Foundation & Footing Drainage in Tacoma, WA
Relieving the water pressure that Tacoma's hardpan holds against foundations, replacing the century-old tile drains under the North End, and handling water on bluff and hillside lots.
Perched Water Is Tacoma's Version of Hydrostatic Pressure
A foundation in Tacoma spends five months a year sitting in saturated ground.
Foundation drainage is a footing drain, curtain drain, or regrading that relieves water pressure against a foundation. Tacoma needs it because hardpan under most hillside and North End lots perches winter groundwater at footing depth, and the clay-tile drains on pre-1950 houses have usually failed.
The Alderwood-type soils that cover Tacoma's hills have a dense layer 20 to 40 inches down, and the soil survey describes a perched seasonal water table above it from December through April. That is the depth of a crawl space wall and the upper part of a basement wall. Saturated soil weighs more than dry soil and exerts pressure in every direction, so the water pushes on the wall, works under the footing, and lifts against the slab. Engineers call the force hydrostatic pressure. Whatever the name, it cannot be sealed out from inside; it has to be drained away outside or collected inside and pumped.
The Tile Drains Under the North End
The City's own housing report puts 64 percent of Tacoma's homes before 1980, and the North End, Stadium District, Proctor, and the blocks above Old Town are older still, largely built between the 1890s and 1930. A house of that age with a basement was drained with clay tile: short lengths of unglazed pipe laid end to end at the footing, water entering through the joints. A century later two things have happened to most of them.
- Roots and silt have filled them. The big maples, cedars, and firs on those streets send roots into every joint, and Tacoma's fine glacial silt follows the water in. A camera down a North End tile drain often finds it packed solid.
- The joints have shifted. Settling, roots, and a few earthquakes move the tiles apart, so water leaves the line and goes straight to the footing instead of away from it.
Once the tile stops working, the perched water it used to carry off reaches the foundation and the basement starts leaking after decades of staying dry. The repair is a new footing drain in perforated pipe with fabric and clean rock, run to a real outlet, or, where digging around the house is impractical, an interior perimeter drain and sump that relieves the same pressure from inside.
Bluff and Hillside Lots
Tacoma's waterfront neighborhoods, Old Town, Ruston, the slopes above Commencement Bay, Browns Point, Dash Point, and the shoreline of Gig Harbor, sit on marine bluffs. Geologists describe those bluffs as permeable glacial sand and gravel over less permeable clay or till, and water moving down through the sand perches on that contact and seeps out of the slope. A house cut into a bluff lot gets that seepage delivered to its uphill wall all winter. A curtain drain across the slope above the house, catching the water before it arrives, is usually the highest-value single piece of drainage on these lots. Work on steep slopes may fall under the City's critical-areas rules, and a contractor who works the waterfront knows where that line is.
The Tools
Footing drain
Perforated pipe at footing depth around the outside of the foundation, intercepting groundwater before it reaches the wall. The modern replacement for a failed clay-tile line.
Curtain drain
A trench across a slope above the house that cuts off water moving downhill. On Tacoma's bluff and hillside lots it often stops the problem before the foundation is involved at all.
Regrading and roof water
Ground that slopes toward the house, downspouts that discharge at the wall, and patios pitched the wrong way all feed the pressure. Correcting grade and piping roof water away are the cheapest moves in the trade and should be checked before any excavation is quoted.