Why Your Basement Is Wet
Wet basements and crawlspaces are among the most common complaints in Seattle-area houses, and the most common mistake is buying a fix before finding the cause. Water below grade comes from a handful of sources, each with its own signs and remedy. Diagnosis costs little; the wrong repair costs a lot.
The five usual sources
| Source | Typical signs | Typical fix |
|---|---|---|
| Roof water discharged too close | Water along one wall after heavy rain; wet soil by downspouts | Downspout extensions, tightlines, gutter repair |
| Surface grading toward the house | Water at the base of walls; puddles against the foundation | Regrading so ground slopes away |
| Failed footing drains | Seepage at the floor-wall joint across a wall; old house | Footing drain replacement or interior drain |
| Groundwater | Water rising through the floor or crawlspace in winter, even without rain that day | Curtain drain uphill, interior drain and sump |
| Condensation and plumbing | Damp walls in summer, sweating pipes, wet spot below a bathroom | Dehumidification, insulation, plumbing repair |
Start outside, in the rain
Walk around the house during a heavy rain with a flashlight:
- Are gutters overflowing, or clogged with needles?
- Where does each downspout empty? Within a few feet of the foundation is too close.
- Does the ground slope toward the house anywhere, including under decks and along patios?
- Is water running down a slope toward the house from uphill?
Many wet basements are fixed at this stage for a few hundred to a few thousand dollars.
Then look inside
Where does water appear? At the joint between wall and floor along one wall suggests drainage on that side. Through a crack in the wall suggests a localized path. Through the middle of the floor or across a whole crawlspace suggests groundwater.
When does it appear? During or right after heavy rain points to roof and surface water. Days after rain, through the winter, points to groundwater. In summer, condensation.
What does it leave? White powdery efflorescence on concrete shows water moving through it. Rust stains below pipes point to plumbing.
A simple test: tape a square of plastic to the wall. Moisture on the room side means condensation; on the wall side means water coming through.
Why the Puget Sound region is prone to this
Much of the area sits on glacial till, a dense soil that water cannot easily soak into. Rain moves along its surface underground until something blocks it, often a foundation. Hillside lots channel groundwater toward houses on the downhill side. Many houses built before the 1970s have footing drains of clay tile that have since cracked, filled with silt or been invaded by roots. And eight months of steady rain keep soil saturated far longer than in drier climates.
Matching fixes to causes
Roof and surface water: extend downspouts to tightlines that carry water away, clean and guard gutters, regrade soil. Always first.
Footing drains: camera-inspect the existing drain if accessible. Replacing it means excavating around the foundation, which also allows waterproofing the outside of the wall. An interior drain along the inside perimeter, discharging to a sump, is an alternative when excavating outside is impractical.
Groundwater: a curtain drain uphill intercepts it before it reaches the house; an interior drain and sump handles what still arrives. A backup pump or battery is worth having.
Condensation: a dehumidifier, insulation on cold pipes and walls, and ventilation for crawlspaces. Encapsulating a crawlspace controls humidity but does not stop flowing water.
Cracks: injection seals cracks that leak but are not moving. Cracks that widen or offset need an engineer.
What not to do first
- Paint the inside of the wall with waterproofing coating; water pressure pushes it off.
- Buy a full encapsulation for a crawlspace with standing water.
- Accept a pier proposal for a water problem.
- Route drainage onto a neighbor's lot or a slope.
When to call an engineer
If you see widening cracks, doors that stick, floors that slope, or water appearing with signs of movement, have a structural engineer assess before choosing a contractor. On steep slopes, a geotechnical engineer may be needed.