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Uphill Battle: A Seattle house defies gravity to manage stormwater.

Peter Cohan

By David Sokol
March 2012
Photo © Lara Swimmer

Architect Peter Cohan is quick to note that Cedar Park House is more dream than green. Designed for a former Microsoft executive, her scientist husband, and their combined family, the residence tops 5,000 square feet and replaces a one-story ranch house overlooking Seattle’s Lake Washington. Yet the house employs several sustainability strategies, particularly with regard to stormwater, that helped make the couple’s dream a reality.

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Without stormwater management, outcomes could have been nightmarish. Cedar Park House’s generous lot gradually descends toward the east, then drops off dramatically. Below this precipice lies the shore of Lake Washington, ringed by small private cottages and the Burke-Gilman Trail, a public path popular with walkers and bikers. The entire area is prone to serious erosion and minor landslides have previously occurred here in saturated soil conditions.

The Cedar Park design team was caught between a slough and a soft place. If the site were to absorb too much rainfall, it would become highly unstable. The most obvious solution would have been to channel the water downhill to the lake, but Seattle Parks and Recreation would not allow a drainage pipe to be installed underneath its rail-to-trail conversion.

That left Cohan with the only option of draining rainwater “uphill” to the street. He says this dilemma became a major factor in generating the design of the house.

“The principal way we dealt with it is by respecting the slope. The house is 45 feet from the steep dropoff,” Cohan says. “We had no problem holding back, because we treated the setback as the great room of the house.” The large lawn is further protected from erosion by a soldier pile wall, located close to the edge of the steep slope.

In plan, Cedar Park House assumes a Y shape with the stem reaching west to the street. The building becomes taller and splits into two volumes as the site slopes gently downward toward Lake Washington. The effect is to demarcate and protect the lawn, hiding it from the street above. Because the house’s east-facing branches also are narrow in width, “when the sliding doors open, it’s almost as though you’re actively participating in the outdoor space,” Cohan says.

In section, Cedar Park House derives character from its standing-seam galvanized steel roofs, whose shapes were created precisely to collect rainwater. The garage, which forms the stem of the Y, has a shed roof that cants upward to create space for a second-floor bedroom and storage loft. It channels rainwater toward the street, into a gutter that projects beyond the building and over the entry walk; in a proscenium-like gesture, the water cascades into a rock box, and then flows naturally to the street.

 Rather than parallel the open branches of the Y, the main roof of the house forms a straight bar oriented toward the view. This misalignment creates exterior spaces on both levels: a lake-facing deck on the second-floor and a south-facing patio outside the ground-floor kitchen. Cohan says the project is influenced by Northwest modernism, which traditionally employs deep eaves and other features to extend outdoor activities beyond the region’s few sunny days.

The butterfly shape of this roof forms a natural gutter that collects the water and transfers it through an extended gutter and down a rainchain into a series of three off-the-shelf commercial-grade cisterns installed at grade. These tall concrete vessels usually would be buried well out of sight, but because this main part of the house is too far downhill to drain naturally to the street, “once they’re full and overflowing, rainwater is at an elevation that it can gravity-drain to the street without requiring a motorized pump. It’s an artificial gravity system,” Cohan says. The cisterns also form powerful site elements that visually shield the south patio from the street.

Before cistern water reaches the overflow point, it is used for flushing toilets, laundry, and irrigation. Of course there is much more rainwater than the Cedar Park House can realistically use. Collecting it all would require many times the number of cisterns. But in this anti-gravity system, all of the rainwater that does strike the house is directed off-site in a controlled manner, inflecting no further harm on the fragile hillside.

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