Crawl Space Repair and Waterproofing in Seattle

Two ZIP codes where the median home predates 1950

The Census publishes housing age for ZIP code areas as well as for the city, and two Seattle ZIP codes stand apart. In 98112 the median home was built in 1942: 48.5% of its 11,513 housing units predate 1940 and 54.8% predate 1950. In 98117 the median is 1950, with 36.3% built before 1940 and 50.6% before 1950. Both lie wholly inside the city.

Several more ZIP codes hold an old core that their median hides. Before 1940 is the build date for 34.7% of housing units in 98103, 31.3% in 98144, 31.1% in 98122, 30.9% in 98102 and 30.1% in 98136. Yet the median year built is 1988 in 98122 and 1972 in 98102, because new building has gone up beside the old: 41.8% of 98122’s units date from 2000 or later. In ZIP codes like these, a house can be decades older than the median for its area, and the neighbor’s experience is a poor guide to what is under it.

For a house of that age the first questions are practical ones. Is there any ground cover at all, and if there is, when was it laid and what is left of it? How much clearance is there to work in? Has the original framing been sistered, shimmed or replaced, and by whom? Where a crawl space has never had ground cover, covering the soil is the first step and the cheapest one with the largest effect on the moisture load. A proposal that goes straight to full encapsulation on such a house is skipping the step that would show how much of the problem was the soil.

ZIP codes built mostly in the 1940s and 1950s

A second group of ZIP codes is younger than the oldest core but older than the city’s median, and the decades after 1940 dominate it. Homes built from 1940 to 1959 make up 31.9% of the housing units in 98199 (median 1960), 28.5% in 98115 (median 1962), 27.9% in 98126 (median 1965) and 25.4% in 98125 (median 1974), against 15.6% for the city as a whole. In 98125 only 7.6% predate 1940, so most of its older stock is from those two decades.

Houses of that era have had seventy years or more for ground cover, insulation and framing to be added, replaced or left alone, and the same street can hold all three. What a crawl space needs here depends less on the date the house was built than on what previous owners did under it, which is the part no figure can show.

Four ZIP codes reach into Seattle but lie mostly outside it, so their Census figures mostly describe other places: under half of 98133 (41.6%) and of 98177 (38.9%) is inside the city, and under a quarter of 98178 (22.4%) and of 98146 (19.6%). The larger part of 98133 and 98177 is in Shoreline, and the largest part of 98146 is in Burien. If your ZIP code is one of these, the figure for it is not a Seattle figure.

No soil rating for most of the city, and hardpan underneath

The federal soil survey has no drainage class for 87.1% of Seattle’s mapped area, the highest unrated share of the nineteen places, ahead of Burien (82.4%) and Shoreline (82.3%). That is not missing data so much as a description of the city: Seattle has its own urban survey area, and its largest map units are Urban land and Urban land-Alderwood complexes, which carry no drainage class. The 0.3% the survey does rate as poorly drained says nothing about the rest, and Seattle should not be read as well drained because of it.

What the soil survey cannot show, the city’s own landslide study describes. Seattle’s glacial deposits include Vashon till, “also known locally as ‘hardpan’”: very dense and relatively impervious, so water soaking down through the looser deposits above it normally perches on top of it. The study puts that contact normally less than 10 feet down, and says the water perched there normally dries up in spring or summer and returns in the winter months. Where sand sits on the Lawton Clay, groundwater perches in the same way at a boundary the study says is commonly termed “The Contact”, and springs come out along it.

For a crawl space, that is a seasonal water source that leaves no trace in August. A crawl space that is dry at a late-summer inspection and wet in January may not have a leak or a failed gutter; it may have perched groundwater rising and falling on the till, and the diagnosis needs to be made in the wet months. It also changes who owns the problem: Seattle Public Utilities says property owners are responsible for managing surfacing groundwater on their own property.

Landslide-prone ground: check the map before trenching or moving roof water

The Seattle Landslide Study counted 1,326 landslides reported in the city through June 1999, with records going back to 1890. About 84 percent had some human influence. Landsliding happens in only about 1 percent of the city, with the highest densities in Southwest Magnolia, Southwest Queen Anne, Madrona, Interlaken and Alki. The city’s emergency management office identifies 8.4% of Seattle’s surface as slide prone.

The city code’s definition of a landslide-prone area reaches beyond slides that have already happened. It includes areas with springs or groundwater seepage, and slopes of 40 percent or more over at least 10 feet of rise. That matters for crawl space work in two ways. Drainage work moves water, and Seattle’s stormwater manual says dispersing roof water is not typically permitted in landslide-prone areas or within a setback of ten times the height of a steep slope. And a usual fix for a wet crawl space, sending water away from the house, can send it somewhere it should not go on a hillside lot.

Before a drain line is trenched or a downspout discharge moved on sloping ground, look the parcel up on SDCI’s GIS map with the environmentally critical areas layer turned on. SDCI says most of those layers are advisory and that a property can contain a critical area the map does not show, so a clean map is a starting point, not a clearance.

Peat and liquefaction-prone ground: when lowering the water matters

Seattle’s code also maps peat settlement-prone areas, drawn from the city’s identified bogs with a 50-foot or 25-foot buffer around each. SDCI’s guidance on them is short and directly relevant under a house: peat-rich soils settle when loaded by structures or fill, or when the groundwater table is lowered. A sump or perimeter drain can lower the water table around a foundation; often that is what it is for. The city’s comprehensive-plan review found northwest Seattle to be the only analysis area with Category 1 peat areas, and northeast Seattle to hold the most Category 2. On a parcel the map shows as peat settlement-prone, ask SDCI about dewatering before any drain or pump is installed.

Liquefaction-prone areas, in the code’s words, are typically underlain by low-density cohesionless soils, usually with a shallow water table, that lose substantial strength in an earthquake. The same review classifies almost all of the Duwamish River and Waterway corridor, and the center of the Interbay valley, as liquefaction-prone, and the emergency management office puts the soil prone to ground failure in an earthquake at about 15% of the city’s area.

Earthquake work is crawl space work. Seattle issues a separate Earthquake Home Retrofit Permit for anchoring a house to its foundation, bracing the wall between the foundation and the first floor, and connecting it to the floor framing, with prescriptive plan sets and reduced fees. If a crawl space is being opened up for moisture work anyway, it is worth asking whether the house has had that retrofit; the two jobs share the same access.

Three rain gauges, and three quarters of the rain in six months

Three NOAA stations with a long enough record sit within 15 miles of the Census Bureau’s internal point for Seattle: Sand Point, 6.5 miles away, with a 1991–2020 annual normal of 37.82 inches; the Renton airport at 10.8 miles, 34.69 inches; and Sea-Tac at 12.2 miles, 39.34 inches. The 4.65-inch spread between them is among the smallest of the nineteen places; for Sammamish and Issaquah the stations within 15 miles span 28.59 inches. Even so, the dataset behind these pages points each city to its nearest station rather than giving it a rainfall figure of its own, because a gauge describes one instrument, not a city.

What matters more than the total is the shape of the year. At Sand Point 27.59 of the 37.82 inches, 73.0%, falls from October to March; at Sea-Tac the share is 75.4%. Sand Point averages 94.5 days a year with a tenth of an inch of rain or more, and only 4.2 days with an inch or more. Seattle’s wet season is a long run of moderately wet days rather than a few large storms.

The cold is short and mild by comparison. Sand Point averages 25.0 days a year with a low at or below freezing and 0.8 with a low at or below 20°F. Seattle Public Utilities still tells homeowners to close their foundation vents for the cold and to reopen them in spring to prevent dry rot. A vent closed in December and forgotten until July turns a vented crawl space into an unventilated one for the warm months, and is one of the first things worth checking under a house with a damp-smelling summer.

Roof water, combined sewers and where pumped water may go

Seattle Public Utilities says approximately one third of the city is served by a combined sewer, which carries sewage and stormwater in the same pipes. King County adds that in some older Seattle neighborhoods stormwater from roof downspouts and foundation drains flows into the sewers, while in other areas those connections are not allowed. So the first question about a wet crawl space under an older house is often where its roof water goes, and whether the answer has changed since the house was built.

Where roof water is released on the lot, the city is specific. SPU’s homeowner guidance is to direct downspout flows at least 5 feet from the foundation (10 feet is better), not onto a neighbor’s property or over a steep hill, and Seattle’s 2026 stormwater manual requires a splashblock to be at least 5 feet from a structure and serve no more than 700 square feet of roof. A splash block that has sunk, an extension moved for the mower, or a yard that now falls toward the house can put that water against the foundation. Checking those is cheaper than a perimeter drain, and belongs before one is priced. In combined sewer areas, RainWise offers rebates for rain gardens and cisterns on properties whose roof water is confirmed to enter the combined sewer; eligibility is by address, not neighborhood.

Water pumped out of a crawl space has rules of its own. Seattle’s stormwater code lists uncontaminated water from crawl space pumps and foundation drains among discharges allowed to the drainage system; sending drainage water to a sanitary sewer needs case-by-case approval from SPU. The side sewer that carries it, all the way to the public main, belongs to the property owner. Any bid that includes a sump should say where the discharge line ends.

Radon: low county results, and a code rule that follows sealed crawl spaces

King County is in the EPA’s lowest radon zone, and the state’s test results for it are low beside the statewide figure. Of the laboratory radon tests from King County submitted to the Washington State Department of Health in 2025, 5% were at or above 4 pCi/L (63 of 1,177); across 2021 to 2025 it was 195 of 4,863, or 4.0%. The statewide figure for 2025 was 14%. Those are tests people chose to send in, not a sample of homes, and the state publishes them by county and census tract only, so there is no Seattle or ZIP code figure to quote. The EPA says every home should be tested whatever its zone.

The crawl space point is narrower. King County is not one of the seven high radon potential counties in Washington’s residential code, but the code applies its radon appendix to any building constructed with an unvented crawl space, in any county. The rule is written for new construction; for an existing house being sealed, the practical step is a radon test in the living space before the work and again after it. The Department of Health lists a crawl space vapor barrier among simple radon-reduction steps, and notes that a test taken in the crawl space itself says little about what the people upstairs are breathing.

Three permit desks, and sales tax on top of every range

Inside Seattle, building permits come from the Seattle Department of Construction and Inspections (SDCI). Its residential code requires a permit to construct, alter or repair a building, with exemptions that matter here: minor repairs valued at $6,000 or less in any six months, provided no loadbearing support is removed, altered or relocated and ventilation is not reduced; and, provided the building envelope is not changed, insulating an existing building and in-kind repair of deteriorated members. Adding posts or beams where there were none alters loadbearing support. How SDCI classifies a ground cover, a sealed crawl space conversion (which changes ventilation) or a sump basin is not stated in anything it publishes that this site has read, so ask SDCI before assuming a scope is exempt.

The other trades have their own desks. SDCI issues an electrical permit whenever wiring is installed, altered, extended or connected, which covers a new circuit or receptacle for a sump pump or dehumidifier. Plumbing permits in Seattle come from Public Health — Seattle & King County, not SDCI. Side sewer permits, including the pipe that takes a footing drain to the drainage system, come from Seattle Public Utilities. Ask any contractor to name in writing which permits their price includes and who will pull them; permit fees sit outside every range this site publishes.

So does sales tax. In Washington, retail sales tax applies to the whole contract price for repair work on a home, labor and materials together, at the rate for the job site. In Seattle that rate is 10.55%. Every range on the cost guide is before tax, and a written quote should show the tax as its own line.