“Spongelike” would be the last descriptor for soil conditions in Austin, as the mostly-limestone ground only sips the thunderstorms endemic to this part of Texas. In a “Best Green Houses” column earlier this year, Cascading Creek House by Austin-based Bercy Chen Studio, embodied one architect’s response to excessive runoff: incorporating reservoirs into the infrastructure and aesthetics of a residential property. West Lake Pool House, designed by Murray Legge, the Toronto-born principal of Austin-based LZT Architects, represents another. The 650-square-foot structure and its environs enhance the site’s absorptive capability.
West Lake Pool House is located in West Lake Hills, Texas, a small city just outside Austin proper. To prevent excessive stormwater runoff here, the municipality has strict regulations regarding impervious cover. Legge’s client had largely reached those limits by the time she contacted him to create a pool house from scratch. An earlier renovation of the existing house included significant hardscaping of the front yard.
The house perches over a box canyon, so the rear of this acre property slopes downward. Recognizing that the rooftop of any pool house would be seen from the kitchen and living room, Legge decided to impart it with a drumlin shape that evokes local topography. “It made sense to think of the project from the roof downward, and to have a conversation with the surrounding hills.” Intensive plantings were considered part of the conversation—and an exact replacement for the small building’s impermeable footprint.
The roof structure is faceted, with tubular steel members supported by additional light-gauge steel. Above it, the multilayered garden assembly smoothed out the armature into the compound curving appearance of a drumlin (an elongated oval mound that looks like an inverted spoon). Legge admits, “Austin is not the best climate for green roofs, because, between downpours, it gets extremely hot and dry. Roofs struggle. But that’s not a reason to avoid them; it just takes more technical work.” The architect collaborated with American Hydrotech—the consultant responsible for the mogul-like green roof on the California Academy of Sciences in San Francisco by Renzo Piano and, closer to home, the Antoine Predock–designed Austin City Hall—to devise a solution. Among other components, the assembly includes a moisture retention layer and a mesh system that geotechnical engineers usually specify to construct steep roads for the military.
To maximize roof performance, Legge avoided penetrations. All pool and spa equipment is located in a floor underneath the living space, and kitchen and bathroom venting pulls to the sides of the building. Due to the grade of the site, the vertical solution is not even revealed unless a visitor walked around the pool house. Hatches and other access points are cleverly concealed.
Legge had intended to finish the surface underneath the armature in plaster, but the client had envisioned using wood, a material less forgiving to the drumlin shape. Legge realized that traditional canoe construction could be applied to this unique situation. Buda, Texas–based Escobedo Construction, which performed most of the stone fabrication and ceiling woodwork on the project, employed cedar for this feature. “You start on the gunnel edges of the canoe and you work your way up to the keel line. Escobedo started on the long edges of the vault, which are straight and flat, then the carpenters worked their way up into the middle as you would on a boat.” The ceiling sits atop a clerestory.
The combination of high- and low-tech strategies has optimized performance. Even prior to insulation of the roof, Legge noted a 120-degree difference between the temperature of the pool house’s copper flashing and the temperature of the garden assembly’s underside. Several extra layers of felt are making it easier for plants to establish roots in the tricky middle part of the drumlin slope, and storm-water runoff is minimal. Legge also reports that the overall compound curvature enhances interior daylighting via the clerestory.
The pool house overlooks a shotcrete, mosaic-finished pool. And, instead of sitting atop a concrete pad, its surrounding Hadrian limestone terrace is constructed like a wood deck or catwalk in which the frame is shop-fabricated and field-bolted galvanized steel; plastic pedestals were mounted to the frame, and pavers set on top of them. Storm water percolates through eighth-inch joints, thereby accommodating the slow absorption rate of the soil, although gutter system collects any excess water from squalls or the pool’s infinity edge for recirculation.
Legge admits that he had his doubts about employing a green roof in this runoff-sensitive situation. He had seen several systems that behaved more like hanging planters than the ground cover that retains and transpires moisture. Moreover, the City of West Lake Hills’ code for pervious surfacing does not include incentives for green roofs; Legge campaigned strongly for a variance, which the city then granted. The completed pool house affirms thoughtfully conceived green roofs as a sound sustainability technique. It may also open the minds of municipalities elsewhere.