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Yard Drainage & Erosion Control in Seattle, WA

Quick answer

Most Seattle-area drainage problems come from long winter rains, roof water released near the house, and dense glacial till that water perches on. Common fixes include tightlining downspouts, regrading, curtain or French drains, swales, rain gardens or cisterns, and deeper amended soil. On mapped steep slopes, check city rules before grading or clearing.

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Drainage is the problem Seattle-area homeowners ask about most from November to March. Rain arrives as long, steady soaks rather than short bursts, and much of the region sits on glacial till, a dense layer that water barely moves through. Water perches on top of it, so lawns stay spongy for months, crawlspaces get damp, and moss takes over. On the region's many hillsides, saturated soil also raises the risk of erosion and slope movement.

Signs you have a drainage problem

  • A lawn that squishes underfoot for days or weeks after rain
  • Moss and thin grass in the same low or shaded areas every year
  • Water in the crawlspace or against the foundation
  • Soil, mulch or gravel washing onto paths and driveways
  • Seeps or wet spots partway down a slope
  • Cracks, slumps or leaning fences and walls on a hillside

Common solutions

  • Downspout management: tightlining roof water in solid pipe to an approved outlet. Roof runoff is often the largest single source of yard water.
  • Regrading: restoring a slope away from the house and toward a safe outlet.
  • Curtain and French drains: gravel trenches with perforated pipe that intercept water moving over the till layer.
  • Swales: shallow channels that carry surface water around a house.
  • Rain gardens and cisterns: features that hold roof water and release or absorb it slowly, where soil and site allow.
  • Soil improvement: deeper, compost-amended topsoil that holds more water before it runs off.

Rain gardens, RainWise and infiltration limits

In parts of Seattle served by the combined sewer system, the RainWise program run by Seattle Public Utilities and King County has offered rebates for rain gardens and cisterns built by RainWise-trained contractors. Eligibility depends on your address, so check the program's eligibility map before planning around it. Note that rain gardens need soil that drains; on dense till, a site test matters, and a cistern may be the better fit.

Slopes and critical areas

Many Seattle hillsides are mapped as steep slope or potential landslide Environmentally Critical Areas. SDCI generally restricts grading and vegetation removal there, and its guidance notes that trees and plants help hold these slopes together. Other cities in the region have similar critical areas rules. Before regrading, adding drains, or clearing brush on a slope, check your city's critical areas map and ask the permit office what is allowed. Never discharge concentrated water onto or over a steep slope.

Where the water goes matters

You generally can't fix your drainage by sending concentrated water onto a neighbor's property. Many lots drain to streams that support salmon, and cities protect stream and wetland buffers. A good plan names a legal outlet: a city storm system connection where allowed, a dispersion area on your own property, or a properly designed infiltration feature.

Before you hire

  • Take video during a long rain showing where water flows and collects.
  • Note where every downspout goes and whether gutters overflow.
  • Ask the contractor where the water will end up and whether a permit is needed.
  • Make sure a Washington 811 locate is requested at least two business days before trenching.

For a deeper look, read how to fix a soggy yard on glacial till. If a slope or wall is actively moving, contact your city and mark your request urgent. Request a drainage company.

See how it works

How a French drain carries water away from a yard. Press the buttons to run each step and see what happens inside. Made with Colorfigures ·
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Frequently asked questions

Why is my Seattle yard soggy all winter?

Much of the region sits on glacial till, which has a very dense layer that water barely passes through. Rain perches above it, keeping the surface saturated from fall through spring, especially where roof water is added.

Can I get a rebate for a rain garden in Seattle?

The RainWise program from Seattle Public Utilities and King County has offered rebates for rain gardens and cisterns in eligible combined sewer areas, installed by RainWise-trained contractors. Check the eligibility map for your address.

Can I work on a steep slope in my backyard?

Possibly, but Seattle and nearby cities restrict grading and vegetation removal in mapped steep slope and landslide-prone areas. Check your city's critical areas map and ask the permit office before starting.

Sources and further reading

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