Soft as Steel

Kengo Kuma finds a poetic alternative to concrete in the iron alloy.

“The biggest problem facing contemporary architecture is concrete,” Tokyo-based Kengo Kuma says. Practically synonymous with Western Modernism, concrete became the construction material of choice for many architects in Japan during the post–World War II period. Over time they used it for everything from massive institutional structures to tiny urban dwellings. But Kuma, who strives for a perfect match between materials and project particulars, builds with just about anything except concrete—especially when it comes to houses.

Completed last November, the Steel House originated when a university professor asked Kuma to design a central Tokyo home for him, his wife, and his teenage daughter. The two main considerations were the client’s vast collection of model trains and the unusual characteristics of the property. Straddling a 20-foot drop, the L-shaped parcel abuts a pedestrian ramp that separates it from a multilane thoroughfare lined with five-story buildings to the south; to the west, it backs up against a ridge of small houses. “I could have used concrete, but then the excitement of the site’s abnormality would have disappeared,” Kuma says of the formidable material. Instead, he opted for a less heavy-handed solution in the form of a monocoque shell of corrugated steel normally used for retaining walls and bridges. While an existing retaining wall meant Kuma had a flat plane to build on, he chose to engage both levels of the site.

Steel House is made of 0.13-inch-thick galvanized ribbed panels bolted together on-site, with an all-in-one skin and structure. In ad­di­tion to maximizing the narrow plot by eliminating interior columns and beams, the plates allowed Kuma to cut windows wherever he wanted—a valuable asset given the surroundings. “I always try to bring out the essence of a material,” Kuma says. “Corrugated metal has lots of corners that reveal its thinness and ability to bend.” Nowhere is this more evident than at the roof, where a cantilevered sheet juts out as a canopy.

While flaunting the material outside, Kuma had to modify his approach inside to make the steel shell into a habitable home. The house contains a generous foyer and a traditional-style tearoom on the first floor; a continuous second-story living, dining, and kitchen area; and sleeping berths, a bathroom, and a wood deck on the third floor. Though the stairs and ceilings are exposed metal, Kuma padded the inner walls. White plasterboard adds warmth and intimacy to the sleeping quarters, and the living spaces are wrapped in three-quarter-inch-thick insulation thinly masked by transparent polycarbonate panels that impart a soft greenish glow. The client likens the effect to the color of a bamboo grove. Built-in components form yet another layer: a 30-foot-long glass-shelving showcase for the model trains dominates the second floor, and downstairs the tatami-floored tearoom is surrounded by movable washi-paper screens. “My method is like a traditional junihitoe twelve-layered kimono that separates the body from its environment,” Kuma says.

Because of code restrictions, Steel House has to sit on a concrete base, but the rest of Kuma’s palette—wafer-thin steel plates, plastic panels, and washi paper—all respond to the site’s topography or the vernacular wood-frame houses nearby. “Fragile materials create the Jap­anese lifestyle, but concrete destroys it,” he laments. “We should recover our sensitivity by going back to fragile materials.”

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