Downspouts and Roof Drainage in Kent, WA
Getting the water out of the gutter and away from the house, the way the City of Kent and King County say to do it.
Where the Roof Water Should End Up
A downspout should discharge away from the foundation: onto a splash block with lawn beyond it, through buried pipe to daylight, into a dispersion trench or drywell, or to the storm system. King County allows one splash block per 700 square feet of roof, given 50 feet of lawn beyond.
A gutter that works only moves the problem to the bottom of the downspout. A 1,500-square-foot Kent roof sheds around 37,000 gallons a year, and a single downspout that ends at the foundation concentrates a quarter of that on one patch of ground. On the valley floor that patch is already close to the water table; on the hills it is the top of a slope. Either way the crawl space, the basement, or the neighbor downhill gets the water. The downspout half of a gutter job is deciding where each outlet discharges.
What the City of Kent Says
Kent Public Works' private-drainage guidance gives homeowners four options, in roughly rising order of cost. Extend the downspout with corrugated pipe to a lawn area that will not pond. Route it into an infiltration trench or drywell that soaks the water into the ground. Connect it to a private storm system serving the neighborhood, where one exists. Or connect it to the public storm system. The same page carries a warning worth taking seriously: once a downspout goes underground, uncleaned gutters send leaves into the buried pipe and the facility downstream, and the pipe clogs and floods the yard it was meant to protect.
King County's Numbers
The King County Surface Water Design Manual spells out what a simple residential setup needs (Appendix C, basic dispersion):
- A splash block may take up to 700 square feet of roof, and it needs at least 50 feet of well-established lawn or groundcover beyond it for the water to spread across before it reaches a ditch, pipe, or property line.
- Where 50 feet of lawn is not available, a gravel dispersion trench does the spreading: 10 feet of trench per 700 square feet of roof, at least 2 feet wide and 18 inches deep, filled with three-quarter to inch-and-a-half washed rock, and then 25 feet of vegetation past it.
- Trenches sit at least 10 feet from any building and 5 feet from the property line, running along the contour of the ground.
- No dispersion device within 50 feet of a steep-slope, erosion, or landslide hazard area, and none aimed at a slope over 15 percent without a geotechnical sign-off.
Those are county rules written for new development; on an existing house they are the design standard a careful contractor works to. Connecting a downspout to the public storm main is the exception and does involve the city.
Valley Floor Versus the Hills
On the Kent Valley floor the ground is flat and often slow to infiltrate, so a drywell may fill in December and stay full into February. Piping to daylight is rarely possible on a flat lot, which leaves a long dispersion trench, a tie-in to the street system where the city allows it, or at minimum a solid extension of six to ten feet onto lawn that slopes away from the house. On East Hill and West Hill the concern reverses: there is plenty of fall, and the risk is sending concentrated roof water onto the lot below or toward a slope. Extensions there should discharge across the contour onto vegetated ground, never straight downhill at the property line.
Sizing and Count
A 5-inch gutter uses 2-by-3 downspouts; a 6-inch system uses 3-by-4, which passes nearly twice the water and, more usefully, passes the wet maple leaves that jam a 2-by-3 outlet. Published spacing guidance runs from one outlet per 20 feet of gutter to one per 40; the practical rule is that a single 2-by-3 outlet should not be asked to carry much more than 600 to 700 square feet of roof, and a long run with one downspout at the far end is the first thing to change when a gutter overflows at the corner.