Basement Waterproofing in Mercer Island, WA

Basement waterproofing in Mercer Island: what to know

Water behaves differently from one hillside to the next. A geotechnical report on West Mercer Way places that lot near the contact between Vashon advance outwash and Lawton Clay, and says light groundwater could sit on or within the clay, shifting with rainfall and irrigation. A report for another island lot describes glacial till with possible seepage where it meets unweathered till. A third saw no seepage at all.

That spread is why a damp foundation here gets a diagnosis, not a template. The West Mercer Way report recommends a perimeter drain unless the interior slab sits at least 12 inches above exterior grade, so we measure slab height against outside grade early.

Permits and rules in Mercer Island

Waterproofing is not on the city’s exemption list, so we ask Community Planning and Development whether a given repair needs a permit. Land-disturbing activity falls under the stormwater chapter, MICC 15.09. On a lot mapped as an erosion, potential slide, or steep slope hazard, foundation work and grading are off limits from October 1 to April 1 without a waiver. We look up which of these reach your lot before we quote.

Rules change and vary by parcel, so we confirm the current requirements with the city before we start.

How we approach it

Compare slab height with outside grade before assuming water comes through the floor
Dig a test pit to see whether clay, till, or sand holds the water
Pipe roof drains to the storm system separately from the footing drain
Keep excavation along hazard-mapped foundations to the dry months unless a waiver exists

For the full process, see our Basement Waterproofing page.

Our basement waterproofing work, serving Mercer Island

Examples of our work. We build basement waterproofing in Mercer Island and the cities around it. See all of our work

Questions from Mercer Island homeowners

Why is my Mercer Island basement wet only after long rains?

Rain-linked dampness points to groundwater or runoff, not a plumbing leak. Island geotechnical reports describe groundwater on Lawton Clay that moves with precipitation, and seepage where till meets unweathered till on another lot. Which applies to yours is a parcel question, so we open a test hole first.

Can I dig down my Mercer Island foundation in January?

A lot the city maps as an erosion, potential slide, or steep slope hazard cannot have foundation work, grading, or filling done between October 1 and April 1 without a waiver. Other lots are not under that limit. We check the map before scheduling.

Can my Mercer Island footing drain connect to the street storm drain?

That is the city’s decision. Its permit conditions require roof drains on permitted projects to connect to the storm drain system and pass inspection before backfill, while one island geotechnical report advises keeping roof and surface water out of the foundation drain. We confirm the outlet first.