Why does my Seattle yard flood even though the rain isn't that heavy?

Seattle's glacial till — a dense, compacted layer left by the Vashon glacier — sits beneath most of the city and is nearly impermeable. Even light, sustained rain saturates the soil above the till, and because water can't drain downward through it, the water pools on the surface or flows laterally into low spots. Six months of near-daily rain keeps soils perpetually saturated, so even modest rainfall has nowhere to go.

Do I need a permit to install a French drain in Seattle?

A French drain that stays entirely on your property and doesn't connect to the city's storm drain system generally does not require a permit. However, if you need to discharge to the public storm system or the right-of-way, you'll need a side sewer permit from Seattle Public Utilities. Work in Environmentally Critical Areas may require additional review.

What is the RainWise program and do I qualify?

RainWise is a Seattle Public Utilities rebate program that pays for rain garden and cistern installations on residential properties. Eligibility depends on your property's location within specific drainage basins that contribute to combined sewer overflows. Rebates can cover 50–100% of installation costs, often $4,000 or more. You can check eligibility on the SPU website.

How deep is the glacial till layer in Seattle?

The depth of the Vashon till varies significantly across Seattle, from as shallow as 2 feet in some areas to 20 feet or more in others. The depth depends on how much outwash and recessional material was deposited on top of the till after the glacier retreated. A test pit or geotechnical investigation is the only reliable way to determine the till depth on a specific property.

Is my Seattle home at risk for a landslide?

Seattle has over 1,200 mapped landslide-prone areas, primarily on steep slopes where saturated soils above the glacial till become unstable. Neighborhoods like Magnolia, West Seattle, Beacon Hill, and areas along the Duwamish bluffs have the highest concentration of landslide hazards. You can check your property's status on the city's ECA maps through SDCI.

Why is my crawl space wet in winter but dry in summer?

Seattle's wet season (October–April) delivers roughly 80% of the city's annual rainfall, saturating soils and raising perched water tables above the glacial till. Water migrates laterally through the soil and enters crawl spaces through foundation walls or the soil floor. During the dry summer months, the water table drops and soils dry out, giving the false impression that there's no drainage problem.

Should I use corrugated black pipe for my French drain in Seattle?

No. Corrugated flex pipe is prone to crushing under soil pressure, root intrusion, and sediment accumulation — all of which are accelerated in Seattle's wet, root-rich environment. Use rigid, perforated PVC or HDPE pipe for any permanent drainage installation. The cost difference is minimal compared to the cost of replacing a failed system.

How much does it cost to fix drainage problems in Seattle?

Costs vary widely depending on the scope. Simple downspout extensions cost $20–$100 each. French drains run $50–$100 per linear foot. Foundation perimeter drains cost $8,000–$25,000. Retaining wall replacement with proper drainage can exceed $50,000. Seattle's high labor costs and difficult access on many lots push prices above national averages by 20–40%.

Can I discharge my sump pump into the Seattle sewer system?

No. Discharging sump pump water into the sanitary sewer is prohibited in Seattle. Sump pump discharge must go to the storm drain system (with appropriate permitting), to a rain garden or infiltration area on your property, or to a surface outlet that doesn't impact neighboring properties. SPU enforces this requirement.

What soil type is most common in Seattle neighborhoods?

The Alderwood series is the most common soil type mapped across Seattle. It consists of gravelly sandy loam over dense glacial till and is classified as having a densic contact that severely restricts downward water movement. Other common series include Everett (well-drained gravelly soils) and Indianola (sandy soils), but Alderwood dominates most residential areas.

Does Seattle require stormwater management for residential projects?

Yes. Seattle's Stormwater Code requires on-site stormwater management for projects that add 750 square feet or more of new plus replaced impervious surface. This applies to residential additions, new driveways, patios, and other hardscape. The code prioritizes infiltration and green stormwater infrastructure before conventional piped systems.

Are there flood zones in Seattle?

Yes, though they are more limited than in surrounding King County. FEMA-designated flood zones exist along the Duwamish River corridor, portions of Thornton Creek in north Seattle, areas near Lake Union and the Ship Canal, and parts of the Rainier Valley. Properties in Special Flood Hazard Areas require flood insurance for federally backed mortgages.

When is the best time of year to do drainage work in Seattle?

July through September is the optimal window for drainage construction in Seattle. Soils are drier, excavation is easier, and concrete and grading work can cure and compact properly. However, Seattle's contractor market is busy during this period, so you should get bids and schedule work in the spring for summer execution.

What should I look for in a drainage contractor in Seattle?

Look for Washington State contractor registration, general liability and workers' compensation insurance, specific experience with Seattle's glacial soils, familiarity with SPU permitting requirements, and willingness to work with a geotechnical engineer when needed. Avoid contractors who propose solutions before investigating the problem or who plan to use corrugated flex pipe for permanent installations.

Do I have to disclose drainage problems when selling my Seattle home?

Yes. Washington State law (RCW 64.06) requires sellers to complete a disclosure statement that includes questions about drainage problems, flooding, water intrusion, and landslide or erosion issues. Failure to disclose known problems can result in legal liability. Buyers should also independently assess drainage conditions, as sellers may not be aware of all issues.

Seattle WA Drainage Guide: Glacial Till & Rain Solutions

Expert Seattle drainage guide covering glacial till soils, steep slopes, stormwater rules, and practical solutions. Get a Drainage Intelligence Report™ today.

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