Mercer Island Crawl Space Repair

The island’s decade counts show where its older stock comes from: 354 housing units built before 1940, 1,952 from the 1940s and 1950s, 2,506 from the 1960s and 1,687 from the 1970s. Together the years from 1940 to 1979 account for 57.9% of the island’s housing. Since then building has been slower, with no decade adding more than 1,350 units.

A crawl space under a house from the 1940s or 1950s may have been opened many times over the decades, for plumbing, wiring, heating and insulation, and each visit can leave its mark on whatever ground cover there was. The first job is to find out what is there now: a continuous ground cover, remnants of one, or bare soil. After that comes the framing nearest the soil: sills, the ends of joists and any posts set on the ground or on small footings. Replacing insulation before either is settled is spending in the wrong order. Wood rot in a crawl space covers what to look for in older framing.

Kitsap silt loam covers 55.1% of Mercer Island’s mapped area: 28.6% on 15 to 30 percent slopes, the largest single map unit, 25.1% on 2 to 8 percent slopes and 1.4% on 8 to 15 percent slopes. Map units the survey names “Arents, Alderwood material” add 26.2%, and Alderwood gravelly sandy loam 8.7%. Almost all of it is rated moderately well drained, and the island has no map units of open water inside its boundary at all.

A good drainage rating does not settle the slope question. The landslide hazard assessment the city commissioned in April 2009 treats as potential slide areas slopes of 15 percent or steeper where coarse-grained deposits lie on top of fine-grained ones, and it treats spring lines the same way. Neither condition shows in a soil drainage class, which describes how the soil itself drains, not what lies beneath it or where a spring comes out. On a lot the map marks, or any lot with a seep or spring near the house, a crawl space that is wet each winter deserves a look at where that water is coming from before anyone proposes where to send it.

The usual answer to a wet crawl space is to collect the water and send it away from the house. On a slope, “away” usually means downhill, toward the next house or onto the slope itself, and on ground the 2009 assessment flags that is a question for the city before it is one for a contractor. So a drainage proposal on Mercer Island should say where the collected water ends up, not only how it is collected, and the city’s Community Planning & Development department should be asked what applies to the parcel before anything is trenched.

Where water reaches the house from uphill, a curtain drain that intercepts it before it reaches the foundation is one option; on this site’s cost guide it runs $2,250 to $5,375 as an add-on to an interior drainage scope of $4,025 to $7,175. Those are metro-wide modelled ranges before sales tax, not Mercer Island figures; the curtain-drain add-on is the one priced for a hillside lot. What crawl space drainage covers explains the parts.

There is a rain gauge on Mercer Island, but it is not used here. The station called Mercer Island 1.5 NW, 1.8 miles from the Census Bureau’s internal point for the city, has only 10 years of record behind its annual figure, and Seattle’s Boeing Field station, 4.0 miles away, has 19. The dataset publishes a station only with 20 years or more, so both are left out.

The published stations within 15 miles are the Renton airport at 4.9 miles, with a 1991–2020 annual normal of 34.69 inches; Seattle’s Sand Point station at 8.6 miles, 37.82 inches; and Sea-Tac at 9.1 miles, 39.34 inches. Their 4.65-inch spread is the same as Bellevue’s. The Renton airport averages 30.4 nights a year at or below freezing and 73.5% of its rain falls from October to March.

Mercer Island is a single ZIP code area: 98040 covers all of the city and lies entirely inside it. Building permits come from the city’s Community Planning & Development department, through MyBuildingPermit.com, and the city issues electrical permits as well, with single-family electrical permits applied for online through the same portal. L&I lists Mercer Island among the cities that run their own electrical permitting. This site has not read which crawl space work Mercer Island exempts from a building permit, so ask the department before assuming a scope needs none.

The Department of Revenue lists one rate for the island, 10.3% for October 1 to December 31, 2026. In Washington it applies to the whole contract price for repair work on a home, labor and materials together, and the cost guide’s ranges are all before it.