. This is the number-one cause in Laurelhurst, Windermere and the older streets of Magnolia, where mature trees sit right over the line.
A sag (belly)
A section that has settled below grade holds water permanently. Solids drop out in the low spot, the effective diameter shrinks, and the house slows. Cleaning restores it for a while; only re-grading or replacing the section fixes it. See what a sewer belly is.
Scale and corrosion in cast iron
Under-slab drains in houses from the 1940s to the 1970s are often cast iron. The inside surface rusts into a rough, scaled bore that grabs grease and paper. Cabling knocks off some scale; jetting cleans it properly; badly channeled pipe eventually needs lining or replacement.
Partial collapse
Orangeburg fibre pipe deforms into an oval and then flattens. Concrete pipe spalls from the crown down. Either produces a house that slows, clears briefly, and slows again — with less and less time between.
Why the fix is not just "snake it"
A cable through a root mass opens a channel and the house drains again — until the roots regrow, typically months. The same goes for a belly: it fills again the moment solids reach it. The question worth answering is not "can we get it flowing" but "what is in there". A camera inspection after clearing answers it in an hour and lets you match the fix to the cause:
| Cause | What actually resolves it |
|---|---|
| Roots at a few joints, pipe otherwise sound | Cut and jet, then a liner to seal the joints against roots |
| Belly under lawn | Excavate and re-grade that section; line the rest if needed |
| Belly under driveway or structure | Weigh spot excavation against bursting the run |
| Scaled cast iron, still round | Descale by jetting, then line |
| Orangeburg or crushed concrete | Replace — trenchless where the line allows |
What you can do tonight
Cut water use to the minimum: no laundry, short showers, no dishwasher. Find your cleanout so whoever comes does not have to hunt for it. Note whether the lowest drain in the house holds water. And skip the chemical opener — it does not reach the problem and it makes clearing the line messier for everyone.
Questions people also ask
How do I know it is the main line and not several clogs?
Several clogs appearing at once in unrelated fixtures is very unlikely. If the tub, the toilet and the kitchen sink all slowed together, they share one cause, and the only thing they share is the main.
Will drain cleaner help a whole-house slowdown?
No. Chemical cleaners act on what is in the trap directly below a fixture. A main-line restriction is far downstream, and the chemical arrives diluted, sits in the pipe and damages old material.
How urgent is this?
A slow house is one heavy laundry day from a backup. It is worth clearing within days rather than weeks, and the clearing is a scheduled visit rather than an emergency if you act while things still drain.
Why did it happen suddenly after months of nothing?
Restrictions build slowly and then reach the point where paper and grease catch. The last few percent of pipe diameter make the difference between fine and slow, which is why it feels sudden.
More of the questions we hear every week are on the FAQ page .
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