Where should downspouts drain?

Last Updated: October 23, 2026

The short answer for a Kent lot

Roof water has four possible destinations: the ground near the house, the ground farther away, a buried line to the storm system, or the sanitary sewer. Kent allows the middle two under conditions and rules out the last. Gutters of Kent installs and reroutes downspouts across Kent and South King County, with the work done by licensed and insured contractors, and every downspout’s discharge is traced during the walk-around before anything is changed. This guide explains each option, the City of Kent and King County rules behind it, and how the valley floor and the hillsides change the answer.

The goal underneath the rules is simple. A rainy-season roof sheds a great deal of water in a few spots, and each downspout concentrates it. NOAA’s 1991 to 2020 normals for the Kent station show 40.09 inches of precipitation a year, with 6.23 inches in an average November, so every discharge point gets tested repeatedly from October through January.

Not the sanitary sewer

Kent City Code 7.04.180 says no one may discharge stormwater, surface water, groundwater or roof runoff to the sanitary sewer unless the director expressly authorizes it. Sanitary sewers carry household wastewater to treatment; roof water sent into them takes up capacity that wastewater needs during the same storms.

On an older house, a downspout that vanishes into the ground may lead somewhere no one remembers. If there is a chance it reaches the sanitary side, it is something to trace and resolve, not reconnect. The underground downspout drains guide explains how buried lines are checked.

Splash blocks and basic dispersion

The simplest discharge is above ground: a downspout elbow lets go onto a splash block or an extension, and the water spreads across vegetated ground where it can soak in. Kent’s Surface Water Design Manual adopts the King County manual with Kent changes, and the King County manual Kent adopts sets limits for this kind of basic dispersion:

A splash block’s job is to break the fall and turn the stream sideways so it does not dig a hole at the wall. The flowpath is what actually disposes of the water. A splash block at the foot of a downspout with a flower bed against the house and a patio three feet out has not dispersed anything; the water just pools nearer the corner. Because each splash block is capped at 700 square feet of roof, the limit can also set how many downspouts you need on a large roof side.

Extensions are how most lots meet the distance. A hinged or fixed extension, or a short run of pipe on the surface, carries the water past foundation plantings to where the ground falls away. The downspout extensions page covers the options.

Buried connections to the storm system

Where dispersion will not work, roof water can go underground to the local drainage system. The King County manual Kent adopts states that any pipe connection of roof downspouts to the local drainage system must be a perforated pipe connection: a length of perforated pipe in a gravel trench that lets some water soak into the soil before the remainder reaches the storm system. The same manual says these trenches are not placed in critical area buffers or on slopes steeper than 25%.

Kent City Code 7.05.070 requires a permit for all work to construct, install or modify any storm or surface water drainage structure or facility in the city. A new buried line, or a change to an existing one, is planned as drainage work with that permit in mind. Kent’s residential site plan checklist asks new-home applicants to show a roof downspout infiltration system and its connection to the storm stub-out, so newer permitted houses may have been built with these lines in place.

Kent City Code 7.14, the illicit discharge chapter, excludes roof drains and foundation drains from its definition of illicit connections, so a properly built roof drain connection is not treated as one. The same chapter prohibits hazardous materials from entering the storm system, which is why debris, wash water and chemicals do not belong in a downspout line. Yard drains, French drains and drainage systems beyond the downspout tie-in are not offered.

Driveways, walkways and the neighbor’s yard

A downspout that empties onto a driveway is common on houses where the garage sits at a corner. No Kent rule found bans it, but a driveway does not disperse anything: the water runs across the pavement to wherever the driveway drains. If that is the street gutter, it reaches public storm drainage. If the driveway slopes toward the garage, a walkway or the house, the water returns to the foundation. During an occasional freeze, a sheet of water across a walkway can also become a sheet of ice; NOAA normals for SeaTac show about 1.9 days a year with an inch or more of snow, so this is uncommon rather than routine.

Water aimed at a neighbor’s lot is a different problem. Concentrated roof water crossing a property line can erode a slope, flood a side yard or start a dispute, and a dispersion flowpath that runs out at the fence after 15 feet is not doing the job the manual describes. Plan the route so water soaks in on your own lot or reaches the street’s drainage.

Valley floor and hillsides: opposite problems

Kent’s terrain splits into a flat Green River valley floor and the slopes of East Hill and West Hill, and the discharge problem changes with it.

On the Kent valley floor, the challenge is getting water to leave. City of Kent critical areas mapping puts 24.3% of the Green River region inside a FEMA flood zone, and the City describes the valley aquifer as recharged by direct infiltration through the land surface. A flat lot gives an extension little fall to work with, so water that lands near the house tends to sit. Longer extensions to the lowest part of the yard, or a permitted connection to storm drainage, are the options to weigh. Whether a given lot can soak water in depends on its own soil and groundwater, which this guide cannot judge from a map.

On the hillsides, the challenge is the reverse. City of Kent mapping shows 13.8% of West Hill on landslide-hazard soils and 8.1% in steep slopes. Spreading roof water on or above a slope like that can add water exactly where it is least wanted, which is why the King County manual keeps dispersion off steep and hazard areas without geotechnical approval. On those lots a tightline carrying water to the storm system, under a Kent drainage permit, may be the route, and the geotechnical question belongs to an engineer, not a gutter installer.

Outside Kent

The rules change at the city line. Renton has its own surface water design manual, Federal Way uses the King County manual with a city addendum, and Auburn uses its own manual based on the Washington Department of Ecology’s western Washington manual. In Covington, Maple Valley, Des Moines and Black Diamond, the discharge rule is confirmed per address. The Renton page and the service areas page cover each city.

Choosing the discharge for each downspout

Take each downspout on its own and ask, in order: is it connected to anything below ground, and do you know where that goes; is there 50 feet of vegetated ground downhill of it; is the ground near it steep or mapped as a hazard; and does the water, once it lands, move away from the house, the driveway and the neighbor. The answers usually point to one of three routes: an extension onto a dispersion path, a permitted buried connection, or a re-placed downspout that drops at a better corner.

If water is already reaching the basement, the guide to gutters and basement water explains what gutters can and cannot change, and the gutters and downspouts guide follows the whole system. When you want each downspout traced and a route planned, Get an Estimate.