How to Fix a Soggy Yard on Puget Sound Glacial Till

Quick answer

Seattle-area yards stay soggy because long winter rains sit on top of dense glacial till that water barely moves through, often with roof water added. Start by watching where water goes in a long rain. Common fixes are tightlining downspouts to a legal outlet, regrading, curtain or French drains that intercept water over the till, deeper amended soil, and rain gardens or cisterns where they fit. Check critical areas rules on slopes.

Last updated October 22, 2026

If your lawn squelches from November to April, your crawlspace gets damp, or a corner of the yard turns into a moss bed every winter, you're dealing with one of the most common landscape problems in the Seattle area. The cause is mostly geology. The last ice age left much of the Puget Sound lowland covered in glacial deposits, and on many lots the soil a few feet down is dense enough that water can barely pass through. This guide explains why, how to diagnose your yard, and the fixes that tend to work.

Why water sits on Seattle-area yards

Glacial till is a mix of clay, silt, sand, gravel and cobbles that was compressed under thick ice. Geotechnical reports from around the region commonly describe two layers: an upper, weathered layer that drains moderately, and a deeper layer so dense it's often called hardpan and is nearly impervious. In the wet season, rain soaks through the upper layer and then perches on the hardpan, keeping the root zone saturated. Not every lot is on till. Some sit on sandy outwash that drains well, and river valleys have their own silty soils. But till is common enough that it explains a lot of winter mud.

Two things make it worse. First, construction. Many lots built in recent decades had topsoil scraped off and the subsoil compacted by equipment, leaving a thin skim of soil over a surface that acts almost like pavement. Second, the rain pattern. Seattle's rain usually comes as long, steady soaks over many days rather than short cloudbursts, so the ground stays wet week after week.

Step one: diagnose in the rain

The best time to understand drainage is during a long rain in winter. Walk the property and note:

Where each downspout goes, and whether gutters overflow.
Where water enters the lot: from the roof, the street, a neighbor's yard or a slope above.
Where it collects, and how long it stays after the rain stops.
Seeps on slopes or along a wall, which suggest water moving along the till layer.
Water in the crawlspace or basement.

Take video. A contractor visiting on a dry day in August won't see what you see in January. If you have a slope, also look for cracks in the soil, tilting fences, or bulging walls, and treat those as urgent.

Surface water vs. perched water

Surface runoff is water flowing across the ground. Grading and swales usually solve it. Perched water is water sitting in the soil above the dense layer. It keeps a lawn soggy long after rain stops and is usually handled with subsurface drains, deeper soil, or by changing how the area is used. Many yards have both.

Common fixes, simplest first

1. Take care of the roof water

A roof sheds a lot of water in a long rain. Downspouts that splash next to the foundation keep the surrounding soil saturated. Tightlining, connecting downspouts to solid pipe that carries water to an approved outlet, is often the single most effective fix. Where the water can legally go depends on your city: some properties connect to the city storm system, others must disperse or infiltrate on site. Ask your city or contractor, and never dump it over a steep slope.

2. Correct the grading

Soil should slope away from the house. Over time, settling and added mulch or bark can flatten or reverse that slope. Regrading with soil, not bark, restores positive drainage. Larger regrading may need a permit depending on the amount of soil moved and the site, so check with your city.

3. Curtain and French drains

A curtain drain is a gravel trench with perforated pipe placed uphill of a problem area to intercept water moving along the till before it reaches the house or lawn. A French drain collects water from a saturated area. On till, these work well when they have a clear downhill outlet and filter fabric to keep fine soil out of the gravel. Without an outlet, a drain just becomes an underground pond.

4. Build deeper soil

Rebuilding a thin, compacted lawn area with several inches of compost-amended soil gives roots more room and holds more water before it runs off. Local stormwater guidance in the region encourages amended soils for exactly this reason. It doesn't break the hardpan, but it changes how the surface behaves.

5. Rain gardens and cisterns

A rain garden is a shallow, planted basin that collects roof water and lets it soak in. It needs soil that drains at least somewhat, so a simple infiltration test matters on till. A cistern stores roof water and releases it slowly, and works even where soil won't absorb much. In parts of Seattle on the combined sewer system, the RainWise program from Seattle Public Utilities and King County has offered rebates for rain gardens and cisterns built by RainWise-trained contractors. Check the program's eligibility map for your address before you plan around a rebate.

6. Change the use

Sometimes the best answer for a perpetually wet corner is to stop asking it to be lawn. Moisture-tolerant natives, a gravel path or a planted swale can turn a problem into a feature.

Rules to know before you dig

Call 811. Washington's dig law requires a locate request at least two business days before excavating, not counting weekends and holidays. Mark the dig area in white first. Private lines, such as a sprinkler main or power to a shed, aren't marked by utilities.
Check critical areas. Seattle maps steep slopes and potential landslide areas as Environmentally Critical Areas, and SDCI restricts grading and vegetation removal there and generally requires permits for drainage installations and retaining walls in those areas. Bellevue, Kirkland, Redmond, Sammamish, Renton, Tacoma and Everett have their own critical areas codes. Look up your lot on your city's map.
Respect streams and wetlands. Many lots drain to salmon streams, and cities protect buffers around streams and wetlands.
Don't move the problem next door. Concentrating water onto a neighbor's property can create disputes and liability.
In the Tacoma Smelter Plume, which covers parts of Pierce and King counties, surface soils can contain arsenic and lead. Ecology offers sampling and guidance before you dig large areas.

What affects the cost

Drainage pricing depends on the scope, so it's more useful to know what moves the number than to rely on a single figure. Big factors include the length and depth of trenches, how far water has to travel to a legal outlet, whether a storm connection or permit is needed, access for equipment on a tight or sloped lot, cobbles and roots in the till, soil to haul off or bring in, and restoring lawn and beds afterward. Get written, itemized estimates from more than one company and ask each where the water will end up.

When to call a professional

Extending a downspout or adding compost to a bed is a reasonable weekend task. Call a professional when water reaches the foundation or crawlspace, when a slope shows cracks or seeps, when water comes from a neighboring property, or when a fix means trenching, regrading or work near a critical area. If a slope is moving, contact your city's building department first. When you're ready, read about drainage services or request a drainage company.

Frequently asked questions

What is hardpan in Seattle soil?

It's the common name for the dense lower layer of glacial till found under much of the Puget Sound region. Water passes through it very slowly, so rain perches above it and keeps the surface wet in winter.

Do rain gardens work on glacial till?

Sometimes. They need soil that drains at least somewhat, so a simple infiltration test is important. Where soil drains poorly, a cistern or a different outlet may work better.

Can I connect my downspouts to the street?

It depends on your city and your lot. Some properties can connect to a storm system and others must disperse or infiltrate water on site. Ask your city's permit office or utility before connecting.

Sources and further reading

Request help now