Short definition
A storm drain carries rainwater, surface runoff, and clear water from a property to the public storm sewer or directly to a creek, lake, or Puget Sound. No sanitary waste enters a storm drain — that’s what distinguishes it from a sanitary sewer or a combined sewer. In older Seattle neighborhoods, however, downspouts often tie into the combined sewer instead of a separated storm system.
What it is
A separated drainage system has two pipes: one for sanitary waste headed for the treatment plant, and one for rainwater headed for a receiving water body. The storm drain is the rainwater pipe — and because rain doesn’t carry sewer gas, no traps are required. A typical residential connection is a downspout running through buried solid pipe out to the public storm sewer, ending in an inspection pit or simple catch fitting at the property line.
In Washington, stormwater discharge is regulated by the Department of Ecology under municipal NPDES permits (Phase I and Phase II). The metal medallions stamped “Drains to stream” or “Drains to Sound” on storm drain inlets are not decorative — they’re a reminder that anything entering the storm drain reaches receiving waters without treatment.
Why it matters to a homeowner
In newer Seattle, Tacoma, Bellevue, Redmond, and most Snohomish County subdivisions, your roof downspouts connect to the storm sewer system. In older Seattle combined-sewer neighborhoods — Ballard, Fremont, Wallingford, Capitol Hill, the Central District, downtown, parts of West Seattle and Beacon Hill — downspouts often connect to the combined main instead. That matters because heavy rain that overwhelms a combined main can also back up through interior fixtures.
Older buried galvanized-steel downspout drains (1950s–70s WA installs) eventually rust through and collapse, sending roof water back to the foundation instead of away. A saturated yard during 0.5+ inches of rain, water pooling at the foundation, or a long-running gutter overflow are all signals to check the buried run.
Common variants / not the same as
- Storm drain vs. sanitary sewer. Storm = rain only; sanitary = waste only.
- Storm drain vs. combined sewer. Combined carries both. Legacy Seattle has a combined system in roughly a third of its area.
- Storm drain vs. drywell or soakaway. A drywell infiltrates on-property; a storm drain conveys runoff to a public system.
- Storm drain vs. french drain. A french drain is perforated pipe in gravel for subsurface yard drainage; a storm drain is solid pipe carrying captured runoff.
Washington note
Puget Sound’s high winter rainfall (roughly 38 inches per year in Seattle, spread across about 150 rainy days) makes stormwater a continuous homeowner concern. The practical implications:
- Older Seattle homes in combined-sewer areas often have downspouts tied into the combined main. A backwater valve helps protect interior fixtures from heavy-rain backups.
- RainWise (an SPU + King County program) offers rebates for rain gardens and cisterns that disconnect downspouts from the combined system. Check eligibility by address before assuming you qualify.
- Newer suburbs (Bellevue, Redmond, Sammamish, most of east King County) are fully separated and don’t have the combined-system risk.
- “Downspouts daylight at grade” — common in older WA homes; legal but creates foundation-water problems in heavy rain.
If you’re east of the Cascades, Spokane has its own stormwater program with different specifics; confirm with the City of Spokane’s stormwater utility before assuming Puget Sound rules apply.